Espresso Coffee
The Science of Quality
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Espresso Coffee
The Science of Quality
Second edition
Edited by
ANDREA ILLY
and
RINANTONIO VIANI
with the assistance of Furio Suggi Liverani
This book is printed on acid-free paper
First published 1995
Second edition 2005
Copyright # 1995, 2005, Illycaffe` s.p.a. All rights reserved.
No part of this publication may be reproduced, stored in a retrieval system,
or transmitted in any form or by any means electronic, mechanical, photocopying,
recording or otherwise, without the prior written permission of the publisher
Permissions may be sought directly from Elsevier’s Science & Technology Rights
Department in Oxford, UK: phone: (+44) 1865 843830, fax: (+44) 1865 853333,
e-mail: [email protected]. You may also complete your request on-line via
the Elsevier homepage (http://www.elsevier.com), by selecting ‘Customer Support’
and then ‘Obtaining Permissions’
Elsevier Academic Press
525 B Street, Suite 1900, San Diego, California 92101-4495, USA
http://www.elsevier.com
Elsevier Academic Press
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http://www.elsevier.com
British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British Library
Library of Congress Catalog Number: 2004113615
ISBN 0-12-370371-9
Printed and bound in Italy
05 06 07 08 09 9 8 7 6 5 4 3 2 1
Contents
Contributors ix
Acknowledgements xvii
1 Quality 1
A. Illy
1.1 Origins and meanings of quality 1
1.2 Definition of quality 2
1.3 Commercial quality 3
1.4 Quality of food products 7
1.5 The experience of coffee consumption 10
1.6 The quality of espresso coffee 15
1.7 Definition of espresso 16
1.8 Conclusions 19
References 20
2 The plant 21
F. Anzueto, T.W. Baumann, G. Graziosi, C.R. Piccin,
M.R. So¨ndahl and H.A.M. van der Vossen
2.1 Origin, production and botany 21
2.2 Variety development 29
2.3 Agronomy 34
2.4 Biochemical ecology 55
2.5 Molecular genetics of coffee 67
References 76
3 The raw bean 87
S. Bee, C.H.J. Brando, G. Brumen, N. Carvalhaes,
I. Ko¨lling-Speer, K. Speer, F. Suggi Liverani, A.A. Teixeira,
R. Teixeira, R.A. Thomaziello, R. Viani and O.G. Vitzthum
3.1 Introduction 87
3.2 Harvesting 87
3.3 Processing of the harvest 91
3.4 Drying 96
3.5 Final processing for export and roasting 101
3.6 Logistics 108
3.7 Defects 116
3.8 Classification: physical and sensorial analysis 134
3.9 Blending 141
3.10 Decaffeination 142
3.11 Raw bean composition 148
References and further reading 166
4 Roasting 179
B. Bonnla¨nder, R. Eggers, U.H. Engelhardt and H.G. Maier
4.1 The process 179
4.2 Roasting techniques 184
4.3 Changes produced by roasting 191
4.4 Volatile aroma compounds 197
4.5 Melanoidins 204
4.6 Contaminants 209
References 209
5 Grinding 215
M. Petracco
5.1 Theory of fracture mechanics 216
5.2 Coffee grinders 218
5.3 Methods for measuring ground product fineness 221
5.4 Parameters influencing grinding 224
5.5 Physico-chemical modifications due to grinding 227
References 229
6 Storage and packaging 230
M.C. Nicoli and O. Savonitti
6.1 Physical and chemical changes of roasted coffee during
storage 230
6.2 Packaging of roasted coffee 245
References 255
7 Percolation 259
M. Petracco
7.1 Conceptual definitions 259
7.2 Physical and chemical characterization of the percolation
process 261
7.3 Modelling of the percolation process 266
7.4 The espresso machine 270
vi Contents
7.5 Parameters influencing percolation 274
References 287
8 The cup 290
M. Petracco
8.1 Physical and chemical characterization of the espresso
beverage 290
8.2 Organoleptic characteristics of espresso (practical aspects) 300
8.3 Espresso definition again 310
8.4 Espresso–milk mixes 311
References 313
9 Physiology of perception 316
R. Cappuccio
9.1 Introduction 316
9.2 Gustation 317
9.3 Olfaction 325
9.4 Human chemosensory psychophysics 332
References 345
10 Coffee consumption and health 352
M. Petracco and R. Viani
10.1 Consumption patterns 353
10.2 Coffee is more than caffeine 357
10.3 Coffee is beneficial to health 357
10.4 Coffee is not harmful to health 365
10.5 Conclusions 369
References 369
Closing remarks 384
E. Illy
Index 385
Contents vii
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Contributors
Francisco Anzueto graduated in 1978 in Agronomy in Guatemala and
obtained a doctorate in Coffee Plant Breeding in France in 1993. He was
regional director of Anacafe from 1978 until 1986, where he is now
research director. He holds a professorship at Landivar University in
Guatemala. He has been co-manager or director of several regional
research programs in Central America. He is the co-author of many
scientific papers.
Thomas W. Baumann received his scientific education in Medicine and
Biology at the University of Zurich, which he completed in 1969. After
training in Molecular Biology at the University of Zurich he became
Research Assistant at the Institute of Plant Biology of the same university,
where he was ‘infected’ for the first time with coffee while chemically
analysing and establishing tissue cultures of several species of Coffea.
After postdoctoral studies at the University of Southern California (Allan
Hancock Foundation) in Los Angeles in 1972 and 1973, he returned to
the Institute of Plant Biology, where he continued his phytochemical
work on coffee and other caffeine-containing plant species. In 1977 he
became Docent, and 1995 Professor, of Plant Physiology. His research
focused on the biosynthesis and ecological significance of caffeine and
related purine alkaloids. He also studied the ecological chemistry of other
plant genera, such as Physalis, Echinacea and Catharanthus, until
retirement in 2001. His hobbies are scientific writing and gardening. In
2001 he published together with his wife a voluminous biography of Henri
Pittier, the ‘Humboldt of Switzerland’ (Basle: Friedrich Reinhardt
Verlag). At his home near Zurich he houses a living collection of almost
all caffeine-containing species consumed in the world. Currently, he is
working on a book dealing with the various facets of little-known fruits of
the Neotropics.
Sarah Bee is a radiation physicist with a doctorate from University
College London, UK. She is a Chartered Physicist and Member of the
Institute of Physics. Since 1998 she has been Engineering Product
Manager within the R&D Department at Sortex. She was responsible for
the technical description of the ‘Niagara’ vegetable sorting machine
(colour and shape sorting at 40 000 objects/sec), which obtained Sortex’s
Queen’s Award for Innovation application in 2001.
Bernd Bonnla¨nder graduated in Food Chemistry at Erlangen University,
Germany. After his doctorate on the characterization of aroma precursors
in wine and spices, he joined AromaLab (R&D illycaffe`), where he
performs research on odour active and physiologically active compounds
in green and roasted coffee as well as beverages.
Carlos Henrique Jorge Brando is an engineer with graduate work at
doctoral level in Planning and Business at the Massachusetts Institute of
Technology. He was the manager and director of leading Brazilian coffee
machinery maker Pinhalense for over ten years. He is now director and
partner of a coffee consulting, marketing and trading company, P&A
International Marketing, which was a technical adviser to the Cafe´ do
Brazil Marketing Program. He has been responsible for coffee processing
and marketing projects in over 60 countries and written and spoken
frequently about the two subjects.
Gianfranco Brumen graduated in Chemistry from the University of
Trieste, and joined illycaffe` in 1972 in charge of Quality Control and
Research, becoming an expert in the raw product (characteristics of coffee
types, production countries, raw coffee selection, blend formulation, etc.),
controlling all the production cycle. As Quality Supervisor he is in charge
of blend formulation, final product conformity, quality system application
and its certification (ISO 9001), and all regulatory aspects.
Roberto Cappuccio graduated in Physics at the University of Trieste in
1997 with a thesis on Near Infrared Spectrometry. Immediately afterwards
he entered the R&D Department at illycaffe` as a researcher, where he
dealt mainly with fluid dynamics. Since 2001 his interests have moved
towards the exploration of the sensory and chemical aspects of taste,
aroma and mouthfeel perception of coffee. Since 2002 he has coordinated
illycaffe` Sensorylab. He has written several scientific publications, mainly
in coffee science and technology.
Nelson Carvalhaes is a lawyer who graduated in Agribusiness and Foreign
Trade at Mackenzie University, Sao˜ Paulo in 1981. He has been a
Director of Escrito´rio Carvalhaes Cafe´ Ltda since 1980 and Porto de
Santos C.E. Ltda since 1989.
x Contributors
Rudolf Eggers graduated and obtained his doctorate in Mechanical
Engineering/Process Techniques at the Technical University in Hanover
in 1972. He was research associate at the Technical University in
Clausthal until 1977. From 1977 until 1984 he was head of Process
Engineering at Thyssen Maschinenbau and Krupp Industrietechnik. Since
1984 he has been Professor for Heat and Mass Transfer at the Technical
University in Hamburg–Harburg. He has conducted research and
published in separation processes (extraction, adsorption, refining); coffee
processing (roasting, extraction, decaffeination); interfacial phenomena
(transport effects across phase boundaries); high pressure technique
(supercritical fluids).
Ulrich H. Engelhardt graduated as a Food Chemist and obtained his
doctorate with work on stimulant drinks at the Technical University of
Braunschweig, where he is now a teacher in food analysis, food toxicology
and chemistry and technology of foods containing polyphenols. He has
published about 70 papers, mainly on coffee and tea chemistry. He is
member of the working groups on tea and coffee of the German Institute
of Standards (DIN). In 1996 he received the Young Scientist award of the
German Chemical Society.
Giorgio Graziosi graduated in Natural Sciences in 1966 at the University
of Trieste, where he was appointed Lecturer in Genetics in 1969. He
obtained a position of Associate Professor in 1976 and of Full Professor in
1986. He was appointed Director of the Department of Biology in 1993
and President of the Faculty of Biological Sciences, University of Trieste,
in 1998. He has been visiting scientist at the Developmental Genetics
Unit, International Embryological Laboratory, ‘Hubrecht’, Utrecht,
Holland, at the Genetics Department, University College London, UK
and for some years at the Genetics Laboratory of Oxford University, UK.
His research interests 1967–86 were developmental genetics of Drosophila
melanogaster and since 1987 have focused on DNA polymorphism in
several organisms including Coffea and genomics and gene expression in
Coffea.
Andrea Illy graduated in Chemistry at the University of Trieste and
completed the Master Executive programme at SDA Bocconi in Milan.
He is the chief executive officer of illycaffe`. His first working experience
was in 1983, at the Nestle´ Research and Development department. In
1990 he began his career at illycaffe` as supervisor of the Quality Control
department, where he instituted a total quality control programme. He
has been president (1999) of the Association Scientifique Internationale
Contributors xi
du Cafe´ (ASIC) and organized the 19th International Colloquium on
Coffee Science in Trieste; he is presently vice-president. Since 1999, he
has been a member of the board of directors of the Italian Association of
Premium Brand Industries. In 2003 he was appointed a member of the
advisory board of SDA Bocconi in Milan.
Ernesto Illy graduated in Chemistry at the University of Bologna. He has
been chairman of illycaffe` since 1963 and an initiator active in
international bodies devoted to the scientific and professional development
of coffee. He was co-founder and a past president of Association
Scientifique Internationale du Cafe´ (ASIC), of which he is now senior
vice-president; co-founder, past president and an active member since its
foundation of the Physiological Effects of Coffee Committee (PEC); cofounder
and president of the Institute for Scientific Information on Coffee
(ISIC); President of the Promotion Committee of the Private Sector
Consultative Body (PSCB) of ICO; President of Centromarca (Italian
Association of Trademark Industries). He has been honoured with
important awards from the international coffee community and has
received from the President of Italy the title of Cavaliere del Lavoro.
Isabelle K¨olling-Speer graduated in Food Chemistry at the University of
Hamburg. She obtained her PhD in food chemistry from the same
university in 1993. Before joining the Institute for Food Chemistry at the
Technical University of Dresden in 1993, where she is Scientific
Researcher, she worked with the Hygiene Institute Hamburg in the
Department of Chemistry and Food Chemistry. She is co-author of two
books and has published over 20 scientific publications. Her research
activities are related to the processing and quality control of foods of plant
origin.
Hans Gerhard Maier studied Pharmacy and Food Chemistry at the
Universities in Freiburg (Breisgau) and Frankfurt (Main), obtaining his
doctorate in 1961. He also studied chemistry from 1963 to 1965 at the
University of Frankfurt, obtaining the doctorate in 1969. After working
from 1961 to 1963 in the Food Research Laboratory of Franck und
Kathreiner in Ludwigsburg, he became in 1963 scientific assistant at the
University of Frankfurt, lecturer in 1970, assistant professor in 1971. After
two years spent at the University of Mu¨nster as Wissenschaftlicher Rat
und Professor, he became full professor and director of the Institute of
Food Chemistry at the Technical University in Braunschweig, from where
he retired in 1998. He is the author of many important publications on
coffee, and, in particular, of a book Kaffee (Berlin: Paul Parey, 1981).
xii Contributors
Maria Cristina Nicoli graduated in Pharmaceutical Chemistry at the
University of Bologna. After a decade of post-doctoral research activity at
the University of Udine as assistant professor she moved to the University
of Sassari, where she was appointed Professor in Food Technology. One
year later she was called back to the University of Udine, where she
continues research and teaching activity. Her research activity is mainly
focused on chemical and physical factors affecting food functionality and
shelf-life with particular attention to processed fruit and vegetables and
roasted coffee. She is author of over 80 scientific papers published in
international journals of food science and technology. She has been in
charge of several Italian and European research projects.
Marino Petracco graduated in Chemical Engineering at the University of
Trieste. After 10 years spent in industrial research working in a
multinational petrochemical company, specializing on catalytic cracking,
he joined illycaffe`, working on topics ranging from plant botany to
industrial transformation processes, beverage brewing dynamics and its
effects on the consumer’s body. He has published widely, including 30
research papers on sensory chemometrics, roasted coffee grinding
dynamics, physicochemical characterization of espresso coffee, beverage
brewing hydraulics and raw coffee sampling, and is co-author of several
books. He is active in several scientific associations: in coffee science,
ASIC and PEC, where he held the chair in 1991 and 1992; in coffee
standardization, the Italian organization for standardization (UNI), the
Association Franc¸aise de Normalisation (AFNOR, commission V33A
cafe´) and the International Standardization Organization (ISO TC 34 SC
15 ‘Coffee’); in food hygiene and microbiology, the Association Africaine
de Microbiologie et d’Hygie`ne Alimentaire (AAMHA) and the Sociedad
Latino Americana de Micotoxicologı´a (SLAM).
Carlos Roberto Piccin graduated in Agronomy at the Universidade
Paulista, Jaboticabal SP, Brazil in 1981, and then attended the Graduate
School in Mineral Nutrition at the Universidade Federal, Lavras, MG,
Brazil. After a career as technical manager with various coffee companies,
in 1999 he created Agropiccin, a technical consulting firm expert in
coffee production systems, with emphasis on plant nutrition linked with
integrated pest management seeking productivity, quality product and
biological equilibrium. He is often an invited speaker at universities and
in the private sector.
Contributors xiii
Oriana Savonitti obtained a Masters in Process Chemical Engineering
from the University of Trieste and has been a researcher in the Research
and Development department of illycaffe` since 1998.
Maro R. S¨ondahl obtained in Brazil a BS degree in Agriculture
Engineering in 1968 and MS in Plant Physiology in 1974. In the USA
he obtained a PhD in Managing Technology and Innovation in 1968 and
in Cell Biology in 1978. His professional career developed in government
(13 years) and private organizations (20 years). Since 1994 he has been
President of Fitolink Corporation in the USA, after working at an
Institute of Agronomy in Brazil from 1970 to 1983, and at DNA Plant
Corp. in the USA from 1983 until 1993. He is the author of five US
patents and more than 40 scientific papers and review articles in the area
of plant physiology and plant cell genetics of tropical plants with
emphasis on coffee, cacao and oil palm. He acts as a consultant to several
private coffee companies in the USA and Europe and to international
organizations. He has been Visiting Professor and/or invited lecturer at
several universities in the USA, Brazil, Argentina, Venezuela, Costa Rica,
Colombia and Chile, and an invited speaker at scientific meetings in
more than 20 countries.
Karl Speer completed his degree in food chemistry in 1985, and his
doctorate at the University of Hamburg, Germany in 1993. He was
affiliated with the Hygiene Institute Hamburg, Department of Chemistry
and Food Chemistry, from 1984 until 1993. He has been with the
Institute for Food Chemistry in Dresden since 1993. In 1997 and 1998 he
was a visiting scholar at the Department of Food Science in West
Lafayette, Purdue University, USA. He received the Josef Schormu¨ller
Award in 1992. He is co-author of two books and published over 50
scientific publications, especially in the area of coffee, honey and the
analysis of pesticide residues and organic contaminants in foods. He is
now professor of Food Chemistry and Food Production, Department of
Food Chemistry, Technical University of Dresden.
Furio Suggi Liverani joined illycaffe` in 1991 in the role of knowledge
manager after having worked for 10 years as a computer science adviser.
Since 1995 he has been managing director of the Department of Research
and Development, directly under the CEO. He has participated in several
projects of industrial and laboratory automation and in the development
of illycaffe`’s production information system. He has developed technologies
and innovative products such as an electronic coffee sorting
machine. In 1993 he introduced the use of Internet and multimedia to
xiv Contributors
the company, actualizing the www.illy.com website in 1996. In 1998 on
behalf of illycaffe` he founded AromaLab, a laboratory dedicated to the
study of coffee chemistry, located in the Area Science Park of Trieste. He
has been a council member of the AI*IA (Italian Association for
Artificial Intelligence), co-ordinator of the workgroup AI*IA in industry,
and is a member of several scientific societies. Currently he is member of
the steering committee of the international conference ‘Cellular
Automata for Research and Industry’ and of the ‘Italian Congress of
Artificial Intelligence’. He is also a board member of Qualicaf and adviser
of the EDAMOK project. He is the author of 30 articles on the subjects of
coffee technology, computer science and artificial intelligence, he has
participated as spokesman in several courses, national and international
conferences and conventions. He is the co-inventor of five industrial
patents.
Aldir Alves Teixeira is an Engineer Agronomist, having graduated at Sao˜
Paulo University in 1959. In 1972 he gained a doctorate in Agronomy on
Cup Tasting from Sao˜ Paulo University. He has been president of his own
company, ASSICAFE´ , since 1992, president of the Scientific Research
Association from 1990 to 1993, president of the Evaluation Board of
Commission on the Brazilian Coffee Prize for ‘Espresso’, promoted by
illycaffe`, since 1991 and a member of the Brazilian Delegation to ISOTC34
SC 15 ‘Coffee’. He has been a scientific consultant of illycaffe` since 1991.
Ana Regina Rocha Teixeira is a Biologist who graduated at Sao˜ Paulo
University in 1988. She worked at the Phytopatologic Biochemical
Department of the Biologic Institute of Sao˜ Paulo from 1986 to 1992.
Since 1992 she has worked with ASSICAFE´ , has been a member of the
Evaluation Board of Commission on the Brazilian Coffee Prize for
‘Espresso’, promoted by illycaffe`, since 1991, she contributes to the
Brazilian Delegation to ISO TC 34 SC 15 ‘Coffee’ and has collaborated
with illycaffe` since 1992.
Roberto Antˆonio Thomaziello is an Engineer Agronomist, having
graduated at Sao˜ Paulo University in 1965. His positions have included:
assistant technician of the Coffee Office, CATI, from 1968 to 1977 and
from 1990/91 to 1998; Project Supervisor for the Coffee Agricultural
Secretariat of Sao˜ Paulo from 1977 to 1986; head of the Department of
Rural Assistance of the Agricultural Secretariat/SP from 1992 to 1995;
director of group techniques at the Agricultural Secretariat from 1995
until 1997; and since 1999 at the Coffee Research Center ‘Alcides
Carvalho’ of the Campinas Agronomic Institute.
Contributors xv
Herbert A.M. van der Vossen graduated in 1964 with an Ir (MSc) degree
in Tropical Agronomy and Plant Breeding and gained his doctorate in
1974 with a thesis on breeding and quantitative genetics in the oil palm
from the Wageningen Agricultural University. He was research officer in
charge of the Oil Palm Research Centre at Kade, Ghana, during
1964–1971 and then head of the Coffee Breeding Unit at the Coffee
Research Station near Ruiru, Kenya, from 1971 to 1981. He was research
manager then director of the breeding programmes for vegetable and
flower seed crops with Sluis & Groot Seed Company at Enkhuizen in the
Netherlands from 1981 until early retirement in 1993. From 1993 until
1996 he was seed policy adviser to the Ministry of Agriculture at Dhaka,
Bangladesh and freelance consultant in plant breeding and seed
production, including assignments in vegetable seeds, cereal crops,
cocoa, oil palm and coffee. He has contributed more than 40 scientific
papers and chapters. He is a member of the board of ASIC.
Rinantonio Viani, after graduating in Chemistry at the University of Pisa,
did postgraduate work at CalTech in Pasadena, California, and Duke
University in Durham, NC. From 1963 until 1974 he was a Research
Chemist at the Research Laboratories of Nestle´ in Vevey and Orbe. From
1975 until retirement in 1998 technical and scientific advisor on
stimulant drinks in the Nestle´ headquarters. He has been Chairman of
PEC, Administrative Secretary and President of ASIC and Chairman of
the Technical Commission of European Decaffeinators, of AFCASOLE
and of ISO TC 34 SC 15 ‘Coffee’ for many years. He has obtained several
patents and contributed many scientific publications in food science and
technology, particularly in the field of stimulant drinks. After retirement
he has worked as consultant with the UN agencies UNIDO and FAO for
coffee projects in producer countries.
Otto G. Vitzthum is a Chemist with a doctorate from the University of
Erlangen and postgraduate studies in Madrid. He was Director of
Scientific Research at HAG AG in Bremen from 1978 to 1986;
Department Manager in Central Research at Jacobs Suchard from 1986
until 1994 and Director of Coffee Chemistry worldwide at Kraft Jacobs
Suchard from 1994 until retirement in 1997; Lecturer at the Technical
University Braunscheig since 1978 and Honorary Professor since 1987.
He has been scientific secretary of ASIC and chairman of the technical
commission of European Decaffeinators for many years, and twice
Chairman of PEC. He is the author of many patents and scientific
publications in the field of coffee.
xvi Contributors
Acknowledgements
The Editors thank Ms Elaine Dentan and Mr Fabio Silizio for the
microscopy photos, Prospero S.r.l. for all figures and electronic drafts, and
Ms Andrea Appelwick for her constant editorial assistance.
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CHAPTER1
Quality
A. Illy
The term ‘quality’ has been abused since the later 1980s, when Western
industries realized that lack of quality accounted for their diminished
competitiveness with respect to those of Japan. This tendency to speak, at
times inappropriately, about quality seldom reflects the true meaning of
the word itself, although this is perhaps understandable and justifiable if
one considers that ‘quality’ can convey just as many meanings as the
‘ability’ an object has of producing effects, as will be seen below.
Quality is highly pervasive, and this prevents anyone who decides to
‘create quality’ from clearly defining a field of action. In companies
recognized on the market for their quality, it is obvious this involves the
entire staff, with no limits in time or space. In other words, quality is
culture.
The following paragraphs will try to sketch at least an outline of this
fascinating concept ‘quality’.
1.1 ORIGINS AND MEANINGS OF QUALITY
The concept of quality is vast and cannot, therefore, be narrowed down to
one meaning. Aristotle distinguished four different families of qualities,
which were subsequently adopted by scholars:
1 Tendencies and aptitudes (examples of tendencies are temperance,
science and virtue, while health, illness, heat, cold, etc. are
examples of aptitudes).
2 Natural abilities and faculties, or active qualities.
3 Sentiments and passions, or passive qualities (sounds, colours, taste,
etc.).
4 Shapes or geometric determination (square, circle, straight line,
etc.).
These families can be classified as:
n attitudinal qualities, which include aptitudes, habits, abilities, virtues
and tendencies, or all the ‘possibilities’ of the object;
n sensorial qualities, which can be perceived by our sense organs and
include sounds, colours, smells, etc.;
n measurable qualities, which can be measured objectively, such as
speed, length, intensity, mass, etc.
The attitudinal qualities refer to the identity attributes of an object, or
what distinguishes it from another, without value hierarchies, whereas the
sensorial and measurable qualities refer to all the value attributes, which
make one thing better than another.
Identity and value are the two poles of meaning of the word ‘quality’. In
a critical analysis, the two poles must be compared with the episteme –
the cognitive dimension of the certain knowledge – and with the taste –
the subjective orientation of individual knowledge. This is of particular
importance for the understanding of quality as innate excellence.
In the modern industrial application, measurable quality prevails: ‘You
can’t have quality if you can’t measure it’ wrote Juran (1951), based on
the scientific method. Alternative and complementary approaches can be
applied based on the product, the user or the production, and the
prevailing dimensions are performance, reliability, conformity, life and
functional efficiency, each with its own units of measurement.
Two ways of considering quality will therefore be compared: the first is
philosophical, where aesthetics concerns itself with sensorial qualities,
and the second is scientific, where everything is related to the
characterization and quantification of measurable qualities.
1.2 DEFINITION OF QUALITY
The range of the term ‘quality’, with all its applications and facets, makes
an exact and concise definition difficult. This explains why there are so
many.
The ‘official’ definition is provided by the International Organization
for Standardization (ISO):
The extent to which a group of intrinsic features (physical, sensorial,
behavioural, temporal, ergonomic, functional, etc.) satisfies the
2 Espresso Coffee
requirements, where requirement means need or expectation
which may be explicit, generally implicit or binding. (ISO, 2000)
There is, moreover, a series of definitions provided by specialists on the
subject. Among the best known are:
Conformance to requirements (Crosby, 1979)
Fitness for use (Juran, 1951)
The efficient production of the quality that the market expects
(Deming, 1982)
The total composite product and service characteristics of marketing,
engineering, manufacturing, and maintenance through which the
product and service in use will meet the expectations of the
customer (Feigenbaum, 1964)
Meeting or exceeding customer expectations at a cost that represents
value to them (Harrington, 1990)
Anything that can be improved (Imai, 1986)
Does not impart loss to society (Taguchi, 1987)
Degree of excellence (Webster’s, 1984)
Although the official definition provides a desirable fusion of the most
important meanings of the original definitions, it must be noted how the
interpretations of these authors make a more or less marked reference to
the various families of qualities described above. Focusing on a few of
these definitions reveals the degree of subjectivity or objectivity of the
qualitative characteristics they take into consideration.
1.3 COMMERCIAL QUALITY
Quality has always been of substantial importance in trade relations and
this is understandable as the value of an exchanged good is the
relationship between its quality and its price. Until a few years ago,
commercial quality was product quality. Nowadays the term total quality
is used to broaden the concept of quality to the service involved in the
exchange.
1.3.1 Historic evolution
In the eighteenth and early nineteenth centuries when production was
limited exclusively to cottage industries, quality, as we know it today, did
not exist. The formal quality audits became necessary only with the
Quality 3
arrival of mass production, mostly in the military field, to such an extent
that, in 1941, the American War Department formed a committee aimed
at enabling the army to procure arms and ammunition in large quantities
without incurring problems of quality. In 1950, only one-third of the
electronic military installations worked properly, and so a working group
of the US Department of Defense was formed to deal specifically with
reliability. This is how reliability came to be one of the ‘chapters’ of
quality.
In the post-war period, with the publication in 1951 of the Quality
Control Handbook by Juran, the concept of ‘the economy of quality’ took
root and is the basis of all industrial quality management today.
In 1956, Feigenbaum introduced the concept of ‘total quality control’,
saying that it must start with the design of the product and end only when
the product has been placed in the hands of a customer who remains
satisfied. So quality control was extended to research and development of
methods and equipment, purchases, production, audit and acceptance
testing, despatch, installation and service.
At the beginning of the 1960s the Japanese quality revolution marked
the end of the previous era of ‘quality control’ and the beginning of the
new era of ‘quality management’. The 1980s saw the birth of a new
concept, that of ‘total quality’. Attention which had till then been
focused on the product alone was shifted to the customer.
Today quality is read as total quality. The concept of ‘fungibility to use’
is replaced by ‘suitability to needs’ and the focal point is no longer merely
the customer, but man, with all his (and her) physical, social and
economical needs, be they explicit or otherwise.
1.3.2 Total quality
From the producer’s point of view it can be said that total quality is
offering products and services in conformity with customers’ needs.
From a consumer’s point of view it is, however, necessary to distinguish
various types of quality:
1 Expected quality is the customer’s expectations from a particular
category of products.
2 Promised quality is the customer’s expectations from a product of one
particular brand. With respect to expected quality, promised quality
is at a higher level, as the customer, who is initially unaware, gains
awareness during his or her analysis of the offer. With the same
product, the factors that differentiate the quality promised by one
4 Espresso Coffee
producer from the other are the elements of communication such as
design, appeal and brand image, price, sales point, advertising, etc.
3 Effective quality is the quality of the product in question, in other
words, the measurable quality. If the promised quality is with regard
to the form of the product, then here we are talking about pure
substance. Effective quality is, in turn, made up of two components.
The first, which is subjective, refers to the excellence of the
characteristics of products and services. Juran’s definition of
‘fungibility to use’ mainly refers to this component. Examples of
product features relating to subjective quality are robustness,
reliability, flexibility, precision, performance, beauty, goodness,
respect for the environment, lifespan, service content, economy of
use and so on. The second component, which is objective, refers,
according to Crosby’s definition (1979), to how the product
conforms to the requirements specified of it in order to respect
the above characteristics, and therefore to the absence of defects.
4 Perceived quality is the sum of the promised and effective qualities,
and therefore both product and communication contribute to this.
This is the most important quality, as the relationship between this
and expected quality determines customer satisfaction.
Satisfaction . Perceived quality / Expected quality
5 Potential quality is how the product can be further improved.
Consumers assess all these qualities one by one. When they decide to
approach a product, they have, or they create, depending on the
information gathered, a ‘map’ corresponding to the expected quality for
that particular category of products. When they must choose between one
brand or another, they will choose the one they perceive as having the
promised quality which is most in keeping with their needs for use and/or
for price. Finally, once the product has been chosen and purchased, they
will be faced with its effective quality, which is not always in line with the
promised quality.
Psychological mechanisms also intervene in this increase in qualitative
appraisal. For instance, it is more difficult for a consumer to realize that
the effective quality of a product is poor if it has a high promised quality.
This is due to the difference between form and substance, which is not
something man is used to measure, as even in nature beauty represents
good. On the other hand, man finds it difficult to recognize even an
existing highly effective quality in a product with a poor image, and,
indeed, even more difficult if one needs to be an expert to assess the
Quality 5
quality of a product. How many people would be able to recognize a fine
Bordeaux wine served in a bottle without a label? And vice versa, how
many could recognize that a fairly good wine poured from a bottle of
premier cru classe´ is not excellent?
1.3.3 Quality certification
Total quality is part of a large movement begun at the end of the post-war
period in the West, aimed at improving the quality of life. This involved
the introduction of more and more restrictive technical regulations
regarding consumer safety and environmental protection, which gradually
reduced the freedom of the producers of industrial goods. In order to
maintain the free circulation of goods in such a restrictive market, it
became necessary to be able to certify the quality of traded goods
objectively and impartially. Therefore, especially in Europe under the
pressure of the European unification, international legislation changed
and quality certification was introduced.
There are two different types of quality certification: conformity
certification and quality system certification.
Conformity certification means the certification of the conformity of the
product to its declared characteristics. This type of certification can either
be binding, as in the case of potentially dangerous products, or consensual,
where the producers wish to attract attention to the qualitative contents
of their product. The certificate is issued by a third party, often on the
basis of tests carried out on the product in question. System certification
seeks to have an official body certifying the ‘quality assurance system’,
which includes all the structural and organizational factors put in place to
obtain and maintain a particular standard of quality. Reference is made to
the International Organization for Standardization ISO 9000 regulations,
which are recognized nearly all over the world. Its latest revision, ‘Vision
2000’, has increased senior management involvement and has also
introduced the concept of ethics in socio-economical activities.
Conformity certification is direct in that the object certified is the
effective quality of the product. System certification, on the other hand, is
indirect, as it implies that a producer who complies with ISO 9000
regulations is capable of respecting the stipulated qualitative standard. In
both cases, in order to be able to issue certificates, the certifying body
must in turn be accredited by an appointed national authority. These
bodies should actually be delegated by the public authorities, in
compliance with a specific legislation on quality, the so-called ‘Country
Quality System’.
6 Espresso Coffee
1.4 QUALITY OF FOOD PRODUCTS
Food products are consumed both owing to man’s need to feed, but also
from the search for pleasure provided by certain foods or drinks. In the
case of unfinished products, one further aspect looked for in food products
is the service level, i.e. how easy it is to use or quick to prepare. These
three components of the expected quality for food products are listed
below.
1.4.1 Nutritional quality
The nutritional quality of a food product depends on the nutritional
content of the food product and how safe it is for the consumer’s health.
The nutritional value implies both a quantitative aspect, in terms of the
number of calories provided by the product per unit weight, and a
qualitative aspect given by the composition in nutritive classes, or the
relative percentages of carbohydrates, proteins, lipids, vitamins and
mineral salts, plus other substances, such as fibres. Each nutritive class
can, in turn, be analysed from a qualitative point of view, by taking into
consideration the content of essential substances (such as amino acids and
fatty acids) and its digestibility.
Wholesomeness, or hygienic quality, corresponds, on the other hand, to
the complete absence of toxicity in food products. Leaving aside cases of
uncontaminated food products, which become toxic due to improper use
by the consumer, it can be said that food products only become toxic by
contamination. This contamination may be chemical or bacteriological
and can take place at any time during the life cycle of the product due to
endogenous or exogenous factors.
Generally speaking, nutritional quality is deemed the acceptable
minimum, that is, an indispensable condition for trading a product.
This means that the nutritional content of a particular product must be
consistent with its category and absolute wholesomeness. As the
acceptable minimum, nutritional quality is at a lower level than expected
quality therefore the consumer tends to take it for granted. Or consumers
only take it into consideration when deciding whether or not to try out
products in that particular category. This is why nutritional quality is
normally standardized by law, especially regarding the aspects affecting its
wholesomeness.
In some cases, however, nutritional quality is a condition for
excellence. This is the case of special products, either of registered origin
or, more generally, the products that, within their category, distinguish
themselves by offering advantages for the consumer.
Quality 7
1.4.2 Sensorial quality
Sensorial quality is a secondary quality, in that it is the effect produced by
certain primary qualities on our sense organs. The primary qualities in
question (organoleptic attributes, according to ISO 5492) are to be
tracked down to those chemical and/or physical properties of food – such
as taste, aroma, texture, aspect (e.g. colour), maybe sound (e.g.
crunchiness) – that impact on consumers’ exteroception, namely on the
senses of gustation, olfaction, haptic (tactile) sensitivity, vision and
audition.
Sensorial quality can be defined as the ability of a product to satisfy the
hedonic needs of consumers. It is, therefore, subjective as it is assessed
through an interpretation by each consumer and cannot be measured in
an absolute sense.
Despite this, products that most consumers like are considered quality
products. As a consequence, the only means of defining the sensorial
quality of a product is to bear in mind the tastes and views expressed by
consumers. These opinions are influenced both by the individual
characteristics of the sense organs of each person and each individual’s
ability to use them, and by the customs and traditions of the various
regions in the world influenced by culture, ethnic groups, religion and
social class.
Sensorial quality varies both in time and in space. Moreover, food
products are very complex, which makes it difficult to attribute a
particular sensorial quality to one or more easily analysed constituents. A
way to solve this difficulty is to define accurately the ingredients used and
the processes applied, as this is done with typical or registered products,
and to proceed to the quality control of the finished product by means of
sensorial analysis.
Consumers attach a great deal of importance to sensorial quality,
indeed it can be said that this is what sells a product. This is due to the
fact that, at least in the West, there is no longer a need to appease hunger;
the Western diet is varied and ‘complete’ foods are no longer needed.
Moreover, Western society, which takes care to guarantee consumer
safety, ensures that only products of high nutritional quality are
commercialized.
1.4.3 Service content
The lifespan of the packaged and/or opened product, the cooking time,
the availability, how easy it is to transport, the quantity and quality of
information that accompanies the product, the encumbrance of the
8 Espresso Coffee
packaging to be thrown away, the environmental impact of the packaging
material, packaging safety etc. are all examples of the service content a
product may or may not have.
At times the service content contrasts with other qualities. Just think
of long-life milk, which has an increased lifespan at the expense of a
reduced nutritional and sensorial quality. At other times, however, the
service content strengthens other qualities; one lucky example of this is,
indeed, coffee, which, if pressurized in inert gas, not only lasts three times
as long, but also undergoes a considerable strengthening of its aroma
content.
1.4.4 Conformity certifications
The fact that our society is more mature regarding producer responsibility
in guaranteeing product safety has led to the development of both
horizontal and vertical regulations in the food sector:
1 European Directive 85/374, 25 July 1985 (EC Official Gazette
n. L210 – 7 August 1985) is concerned with the liability for faulty
products (which applies also to all non-food sectors).
2 The cornerstone of the European regulations on the hygiene of food
products is Regulation (EC) n. 852/2004, 29 April 2004 (EC Official
Gazette n. 139 – 30 April 2004), which focuses on prevention by
prescribing obligatory internal audits, assigning responsibility to
manufacturers by imposing the analysis of critical control points –
Hazard Analysis Critical Control Point (HACCP) – and requiring
the drawing up of a manual of correct hygiene practices as a function
of the type of products and processes and the characteristics of the
manufacturer.
3 All the vertical regulations regarding disparate food products as a
function of their critical state are coupled up with this horizontal set
of regulations. Genetically Modified Organisms (GMOs), which so
far do not include coffee product, are regulated by numerous
directives and regulations; their presence in a food is among the
situations most critically perceived by consumers.
1.4.5 Quality and the general food law
As a consequence of the recent European Regulation 178/2002/EC
(General food law), 28 January 2002 (EC Official Gazette n. L31 – 1
February 2002), which states the principles and general requirements of
Quality 9
the food law, provides the setting up of the European Authority for Food
Safety (EFSA) and lays down procedures for food safety control, the
European standardization organizations – which refer to the ISO – are
drawing up guidelines on the systems of traceability in food companies.
According to the new models of development based on the sustainability
of the whole food chain, traceability is an instrument of health and
hygiene guarantee, which, besides inspiring trust in the consumer, can
help strengthen the identity and quality of the food product.
In addition to the safeguarding of fundamental health and hygiene
requirements, protected by the regulations in force, more restrictive
qualitative requirements, or requirements of typicality, are proposed
through the guarantee of product certifications. These can refer to
binding regulations (voluntary regulated product certifications), or to
voluntary technical regulations defined by the producer companies with
the control bodies. Organic farming production is an example.
Table 1.1 gives some examples of the validity of voluntary regulated
product certifications laid down within the European Union.
The seal of quality is attributed to farm and food products for which a
particular quality, reputation or other characteristics depend on their
geographical origin, and whose production, transformation and/or
processing take place in a certain area of production. At least one stage
of the production process must therefore take place in a particular area.
As far as wines are concerned, they are regulated by national
regulations in their respective production countries, which, in their
turn, have been harmonized with the EC Council Regulation n. 823/87,
16 March 1987 (EC Official Gazette n. L084 – 27 March 1987 – Pg. 0059-
0068 and subsequent amendments).
1.5 THE EXPERIENCE OF COFFEE
CONSUMPTION
Ever since coffee was first consumed in the West, towards the middle of
the seventeenth century, specific local habits and traditions have
developed and overlapped, entailing great differences in coffee consumption.
There is generally a remarkable ritual component, divided into two
very distinct rites, preparation and tasting. Below is a cross-section of
contemporary coffee cultures, which clearly shows the expected and
perceived qualities in the various countries.
n Amsterdam: Essential times for drinking coffee in Holland are in the
morning, at breakfast and around 10 or 11 o’clock, often with guests.
10 Espresso Coffee
It is mostly prepared with a filter machine, even though a growing
number of people now have an espresso machine in their homes.
Much care is taken in the preparation, the presentation and how the
coffee is served. The ‘koffie verkeerd’ is their typical coffee, drunk with
plenty of milk in mugs and often served with sweets, such as apple
pie. Socializing and comfort are two aspects particularly associated
with coffee consumption in Holland and, in public, young people
want to drink coffee ‘in the right way’. Espresso has become the ideal
Quality 11
Table 1.1 Regulations related to quality in the European Union
Product Definition Regulation
Organic
product
A food product for which, throughout
the production cycle, the use of
chemicals (pesticides and fertilizers) is
excluded, and only the use of
environmentally friendly techniques of
cultivation and stock farming is
foreseen. Land is made fertile by crop
rotation and the use of organic manure
and natural minerals, while
environmentally friendly products and
techniques are employed to defend the
crops from parasites. Coffee can be
certified as organic
EC Regulation n. 2092/91
– Official Gazette n. L198
– 22 July 1991 and
subsequent amendments
Protected
designation
of origin
(PDO)
Seal of quality attributed to food
products whose particular characteristics
essentially depend on the territory in
which they are produced. The
geographical environment includes
natural and human factors, which make
it possible to obtain a product, which
cannot be imitated outside its defined
area of production.
All stages of production, transformation
and processing must take place in a
delimited geographical area
EC Regulation n. 2081/92
– Official Gazette n. L208
– 24 July 1992 – pp. 1–8
and subsequent
amendments
Protected
geographical
indication
(PGI)
Seal of quality attributed to farm and
food products for which a particular
quality, reputation or other
characteristics depend on their
geographical origin, and whose
production, transformation and/or
processing take place in a certain area
of production. At least one stage of the
production process must therefore take
place in a particular area.
EC Regulation n. 2081/92 –
Official Gazette n. L208 –
24 July 1992 – pp. 1–8 and
subsequent amendments
after-dinner coffee beverage, and possession of an espresso machine
with all its accessories has become a status symbol.
n Hamburg: Coffee in Germany is associated, first of all, with wellbeing
and euphoria, relaxation and fun, and, despite the fact that it is
consumed throughout the day, it is seldom drunk after a meal. The
day starts off with a steaming cup of coffee and a large breakfast. The
careful preparation of the brew takes place in a relaxed atmosphere,
in which the coffee is served on a perfectly laid table, every day of the
year, and drunk in quantity, followed by fresh bread rolls. Throughout
the rest of the day, plenty of coffee is drunk, both at work and out and
about, in the ‘EisKaffe’, places where beverages are served with cakes,
or in the ‘Steh-kaffee’, bars with high counters looking out onto the
street, where one can drink coffee standing up. The traditional coffee
is mostly prepared with a filter, but for a more modern consumer
espresso coffee consumption is growing. Milk is gradually replacing
cream, which traditionally used to be served with coffee. Germans
pay particular attention to coffee being at the right temperature, and
so they tend to keep it handy in a thermos. In the afternoon, around 5
o’clock, it is time for ‘Kaffeeklatsch’, an important moment for
socializing at home, which is the ideal formula for inviting someone
to the home.
n London: The main reason British people decide to drink a cup of
coffee on a weekday is for its stimulating effect to improve their
performances. Taste and aroma are of secondary importance, so much
so that some take a thermos of coffee with them from home. Lack of
time and space have contributed to the spread of extremely
functional methods of preparing coffee, such as instant coffee
prepared with the kettle that is also used for tea, which still has a
strong hold in British homes. At weekends, when people have more
time, habits change and coffee is then drunk with real pleasure,
because it no longer means only ‘getting on with something quickly’.
Some bars have become popular because they offer coffee in real
settings consumers can relate to. Coffee consumption in the UK is
therefore a modern and cosmopolitan experience, most certainly not
a substitute for the traditional rite of tea.
n Naples: Naples probably represents the city with the richest daily
idiosyncratic coffee culture in the world, having elaborated both a
philosophy and a theory on its use and consumption. It is no accident
if all expressive activities that distinguish this city, from the theatre of
de Filippo to the songs of Pino Daniele, have dealt with coffee by
creating and offering particularly original visions and interpretations
of the experience. At home, coffee is prepared with the Neapolitan
12 Espresso Coffee
coffeemaker and served in the ‘tazzulella’. At the coffee bar – an
authentic omnipresent coffee temple (approximately one for every
450 people) – a very concentrated espresso of no more than 20 ml
(called ristretto) is served, prepared by the expert hand of a
Neapolitan barman, operating on an old-fashioned lever-espresso
machine, with sugar mixed with coffee during percolation.
n New York: The first coffee of the morning, generally prepared with
a filter machine or by the infusion system, is drunk in a hurry in
the traditional mugs accompanied by the traditional muffins or
bagels, and seen as a habit, with no special rite. A common scene in
New York is that of coffee drunk in the street in a paper cup, ‘in
transit’, like street fuel, very basic and always available on every
street corner. There is an enormously wide supply of blends for the
home – besides the various brands, you can find pure origin,
flavoured coffee, organic coffee, etc. Coffee is less commonly taken
after main meals, whereas, especially on holidays, people often take
a break during the day to prepare a coffee for themselves or for
friends. There are no fixed rules about occasions for drinking coffee
as there are in other countries. Espresso, stronger tasting than the
traditional American coffee, and the beverages associated with it,
the so-called ESBAD (espresso-based), such as latte and cappuccino,
are consumed more frequently in the coffee shop chains or in
Italian restaurants and are therefore mainly considered ‘special’
beverages.
n Oslo: The basic Norwegian coffee is what is known as ordinary
coffee, which is black, with no milk or sugar, mostly associated
with home consumption. Preparation with a filter is gradually taking
over from the use of a kettle. Consumption out of the home, which
mainly takes place in coffee bars, is becoming more and more
widespread and this has opened the way for new ways of drinking
coffee, such as espresso and cappuccino. Sometimes these are even
prepared at home, especially when people have guests for the
weekend or in the evening. Consumption is very high – an average
of five cups a day – which makes Norwegians, together with the rest
of the Nordic people, the highest coffee consumers per head in the
world.
n Paris: Coffee is the heart of breakfast; most French people have a real
yearning for it. It is normally prepared using a filter and is drunk with
other typically French products, such as croissants and baguettes.
Another cup is drunk after lunch, without milk. Real coffee lovers
still go to ‘bruˆleries’, if they can find one, where one can enjoy coffee
after choosing the desired strength and aroma. ‘Cafe´’, in its two
Quality 13
accepted meanings, as a beverage and as a place of encounter,
represents an important social and collective rite that reached its
climax, thanks to the likes of Voltaire, Balzac and all the way to
Sartre. In bars and restaurants, especially in the north of the country,
coffee is generally prepared with an espresso machine, although it is
not served as concentrated as in Italy.
n San Francisco: Their first desire in this city is to start the day off with
the rite of a good cup of coffee, that is, to begin the day with an
experience of quality where its careful preparation is something to be
proud of. There are roasted coffee dealers, shops and bars that serve
coffee almost anywhere in town, and they are proud of roasting their
own coffee. San Francisco is an important centre for coffee in the
United States and played a leading role in the 1980s in the revolution
of gourmet coffee. The wide availability of different types of coffee
tempts people to try out all kinds of novelties. Milk and espressobased
beverages are like meals, so at lunch lighter drip coffee is
preferred, while ‘fun’ coffees are consumed after lunch, as a dessert.
The bars are like an extension of the sitting room at home where you
can drink a coffee as you relax, reading the newspaper or surfing on
the Internet.
n Tel Aviv: In the morning, instant coffee, quick and without much
ceremony, is a necessary provision for the working day ahead; no
great interest is taken in preparing the coffee and certainly no time is
dedicated to this. At other times of the day drinking coffee is a way of
spending time together. People in the city love sitting for hours in
roomy, high-tech cafes, where they can go to see and be seen,
especially on Friday, savouring coffee at length with little sips, often
accompanied by dry cakes, biscuits or fruits.
n Tokyo: The pace of life is very fast and people do not have much
time for themselves. For this reason, most people drink instant coffee,
of which there is a wide choice. The most widespread coffee drunk,
especially out-of-home, is American coffee. The coffee is consumed
in bars or from vending machines, mostly as an energizing drink. The
vending machines, to be found everywhere, even in city streets and
on country roads, sell various kinds of soft drinks, but above all, cans
of liquid coffee, for which Japan is at the top of the world market.
Coffee is on the menu in Western-style restaurants, which, together
with the coffee shop chains, are the main places where coffee is
drunk, especially among coffee lovers. At home, this beverage is
indeed little used, people almost only drinking instant coffee once in
the morning. Espresso coffee is very successful, even if it is thought by
many to be too bitter.
14 Espresso Coffee
1.6 THE QUALITY OF ESPRESSO COFFEE
Three coffee experiences can be lived throughout the day:
1 coffee for waking up is still preferred as a hot, dilute beverage;
2 espresso, as has been seen, takes the lion’s share of coffees at breaks,
especially in Latin countries and above all in Italy;
3 in Anglo-Saxon or northern European countries, espresso is
considered a speciality suitable for relaxing, where feeling and
care are involved in its preparation.
The most appreciated characteristics of espresso are its creaminess, body
and the strength of its aroma, associated with stimulating properties,
despite the fact that espresso contains less caffeine than the more dilute
coffees do (see Chapter 8). Espresso therefore represents a benchmark
universally recognized as being a great pleasure and a symbol of Italian
culture, so much so that the highest expression of coffee is that served in
bars in Italy, in restaurants in France, and restaurants and coffee shops in
the rest of the world, precisely because it is coffee prepared with a
professional machine.
As regards the nutritional content (see Chapter 10), the only
expectations one can have from the consumption of an espresso are the
intake of a moderate dose of caffeine, known above all as a stimulating
pleasure, boosting intellectual activity, improving memory and concentration,
quickening reflexes, making it easier to stay awake, improving
one’s mood, etc., and the fact of consuming a product devoid per se of
calories.
The real consumer’s expectations, however, are of a hedonistic nature
(see Chapter 10), because drinking a good espresso is genuinely wonderful.
Espresso is a true elixir, a concentrate of exquisite aromas lasting long after
it has been drunk. Even sight and touch are satisfied, thanks to the striped
hazelnut colour of the foam and to its full body. In moments of relaxation at
home this tasting rite is preceded by the eagerly awaited rite of preparation,
when it is enjoyable to play out the actions of the barman, enriching it
with your own secrets, as you anticipate the pleasure of its taste.
As far as service is concerned, consumers require the roasted and
ground coffee to retain the fragrance of the aromas developed during
roasting until the time the package is opened, and they want to know the
characteristics of the product they are consuming (see Chapter 6). It
should also go a long way, in that only a small amount is needed to make a
good cup of coffee and, once open, the aroma should last until the
contents of the package run out.
Quality 15
Grinding deserves a whole chapter (see Chapter 5). Connoisseurs
believe that in order to be excellent, the beans should only be ground
immediately prior to preparation; unfortunately, it is difficult to find and
take care of top quality household grinders. This is why 90% of packaged
coffee sold in retail shops is ground. The most promising solution for the
future is servings – ground and pressed doses of coffee, sealed in a wrapper
and ready for use. Servings offer the fastest, cleanest way of preparing
espressos, and, most importantly, they guarantee a consistency of quality
that neither coffee beans nor ground coffee can in any way guarantee due
to the serious problems in keeping all the parameters involved in
preparing a perfect espresso cup under control.
Espresso consumption is an aesthetic experience, like tasting a vintage
wine or admiring a painting. It is a search for beauty and goodness for
improving the quality of our life. As it offers such subjectively ineffable
‘goodness’, devoid of defects, the only adequate reaction to it is
astonishment – astonishment that can give birth to enthusiasm, and
therefore intellectual and spiritual enrichment.
The predominance of the experience aspect means that the official
definition of quality may be too limited for espresso. It may be more
appropriate to speak of ‘degree of excellence’. The elements characterizing
effective quality are the subject of this book and will be revealed in detail
as you read. From an organoleptic point of view, we have already seen the
importance of the aroma and the full-body – to a certain extent
represented by the visual component of the foam – while, as regards
the taste, consumers look for a slight bitterness in southern countries or a
slight acidity in northern countries, in both cases accompanied by the
characteristic sweetness of the coffee. On the opposite side, the most
common, serious defects penalizing consumption are the extreme
bitterness and foul flavour of poor quality beans.
1.7 DEFINITION OF ESPRESSO
Everyone in Italy has a clear mental picture of a cup of espresso: a small
heavy china cup with a capacity just over 50 ml, half full with a dark brew
topped by a thick layer of a reddish-brown foam of tiny bubbles, also
known by the Italian term ‘crema’. More than 50 million cups of espresso
are consumed every day in the world: its fragrance and flavour are the first
stimuli in the morning, they crown an excellent meal later in the day, and
act as frequent revivers during lengthy working sessions.
16 Espresso Coffee
1.7.1 Espresso as a lifestyle: brewed on the spur of
the moment
One of the meanings of the word espresso (express) is that it is made for a
special purpose, on the moment, on order (Marzullo, 1965; Hazon, 1981);
therefore it is made for the occasion on express request, extemporaneously
rather than fast. This concept is clarified by the saying ‘the consumer, not
the espresso must wait’. As a direct consequence, once brewed, espresso
cannot be kept and must be drunk immediately, before the foam shrinks
and collapses breaking into patches on the surface. After a while, the
surface of the liquid is completely free from foam, which has dried out on
the walls of the cup above the liquid.
If an espresso is kept waiting, smoothness of taste is lost and perceived
acidity increases with time regardless of cooling. Furthermore, if the cup
cools down, an unbalanced saltiness becomes noticeable.
n Freshness of preparation must be an integral part of the definition of
this very special brew.
1.7.2 Espresso as a brewing technique: it requires
pressure
At the beginning of the twentieth century the need for preparing a cup of
coffee within seconds of a customer’s request led to an increase in the
pressure of the extraction water. Water was heated up to its boiling point
in a sealed kettle, so that the steam in equilibrium created pressure,
accelerating extraction. A drawback of this technique was that brewing
with boiling water provokes over-extraction of astringent and bitter,
usually less soluble, substances, which give a burnt taste to the brew.
Brewing was first improved by separating the water used for brewing,
best hot but not boiling, from the heating water. Pressures as high as 10
bars could be created by a lever, multiplying the force of the arm of the
bartender, producing a thick layer of foam on the cup. The lever has now
been replaced by an electric pump, simpler and more regular to operate.
A pressure field applied within a fluid produces potential energy – what
is known as Bernoulli’s piezometric energy – which can be easily
transformed into kinetic energy, and further transformed into surface
potential energy and heat.
Quality 17
Pressure is important for the definition of espresso, making it different
from other brews. During espresso percolation (see Chapter 7), a small
amount of hot water under pressure is applied to a layer of ground roasted
coffee, the coffee cake, and this very efficiently produces a concentrated
brew, containing not only soluble solids, but also lipophilic substances,
lacking in filter and instant coffees. The foam on the top and the opaque
brew are unique to espresso, owing to the presence of a disperse phase
formed by very small oil droplets in emulsion (Petracco, 1989) (see 8.1.1),
which are perceived in the mouth as a special creamy sensation, the body.
Furthermore, the oil droplets preserve many volatile aromatic components,
which would otherwise either escape into the atmosphere or be
destroyed by contact with water as in other brewing techniques, so that
the rich coffee taste lingers in the mouth for several minutes. If coffee
were percolated under high static pressure only, the pressure would be lost
downstream and no work could be performed on the cake; while, if kinetic
energy from stirring propellers, choke nozzles, sprayers, etc., were applied
downstream from the cake, a smooth layer of foam could be produced, but
it would lack body.
The Latin etymology of the word espresso, literally meaning pressed out
(Campanini and Carboni, 1993), clearly points out the importance of
pressure in espresso brewing, making the technique an integral part of the
definition:
n Espresso is a brew obtained by percolation of hot water under pressure
through tamped/compacted roasted ground coffee, where the energy
of the water pressure is spent within the cake.
1.7.3 Italian espresso: it must be rapidly brewed
Another important feature of espresso, especially as traditionally drunk in
Italy, is the length of percolation (see 7.5.8). The diversified energy input
in espresso pressure-brewing efficiently brings into the cup both
hydrophilic and lipophilic substances. A best mix is reached within 30
seconds; if the extraction is shorter than 15 seconds a weak and
exceedingly acid unbalanced and under-extracted cup is obtained.
Conversely, if extraction lasts longer than 30 seconds, over-extraction
of substances with poor flavour will produce an ordinary harsh-tasting
cup, as can be easily seen by separately tasting the liquid fraction
percolated after the prescribed 30 seconds.
A quantitative definition can now be given:
18 Espresso Coffee
Italian espresso is a small cup of concentrated brew prepared on
request by extraction of ground roasted coffee beans, with hot
water under pressure for a defined short time.
The range of the parameters is:
Ground coffee portion 6.5 1.5 g
Water temperature 90 5
C
Inlet water pressure 9 2 bar
Percolation time 30 5 seconds
The requisite conditions to make a good cup of espresso will be reviewed
in detail in the following chapters.
1.8 CONCLUSIONS
We have seen how varied and complex the concept of quality is and how
difficult it is to use it as an objective measure, particularly in the case of
the espresso. We have, however, understood that the espresso lives almost
solely on the pleasure it gives consumers. Therefore, if the number of
lovers is to be maintained and increased, it is necessary to seek the means
of continually improving its quality, meaning its degree of excellence. In
the following chapters we will try to outline this road to improvement,
setting out everything that is known so far about the quality of the
espresso.
This becomes a stage on the road to improving the quality of the
production of coffee brewed worldwide, which, as the resounding success
of gourmet coffee has shown in the Anglo-Saxon world, can lead to a
significant increase in consumption. This may result in an important
contribution to the rebalancing of the supply and demand of coffee, thus
improving the precarious financial and social situation the producer
countries find themselves in.
Wine has travelled a similar road very successfully and, besides
providing pleasure to consumers, has achieved a significant rise in the
value of overall production. This has resulted in a segmentation of the
market where no one in the world is any longer surprised by the fact that
the price of a bottle can range over several orders of magnitude.
Consequently, there has been a general increase in the well-being and
satisfaction of those along the whole chain. This road was not easy and
involved an almost ‘manic’ search for excellence, the specialization of the
distribution networks and education of the consumers.
Quality 19
Espresso can play the lead role in a similar situation in the coffee field,
thanks to the image it enjoys as a symbolic beverage and thanks to the
commitment of the whole industry to continuous improvement.
REFERENCES
Campanini G. and Carboni G. (1993) Vocabolario latino-italiano. Milan:
Paravia, p. 524.
Crosby P. (1979) Quality Is Free. New York: McGraw–Hill.
Deming W.E. (1982) Out of Crisis. Cambridge, MA: MIT, Center for
Advanced Engineering Study.
Feigenbaum A.V. (1964) Total Quality Control. New York: McGraw–Hill.
Harrington H.J. (1990) Il costo della non-qualita`. Milan: Editoriale Itaca.
Hazon M. (1981) Grande dizionario italiano–inglese e inglese–italiano.
Milan: Garzanti, p. 1348.
Imai M.K. (1986) The Key to Japan’s Competitive Success. New York: McGraw–
Hill.
ISO (1992) Sensory Analysis – Vocabulary. ISO 5492:1992. Geneva:
International Organization for Standardization.
ISO (2000) Quality Management Systems, Principles and Terminology. ISO
9000:2000. Geneva: International Organization for Standardization.
Juran J.M. (1951) Quality Control Handbook. New York: McGraw–Hill.
Marzullo A. (1965) Dizionario della lingua italiana. Milan: Fratelli Fabbri, p. 417.
Petracco M. (1989) ‘Physico-chemical characterisation of espresso coffee
brew’, Proc. 13th ASIC Coll., pp. 246–261.
Taguchi G. (1987) ‘The system of experimental design: engineering methods
to optimize quality and minimize cost’. Unpublished, White Plains, New
York.
Webster’s (1984) New Riverside University Dictionary.
20 Espresso Coffee
CHAPTER2
The plant
F. Anzueto, T.W. Baumann, G. Graziosi,
C.R. Piccin, M.R. So¨ ndahl and
H.A.M. van der Vossen
2.1 ORIGIN, PRODUCTION AND BOTANY
M.R. So¨ ndahl and H.A.M. van der Vossen
2.1.1 Origin and geographic distribution
Commercial coffee production is based on two plant species, Coffea
arabica L. (arabica coffee) and C. canephora Pierre ex Froehn. (robusta
coffee). A third species, C. liberica Bull ex Hiern (liberica or excelsa
coffee) contributes less than 1% to world coffee production. All species
within the genus Coffea are of tropical African origin (Bridson and
Verdcourt, 1988). Many forms of wild C. canephora can be found in the
equatorial lowland forests from Guinea to Uganda, but natural populations
of C. arabica are restricted to the montane forests of southwestern
Ethiopia (Berthaud and Charrier, 1988).
The date of first domestication of arabica coffee in Ethiopia is uncertain
and so is the claim that Ethiopian invaders brought coffee to Arabia in
the sixth century. However, there is written evidence for extensive
cultivation of coffee in the Arabian Peninsula (Yemen) by the twelfth
century. During the following 300 years, the stimulating hot beverage
prepared from roasted and ground coffee beans, called qawha (a general
word for wine and other stimulants) by the Arabs and cahveh by the Turks,
became immensely popular in the Islamic world and from 1600 onwards
also in Europe, with Mocha as the exclusive centre of coffee trade
(Pendergrast, 1999). In Uganda and other central African countries, there
has been a centuries-old tradition of growing coffee (mainly C. canephora)
near homesteads for the purpose of chewing the dried fruits or beans for
their stimulating effect (Wrigley, 1988).
Commercial production of arabica coffee outside Yemen started in Sri
Lanka (formerly Ceylon) in the 1660s, and subsequently in Java around
1700 with coffee plants introduced by the Dutch East India Company
from Sri Lanka or the Malabar Coast of southwestern India. The arabica
coffee in India and Sri Lanka originated probably from seeds brought
directly from Yemen by Moslem pilgrims during the first decade of the
seventeenth century. A few plants taken from Java to the Botanic Garden
in Amsterdam in 1706 formed the basis of practically all the cultivars of
arabica coffee planted in the Western Hemisphere. These early
introductions of coffee plants from Yemen to Asia and Latin America
are of the variety Typica, C. arabica var. arabica. Coffee plants taken by
the French around 1715 from Yemen to the Re´union island (formerly
Bourbon) and subsequently to Latin America and East Africa are C.
arabica var. bourbon. Varieties of the Bourbon type have generally a more
compact and upright growth habit than Typica varieties, are higher
yielding and produce coffee of better quality (Carvalho, 1988).
By 1860 world coffee trade involved some 4 million bags (60 kg, the
standard unit of trade), mostly from Brazil, Indonesia and Sri Lanka. On
account of its mild and aromatic cup quality, C. arabica would have
certainly continued to be the only cultivated species, had it not been so
vulnerable to diseases, particularly to coffee leaf rust (Hemileia vastatrix).
Coffee leaf rust (CLR) was first reported in 1869 in Sri Lanka and within
20 years it had virtually wiped out coffee cultivation in Asia. Sri Lanka
switched to tea cultivation, but Indonesia has continued to be a major
coffee producer based on C. canephora, which turned out to have
resistance to CLR. Robusta was first introduced into Indonesia (Java)
from Congo in 1900 (via Belgium) and a few decades later, high-yielding
plant materials had been developed by selection, which became known as
robusta coffee and have until today remained the basis of most robusta
coffee production in the world.
Coffee cultivation is now widespread in tropical and subtropical regions,
with the bulk of arabica coffee concentrated in Latin America and robusta
coffee predominant in South-East Asia and Africa (Figure 2.1).
2.1.2 Production
World coffee production increased from 86 million bags in 1980 to 112
million bags in 2000/01 (from 10.5 million ha). However, production
graphs show large annual fluctuations, from a very low 80 million bags in
1986/87 to a record crop of some 115 million bags in 1999/2000, mainly
caused by the leading producer Brazil (ICO, 2002). The Brazilian coffee
22 Espresso Coffee
The plant 23
Figure 2.1 Coffee producing regions of the world
crop may vary from 19% to 33% of annual world production as result of
biennial fluctuations, recurrent frosts and droughts. The tremendous
expansion of robusta coffee cultivation during the past decade, in
Vietnam in particular, but also in Indonesia and Brazil, has further
exacerbated the excess in supply over demand on the world coffee market.
Consequently, the share of arabicas in the world coffee supply has also
dropped from the erstwhile 75% to 68–70%. Production data for the 10
largest coffee-producing countries are presented in Table 2.1. About 59%
of the world’s coffee in 2000/01 was produced in Latin America, 21% in
Africa and 20% in Asia. Brazil (80% arabica) alone produced 26%,
Vietnam (97% robusta) 12%, and Colombia (100% arabica) 11% of the
2000/01 coffee crop. Coffee is produced in some 60 countries, including
the coffee-exporting members of the International Coffee Organization
(40 at 30 September 2001: ITC, 2002, p. 21). In most countries,
smallholders are the main coffee producers, but medium to large coffee
plantations (30–3000 ha) can be found in countries like Brazil, El
Salvador, Guatemala, India, Indonesia, Vietnam, Kenya and Ivory Coast.
2.1.3 Taxonomy
The genus Coffea belongs to the family Rubiaceae, subfamily Ixoroideae and
tribe Coffeae. Linnaeus described the first coffee species (Coffea arabica)
24 Espresso Coffee
Table 2.1 Coffee production in the 10 largest
producing countries and world total (millions of
60 kg bags)
Country
Year 2000/01 Annual average
Arabica Robusta Total 1996–2001
Brazil 23.7 6.4 30.1 28.9
Vietnam 0.4 14.8 15.2 8.6
Colombia 10.5 10.5 11.1
Indonesia 0.7 5.7 6.4 7.1
Mexico 5.1 0.1 5.2 5.5
India 2.4 2.6 5.0 4.2
Ivory Coast 4.0 4.0 3.9
Guatemala 4.7 0.05 4.7 4.7
Ethiopia 2.9 2.9 3.2
Uganda 0.4 2.8 3.2 3.3
Total (60 countries) 68.8 42.9 111.7 106.6
Sources: ICO, 2002; ITC, 2002
in 1753. The taxonomy of coffee has been under recent review and may
not yet be completely finalized yet. Bridson and Verdcourt (1988)
have grouped coffee species in two genera: Coffea (subgenera Coffea
and Baracoffea) and Psilanthus (subgenera Psilanthus and Afrocoffea).
The subgenus Coffea includes 80 species, 25 from continental Africa and
55 unique to Madagascar and the Mascarene Islands. The subgenus
Baracoffea was assigned only seven species (Bridson, 1994). Conventional
and molecular genetic studies confirm that five clusters of related species
can be distinguished corresponding to geographical regions (West,
Central and East Africa, Ethiopia and Madagascar), but that the process
of differentiation between these groups has not yet progressed into a stage
of strong crossing barriers (Berthaud and Charrier, 1988; Lashermes et al.,
1997). The earlier taxonomic classifications into sections and subsections
(Chevalier, 1947) or subgenera (Leroy, 1980) do not appear to be justified
any longer. Instead, a monophyletic origin of all species within the genus
Coffea has been generally accepted. Molecular evidence (Lashermes et al.,
1997; Cros et al., 1998) for fairly close genetic relations between Coffea
and the genus Psilanthus (Bridson and Verdcourt, 1988), as well as the
successful hybridization between C. arabica and a Psilanthus species
(Couturon et al., 1998) may lead to a taxonomic revision into one genus
Coffea with more than 100 species.
All Coffea species are diploid (2n . 22 chromosomes) except for the
allotetraploid C. arabica (2n . 44), which probably originated as a
spontaneous interspecific cross between two diploid species, followed by
duplication (Carvalho, 1946; Sondahl and Sharp, 1979). Recent
cytogenetic and molecular studies have indicated that indeed C.
eugenioides and C. canephora (or C. congensis) are the most likely
progenitors of C. arabica (Raina et al., 1998; Lashermes et al., 1999).
The analysis of segregating molecular markers (Lashermes et al., 2000a)
has confirmed earlier genetic and cytogenetic evidence that C. arabica is a
functional diploid. The observed disomic meiotic behaviour is probably
under the control of genes regulating pairing of chromosomes.
2.1.4 Growth, flowering and fruiting
The coffee plant is an evergreen shrub or small tree, which under free
growth may become 4–6m tall for C. arabica and 8–12m for C. canephora.
In cultivation, both species are pruned to manageable heights of less than
2m (less than 3m in mechanically harvested plantations in Brazil) with
one or more stems. The strictly dimorphic branching habit of coffee, as is
schematically presented in Figure 2.2, determines plant growth habit and
The plant 25
has important consequences for agronomic practices, such as pruning for
maintenance or rejuvenation and vegetative propagation (Cannell,
1985). Continuously growing orthotropic (vertical) stems produce a
pair of plagiotropic (horizontal) branches at each new node from the
‘head of series’ buds. The other buds on the same node remain dormant
but will grow out into orthotropic shoots called suckers, when forced by
capping of the main stem. Young seedlings produce the first pair of
plagiotropic branches on the ninth to eleventh node. Branches bear two
opposite leaves per node, with glossy dark green upper surfaces, 10–15
5–10 cm in size, and slightly undulating in arabica. In robusta plants,
leaves are much larger and strongly corrugated. The serial buds present in
the leaf axils on plagiotropic branches may develop into inflorescences or
secondary lateral branches, but never into orthotropic shoots. In
consequence of this strictly dimorphic growth habit in coffee, only
cuttings prepared from orthotropic shoots (suckers) will, after rooting,
grow into trees with a normal upright growth habit.
At the end of the rain period, differentiation of flower buds takes place,
and this phenomenon is frequently associated with lower temperatures
and shorter daylengths. Soon after differentiation, flower buds will grow
26 Espresso Coffee
Figure 2.2 Dimorphic branching habit of coffee (Cannell, 1985, p. 113)
for about 8 weeks until reaching 5mm in length, and then become
quiescent for about 2 months. At the end of this period, they will
complete their differentiation process to form a glomerule (short
inflorescence) bearing a cluster of 2–19 flower buds in arabica
(Carvalho et al., 1969) and up to 80 buds in robusta. Later, flower buds
resume growth until they reach 8–10mm in length, and then enter into a
second quiescent period. At this point, they are quite visible, with a paleyellow
coloration. Arrested flower buds will grow again when exposed to
first showers of the new rainy period. Flower buds are then released from
dormancy, undergo a rapid expansion during 8–10 days, changing colour
from pale-yellow to white. This last growth phase will end with the
opening of the flowers (anthesis), resulting in massive flushes of
blossoming (Alvim, 1973; Cannell, 1985). Depending on the prevailing
dry season, two to five blossoming events will occur per flowering season,
leading to sequential ripening events and differentiated pickings. The
longer the dry season, the more synchronized flowering and reduced
number of blossoming events.
In equatorial regions with bimodal rainfall patterns, flowerings occur at
the beginning of both rainy seasons, and this will result in overlapping
stages of fruit development and fruit ripening extending over several
months. In unimodal rainfall regions further from the equator, there is a
single flowering season, and a much shorter harvesting period.
The white and fragrant bisexual flowers have a corolla tube 8–10mm in
length, with five lobes (five to seven in C. canephora). The five stamens
are fused to the corolla and the bilocular anthers open lengthwise. The
pistil has an inferior, bilocular ovary and a filiform style with two
stigmatic branches (Van der Vossen et al., 2000) (Figure 2.3).
Flowers wither within 2–3 days after anthesis and pollination. C. arabica
is self-fertile and less than 10%of the flowers are naturally cross-pollinated,
mainly by visiting bees (Carvalho, 1988). On the other hand C. canephora
is strictly allogamous with a gametophytic system of self-incompatibility
preventing self-pollination (Berthaud, 1980; Lashermes et al., 1996a).
Pollen tubes take about 24 hours to reach the ovary followed by
fertilization, but the first cell division of the zygote takes place not until
60–70 days later (Dedecca, 1957). Three stages of fruit development can be
distinguished (Cannell, 1985). After an initial period of dormancy with
little growth during the first 6–8 weeks after flowering (pinhead stage)
fruitlets expand in volume almost to the final size over a period of 8–10
weeks (soft-green stage) followed by a period of bean (endosperm) filling for
another 10–15 weeks (hard-green stage). Fruits mature in 7–9 (C. arabica)
to 9–11 months (C. canephora) from date of flowering. The green fruits will
turn yellow or red, depending on the variety, during ripening (Figure 2.4).
The plant 27
The fruit is a drupe, usually called a berry or cherry, containing two
seeds embedded in a fleshy pericarp (fruit skin) and a sweet tasting
mucilage layer. Sometimes the fruit contains only one round seed called
a peaberry. Fruits of C. arabica are 12–18mm and those of C. canephora
8–16mm long. The seeds (coffee beans) are plano-convex in shape and
grooved (centre cut) on the flat side. They consist mostly of endosperm
with a small embryo at the base of the seed and are enveloped in a
rudimentary integument (‘silverskin’) and fibrous endocarp (parchment).
Beans of C. canephora are generally smaller, rounder and with a tighter
centre cut than those of C. arabica.
28 Espresso Coffee
Figure 2.3 Coffee flowering
Figure 2.4 Green, ripe and overripe cherries of C. arabica: (left) yellow variety;
(right) red variety
Coffee fruits are strong assimilate-accepting sinks and the tree is unable
to regulate the crop load effectively by shedding fruits. The key issue in
coffee growing is therefore the prevention of excessive cropping that leads
to biennial bearing and also shoot and root dieback. Crop husbandry is,
therefore, aimed at maintaining enough foliage to sustain the crop (about
20 cm˙ leaf area per fruit) as well as new shoot growth by pruning and other
agronomic practices (Cannell, 1985). Copper and some other types of
fungicides improve leaf retention, particularly in ecosystems with
pronounced periods of water stress. This ‘tonic’ effect of fungicides may
result in considerably higher yields, irrespective of disease control per se
(Van der Vossen and Browning, 1978).
2.2 VARIETY DEVELOPMENT
M.R. So¨ ndahl and H.A.M. van der Vossen
2.2.1 Breeding strategies
Breeding programmes implemented to the two coffee species have
basically the same main objective of developing new cultivars with the
potential of yielding optimum economic returns to coffee growers. Yield,
plant vigour and quality are the main selection criteria, but particularly in
arabica disease resistance and compact growth habit have also been given
much attention. Variation in climate and soil, incidence of diseases and
pests, cropping systems, socio-economic factors, market dynamics and
consumer preferences further define priorities given to selection criteria
applied in specific programmes. Breeding methods depend primarily on
the mating system and plant improvement goals (Van der Vossen, 1985,
2001; Charrier and Eskes, 1997). For the self-pollinating C. arabica the
methods applied include, in increasing order of complexity (Carvalho,
1988):
n introduction of coffee germplasm and pure line selection;
n pedigree selection after intervarietal crossing and often also backcrossing;
n F1 hybrids between genetically divergent breeding lines;
n interspecific hybridization followed by backcrossing to arabica lines
and pedigree selection.
In case of the out-crossing C. canephora, these are (Montagnon et al.,
1998 a,b):
The plant 29
n introduction of coffee germplasm and mass selection;
n family and clonal selection;
n reciprocal recurrent selection;
n interspecific hybridization followed by family and clonal selection.
2.2.2 Traditional and modern cultivars
2.2.2.1 Arabica
World coffee production is still based to a large extent on traditional
cultivars. In C. arabica cultivars were developed long ago by line selection
within the Typica and Bourbon varieties, or in offspring of crosses between
these two types. Besides, cultivars like Caturra, Pacas, San Ramon, Sumatra
and Maragogipe have their origin as single-gene mutants found in C.
arabica populations (Krug and Carvalho, 1951). The traditional cultivars
are often renowned for their excellent cup quality, but most are very
susceptible to the major coffee diseases, which makes them increasingly
obsolete for economic (costs of chemical disease control) and ecological
(chemical pollution) reasons in many coffee regions (Table 2.2).
However, as a result of considerable breeding efforts during the past 50
years, new arabica cultivars with higher yield potential and resistance to
important diseases have now started to replace traditional varieties in
several countries. These include Catimor and Sarchimor type of cultivars
in Colombia, Brazil, Central American countries and India, Icatu in
Brazil, as well as the F1 hybrid cultivars Ruiru II, generally referred as
Ruiru 11 in Kenya, and Ababuna in Ethiopia. Some of the recently
developed cultivars of arabica coffee are described in Table 2.3.
Efforts to obtain resistance to CLR, already started in India around 1920,
have had a long history of initial successes followed by disappointments
because of repeated appearance of new virulent races of the rust fungus.
However, some lines of the cultivar Catimor, which was developed from
crosses between Caturra and Hibrido de Timor (a natural interspecific
hybrid between C. arabica and C. canephora with arabica phenotype), have
shown complete resistance to all physiological races of the CLR pathogen
in most countries. These results were obtained by breeding plans normally
applied to self-pollinating crops, including recombination crosses followed
by backcrossing, inbreeding and pedigree selection (Carvalho, 1988;
Bettencourt and Rodrigues, 1988). The breeding programme in Kenya
demonstrated the advantages of F1 hybrid cultivars also for arabica coffee,
especially the simultaneous combination of compact plant type, high yield,
good quality and resistance to coffee berry disease (CBD) and CLR in one
cultivar (Van der Vossen and Walyaro, 1981). Subsequently, breeding
30 Espresso Coffee
Table 2.2 Commercially important traditional arabica cultivars (disease
susceptible)
Name Country Description
Typica Worldwide The original type of arabica coffee introduced from Yemen into
Asia in the early sixteenth century and after 1720 into the
Caribbean and Latin America
Bourbon Worldwide Introduced around 1715 from Yemen into Re´ union Island
(formerly Bourbon) and subsequently into Latin America and East
Africa; more compact and upright growth, higher yielding and
better bean and cup quality than Typica; red and yellow fruited
Bourbon types
Tekesik C. America High-yielding selection from Red Bourbon made in El Salvador
Kona Hawaii Selection from Typica with large beans, good quality, but low
yield
Mundo Novo Brazil Selection from a natural cross between Sumatra (Typica) and red
Bourbon found in Sao~ Paulo in 1931; replacing much of Typica
after 1960; vigorous growth and high yielding
Blue
Mountain
Jamaica A Typica selection grown in the Blue Mountains and famous for
its cup quality; susceptible to CLR, moderately resistant to CBD
Kent India Selection from Typica in 1911; some resistance to CLR (race II);
good cup quality; still grown at high altitudes
K7 Kenya Selection from Kent in 1936; some resistance to CLR (race II)
and CBD; vigorous growth and high yielding, but lower cup
quality than SL28
SL28 Kenya Selection from Bourbon ‘Tanganyika Drought Resistant II’ in
1935; high yielding and some drought resistance; excellent bean
and cup quality
SL34 Kenya Selected from ‘French Mission’ (Bourbon-type); good yield and
excellent bean size and liquor quality; adapted to highest
altitudes
KP423 Tanzania Selection from Kent similar in yield and quality to K7 in Kenya
N39 Tanzania Selection from Bourbon-type coffee; excellent bean and cup
quality, but moderate yielder
Jimma Ethiopia Dry-processed forest and garden arabica coffees of many origins
and very variable quality
Harar, Gimbi Ethiopia Distinct varieties grown in Hararge and Welega provinces resp.;
dry-processed coffees highly appreciated for their ‘mocha’
flavour
Yirga Chefe,
Limu
Ethiopia Washed coffees of very good quality; names refer to place of
origin (Sidamo and Kefa provinces resp.) rather than being
distinct cultivars
Caturra Worldwide Mutant discovered in Bourbon field in Brazil in 1935; compact
growth (short internodes); medium-sized beans and moderate
cup quality
Catuai Brazil Selection from a cross (Caturra x Mundo Novo) made in 1949;
compact growth; in Brazil higher yielding and better quality than
Caturra
Villa Sarchi Costa Rica Mutant with short internodes found in Bourbon coffee; syn.
Villa Lobos; similar to Caturra in plant stature and yield, but
smaller beans
Pacas El Salvador Caturra-like mutant found in Red Bourbon coffee in 1909
CLR, coffee leaf rust.
32 Espresso Coffee
Table 2.3 Commercially important modern arabica cultivars
(cultivars with disease resistance)
Name Country Description
Catimor Worldwide Selections from crosses (Caturra/Catuai x Hibrido de
Timor); compact growth like Caturra; resistant to
CLR; Oeiras (Brazil), Cauvery (India), IHCAFE90 and
CR95 (C. America) are Catimor-like cultivars
Sarchimor Worldwide Selections from cross (Villa Sarchi x H. de Timor);
similar to Catimor in compact growth and resistance
to CLR; Tupi, Obata and IAPAR59 (Brazil) are
Sarchimor-like cultivars
Colombia Colombia Synthetic variety composed of a number of Catimor
lines; large bean and good liquor quality; resistant to
CLR; some lines with resistance to CBD
S795 India Selection from a natural interspecific hybrid (C.
arabica x C. liberica) backcrossed to C. arabica;
resistant to some CLR races; high yield and good
liquor quality; most important arabica cultivar in
India
Ruiru II (Ruiru 11) Kenya F1 hybrids (seed by hand-pollination) between
selected Catimor lines and selected clones from
multiple crosses (tall plants); resistant to CBD and
CLR; slightly less compact than Catimor; early and
high yielding; most F1 combinations have very good
bean size and cup quality
Ababuna Ethiopia F1 hybrid (seed by hand-pollination) between
selected lines of Ethiopian germplasm with normal
(tall) plant habit; resistance to CBD
Icatu Brazil Selections developed by crossing (tetraploid) C.
canephora x C. arabica (Bourbon) followed by
backcrossing to Mundo Novo; resistance to CLR; tall
growth habit; susceptible to drought and cold; high
yielding and good cup quality. IAC3282 is an early
maturing Icatu with yellow berries
S2828 India Developed by interspecific crossing and backcrossing
similar to Icatu; tall plants; resistance to all (?) races
of CLR; high yielding and good cup quality
1. CLR, coffee leaf rust; CBD, coffee berry disease.
2. Important progenitors for disease resistance:
Hibrido de Timor: arabica-type variety cultivated in Timor; assumed to have developed from a
natural cross between C. arabica and C. canephora; CIFC (Portugal) clones 832/1, 832/2 and
1343 have resistance to all or most races of CLR; CIFC clone 1343 also carries a gene for
resistance to CBD. According to Bertrand et al. (In Press) it should be possible to find lines with
resistance genes and good beverage quality. Selection can avoid accompanying the
introgression of resistance genes with a drop in beverage quality (Bertrand et al., 2004).
Rume Sudan: semi-wild arabica variety collected in 1942 from the Boma Plateau in
southeastern Sudan; some individual plants have very high resistance to CBD.
The plant 33
strategies aiming at F1 hybrid cultivars in arabica coffee have been adopted
elsewhere, e.g. in Ethiopia (Bellachew, 1997) and in Central American
countries (Bertrand et al., 1997).
Selection for bean size and cup quality has received much attention in
arabica coffee breeding programmes in Kenya and Colombia in particular,
because the quality of new disease-resistant cultivars should be at least
equal to that of the traditional cultivars in order to uphold the country’s
high reputation and special position in the world coffee market. This was
obviously achieved in Kenya with the CBD- and CLR-resistant hybrid
cultivar Ruiru 11 (Njoroge et al., 1990) and in Colombia with the
CLR-resistant cultivar Colombia (Moreno et al., 1995), as judged by
international coffee-tasting panels. Rigorous standardization of pre- and
post-harvest practices, bean grading and cup tasting applied in these two
breeding programmes contributed to increased selection progress and
helped to overcome the initially negative effects on quality due to introgression
of disease resistances from exotic germoplasm (cupping characteristics
are, however, different from those of the parent material and
still considered by trade less suitable for espresso-type blends). Bean size
improvement is particularly noticeable in the Pacamara variety (cross
between Pacas Maragogype), developed in El Salvador, with screen size
bean values of 19 andmire.
2.2.2.2 Robusta
In case of the cross-pollinating C. canephora, most cultivars are also
propagated by seed, often from polyclonal seed gardens. Commercially
important robusta cultivars include the BP and SA series in Indonesia,
S274 and BR series in India, the IF series in the Ivory Coast and the
cultivar Apoata in Brazil (Table 2.4). Some breeding programmes in
robusta coffee (e.g. in the Ivory Coast) have recently adopted methods of
reciprocal recurrent selection with distinct sub-populations to increase
chances of producing genotypes superior in yield, quality and other
important traits (Leroy et al., 1997b).
2.2.2.3 Interspecific hybrids
Interspecific hybridization has played a significant role in coffee, such as
crosses between arabica and robusta coffee with the objective of
introgressing disease resistance into arabica, e.g. the cultivars Icatu in
Brazil (Carvalho, 1988) and S2828 in India (Srinivasan et al., 2000), or
improved liquor quality into robusta coffee, such as the variety Arabusta
in the Ivory Coast (Capot, 1972). Other examples of interspecific
hybridization leading to successful robusta cultivars are Congusta in
Indonesia and the CxR variety in India.
2.3 AGRONOMY
M.R. So¨ ndahl, F. Anzueto, C.R. Piccin and H.A.M. van
der Vossen
2.3.1 Climate and soil
Arabica coffee is cultivated at medium to high altitudes (1000–2100m
a.s.l.) in equatorial regions, or at 400–1200m altitudes further from the
equator (9–24
N and S latitudes), where average daily temperatures are
about 18–22
C. In general, the cooler the climate the better will be the
cup quality of arabica coffees. However, temperatures near 0
C will kill
the leaves immediately. On the other hand, long periods of hot weather
will cause abnormal flowering (star flowers) and shoot dieback.
Depending on local evapotranspiration, it requires a minimum of 1200–
1500mm annual rainfall with no more than 3 months of less than 70mm
for good growth and production. In contrast, robusta coffee requires warm
and humid climates of tropical lowlands and foothills (100–1000m
altitude) with average daily temperatures of 22–26
C, absolute minimum
34 Espresso Coffee
Table 2.4 Some robusta cultivars
Name Country Description
BP and SA
series
Indonesia Important robusta cultivars developed on two research
stations in Java in the 1920s; propagated by seed or
grafting
S274, BR
series
India Major robusta cultivars in India; selected from plant material
of Java origin and released in the 1950s; seed propagated
IF series Ivory
Coast
Selected from robusta plant material from Java and the DR
Congo; propagated by seed and cuttings
Kouilou
(Conilon)
Brazil Named after the Kioulou river in the DR Congo; the main
robusta variety (95%) in Brazil; seed propagated
Apoata Brazil Initially used as nematode-resistant rootstock for arabica
cultivars; now also used as robusta cultivar; seed
propagated
Arabusta Ivory
Coast
Developed from a (C. canephora x C. arabica) cross in the
1960s: cup quality better than robusta: limited
commercialization due to persistent low fertility; clonal
propagation
C x R
variety
India Developed from a (C. canephora x C. congensis) cross in
1942, followed by backcrossing to S274 and selection;
released in 1976; bolder beans and better cup quality than
robusta
temperatures not below 10
C and well-distributed annual rainfall of at
least 2000 mm.
The parent material of major coffee soils include lava and tuff (e.g.
Kenya), volcanic ash (e.g. Indonesia, Central America), basalt and
granite (e.g. Brazil, West Africa, India). Soils for coffee cultivation should
be deep (at least 2 m), free-draining loams with a good water-holding
capacity, fertile and slightly acid (pH 5–6) and contain at least 2%
organic matter in the upper horizon.
2.3.2 Propagation and crop husbandry
2.3.2.1 Propagation and planting
Cultivars of arabica and robusta coffee are generally propagated by seed.
Seeds germinate within 4–5 weeks after sowing in wet sand. Removal of
parchment prior to sowing reduces coffee seed germination time by half.
Vegetative propagation by rooted cuttings or grafting has found limited
application in the past to multiply high-yielding robusta clones (e.g., in
Java and Ivory Coast). Grafting of arabica cultivars on robusta rootstock is
very common in certain areas of Brazil and Central America where
nematodes are a serious problem. Recently, more efficient in-vitro
methods of clonal propagation by somatic embryogenesis have been
developed for robusta as well as arabica coffees (Sondahl and Baumann,
2001; Etienne et al., 2002).
Seedlings or clonal plants are raised in shaded nurseries, in polythene
bags filled with a soil-compost mixture, for 6–12 months before
transplanting to the field at the start of the rainy season. Seedlings are
planted in previously dug holes filled with top soil supplemented with
organic and inorganic fertilizers, or alternatively in trenches dug along the
rows by a tractor-mounted implement and refilled with a soil–compost
mixture (Mitchell, 1988). Under optimum field conditions young plants
may flower 12–15 months after planting, producing a first light crop 2.5
years after planting. Conventional plant densities vary from 1300–2600
plants/ha for tall-stature arabica varieties to 1100–1400 plants/ha for
robusta coffee. Higher densities of 3333–10 000 plants/ha are applied with
compact growing arabica cultivars like Caturra, Catuai and Catimor in
Latin America or Ruiru 11 in Kenya.
2.3.2.2 Shade
In South America and East Africa coffee is mostly grown in pure stands
without permanent overstorey shade, except for widely spaced rows
The plant 35
of shade trees acting as windbreaks. In other countries (e.g. Central
America, Mexico, Cameroon, Indonesia, India), coffee is grown either
with temporary or permanent shade trees, or in association with perennial
crops (coconut palms, rubber, clove, fruit trees), or in home gardens
associated with food crops, bananas (e.g. northern Tanzania, Uganda) and
tree crops. In India and Indonesia the stems of shade trees are often
utilized to support the vines of black pepper, and so provide an additional
source of revenue to coffee growers. With intensive crop management
including high inputs (fertilizers and pest/disease control), unshaded
coffee will usually produce higher yields than coffee grown under shade.
Without high inputs or under sub-optimal ecological conditions coffee
usually produces better results under shade. Shade trees have multiple
functions, such as: reducing extreme temperatures, breaking the force of
monsoon rain and winds, controlling erosion on steep slopes, reducing the
incidence of certain pests (e.g. white stem borer), and generally
preventing overbearing and shoot dieback.
Common shade trees are Inga spp. and Grevillea robusta (Latin
America), Albizzia spp. (Africa), Erythrina indica (India) and Leucaena
sp. and Casuarina sp. (Indonesia and Papua New Guinea). In Central
America the density of shade trees varies from 156–204 trees/ha at lower
altitude to 83–100 trees/ha at higher elevations. As a general recommendation,
overstorey shade should not reduce more than 50% of total
irradiances.
2.3.2.3 Pruning
Pruning is essential in coffee production to achieve the desired plant
shape, to maximize the amount of new wood for the next season crop, to
maintain a correct balance between leaf area and fruits, to prevent
overbearing and thus reduce biennial production, stimulation of root
growth, enhancement of light penetration, and also to facilitate disease
and pest control. The following main pruning systems can be distinguished
(Wrigley, 1988):
n single- or multiple-stem, capped at about 1.8m height, accompanied
by regular pruning of lateral branches and removal of suckers;
n multiple-stem with three to four uncapped stems with little
maintenance pruning; rotational system of replacement of old stems
(after three to four cropping years) by new orthotropic shoots;
n ‘agobiado’, which is a multiple-stem system on one main stem that
has been bent over at an early age;
36 Espresso Coffee
n complete stumping to 30 cm above ground level (with or without a
temporary lung branch) to encourage re-growth of suckers, which,
after thinning to two to three vertical shoots, will be brought up to
form the vertical stems for the next cycle of production.
This last pruning system is applied on a rotational basis every fourth or
fifth year in closely spaced arabica coffee blocks, usually planted with
compact-type cultivars (e.g. Caturra, Catuai), or to rejuvenate an old
block of conventionally spaced coffee after 8–12 years of production.
2.3.2.4 Weed control
Weeds should be controlled, as they compete with the coffee trees for
moisture and nutrients. Broad-leaved weeds are generally less harmful
than perennial grasses (e.g. couch, star grass and lalang) and sedges (e.g.
nut- and water-grass), because the roots of the latter also produce exudates
toxic to coffee and they are more difficult to eradicate. Tillage, herbicides,
mulching or interplanting with cover crops can suppress weeds. Tillage
should be done carefully to prevent damage of the superficial feeder roots
of the coffee and also to avoid soil erosion. Herbicide applications in
combination with zero cultivation has the added advantages of not
disturbing the superficial feeder roots of the coffee trees, which can
therefore develop in the layer of decomposing organic matter from dead
weeds and so increase uptake of nutrients. Mulching produces significant
yield responses in low rainfall areas, such as in the arabica coffee regions
of East Africa and Brazil. The interplanting of leguminous cover crops is
less common with arabica than with robusta coffee, even where rainfall is
not a limiting factor. Arabica coffee is more sensitive to competition from
weeds and cover crops. During the first three years after planting, when
the young coffee trees occupy a small portion of the soil surface, betweenrow
interplanting with annual food (e.g. beans, maize, rice) and
commercial (e.g. pineapple, cassava) crops serves as an alternative
manner of controlling weeds, while producing also an additional source
of food and income (Mitchell, 1988).
2.3.2.5 Fertilization
Fertilizer requirements depend on yield level and natural soil fertility.
Nutrients are removed by harvested fruits, but additional nutrients are
also required for sustaining the vegetative growth. Part of the total
nutrients is recycled back to soil by leaf fall, prunings and decaying feeder
roots. The annual nutrient uptake per hectare of a vigorously growing
The plant 37
block of arabica coffee yielding 1.0 t/ha green coffee beans is estimated at
135 kg N, 34 kg P2O5 and 145 kg K2O. Soil and leaf analyses are
important for monitoring the balance of nutrients in soil and plant tissues,
and so provide good guidance for fertilizer application. Generally,
nitrogen fertilizers at rates of 50–400 kg N/ha/year give considerable
increases in yield, especially when applied in split applications. Responses
to potassium fertilizers (up to 400 kg K2O/ha/year) vary from nil in
mulched coffee grown on volcanic soils rich in K (e.g. Kenya) to highly
significant in soils with a low K status (e.g. Brazil). Very high K
applications may induce Mg deficiency. Phosphate is often applied as
compound fertilizer (NPK: 2–1–2), but its effect is faster in foliar
applications. Calcium in the form of lime is used to correct soil acidity.
Deficiencies in Mg and minor nutrients such as boron and manganese are
also best corrected by foliar applications. Organic manures (stable
manure, cover crops, mulch and decaying coffee pulp) are alternatives
to chemical fertilizers and are often the only type available to
smallholders. Organic matter also improves physical (water retention
and aeration) and biological (microflora and soil fauna) properties of the
soil (Willson, 1985; Mitchell, 1988).
2.3.2.6 Irrigation
Irrigation is essential in nurseries and during the first 1–2 years after field
planting to enable the young plants to establish well and to survive the
dry seasons. Irrigation of unshaded adult coffee can produce economic
yield responses, when annual rainfall is less than 1200mm and the dry
season(s) are extended over more than 2–3 months. Critical periods when
irrigation should be applied are at flowering time, at berry expansion stage
and at bean filling stage. Especially moisture stress at the latter (hard
green berry) stage will result in light and poor quality beans (Mitchell,
1988). However, irrigation is only applied on large estates, as it is
expensive, e.g. overhead sprinkler or drip irrigation systems. The latter is
most efficient in water use, but requires a very costly initial investment of
equipment.
2.3.2.7 Organic and sustainable coffee
In several coffee-producing countries there is increasing awareness of the
need to produce sustainable and environment-friendly coffee. This is
achieved by improved land management, increased organic instead of
synthetic fertilizers, reduced pesticide use by integrated disease and pest
management (disease resistant cultivars and biological control of insect
38 Espresso Coffee
pests) and by reduced water use and pollution during post-harvest
handling. Some of these coffees may be certified as organically or
sustainably produced coffees and so attract premium prices in a gradually
developing niche market of health- and environment-conscious consumers,
particularly in the USA and Europe.
2.3.3 Diseases and pests
2.3.3.1 Diseases
Coffee leaf rust (CLR), caused by the fungus Hemileia vastatrix, is the most
important disease of arabica. It reached Brazil in 1970, and is now present
worldwide in all coffee-producing countries (except Hawaii and
Australia). The use of copper and systemic fungicides and resistant
cultivars has reduced the CLR threat. Nevertheless, CLR continues to
cause considerable economic damage to arabica on a global scale due to
increased production costs by fungicide sprays (10%) and crop losses (20%
and more), because most coffee is still produced on CLR-susceptible
cultivars. There is also a problem in some countries, India in particular, of
repeated breakdowns of host resistance on CLR-resistant cultivars by the
development of new pathogenic races of the leaf rust fungus (Eskes,
1989).
The very destructive coffee berry disease (CBD) is still restricted to
arabica in Africa, although climatic conditions in certain high-altitude
coffee areas of Latin America and Asia appear to be favourable to the
fungus Colletotrichum kahawae (coffeanum). CBD epidemics can quickly
destroy 50–80% of the developing crop on susceptible arabica cultivars
during prolonged wet and cool weather conditions. Preventive control by
frequent fungicide sprays may account for 30–40% of total production
costs. New cultivars with high levels of durable CBD resistance have been
successfully developed (e.g. in Kenya), but have not yet sufficiently
replaced old susceptible cultivars (Van der Vossen, 1997). Most robusta
cultivars are resistant to both diseases.
Of the several other fungal and bacterial diseases affecting both coffee
species (Wrigley, 1988), the following four have drawn attention in
recent years. In Central America the leaf spot disease ‘Ojo de Gallo’
(Mycena citricolor) can be a serious problem in coffee grown under shade at
high altitudes, particularly affecting CLR-resistant cultivars like Catimor
and Sarchimor. Black rot (Koleroga noxia) is now the second most
important coffee disease in India after coffee leaf rust (Bhat et al., 1995).
The bacterium Xylella fastidiosa has become a major problem in some
regions in Brazil (Beretta et al., 1996). Fusarium wilt disease or
The plant 39
tracheomycosis (Fusarium xylarioides) has been seriously affecting C.
canephora in DR Congo and Uganda since the 1980s (Flood and Brayford,
1997). The traditional Nganda types are very susceptible, but some
robusta clones appear to be resistant to Fusarium wilt.
2.3.3.2 Nematodes and insect pests
Nematodes (mainly Meloidogyne spp. and Pratylenchus spp.) can cause
considerable problems on arabica in Central America in particular, but
also in Brazil, India and Indonesia. Much progress has been made in
developing arabica cultivars with resistance to certain nematode species
(Anzueto et al., 2001) as well as in utilizing nematode-resistant robusta
clones as rootstock for arabica cultivars (Bertrand et al., 2000).
Among the most damaging and worldwide coffee pests are the coffee
berry borer (Hypothenemus hampei), stem borers (e.g. Xylotrechus quadripes),
scale insects (e.g. green scale, Coccus viridis) and leaf miners (e.g.
Perileucoptera coffeella). Host resistance to insect pests in natural coffee
germplasm is extremely rare, but transgenic coffee plants with resistance
to leaf miners and the coffee berry borer may become available in the near
future (Guerreiro Filho et al., 1998; Leroy et al., 2000). Integrated pest
management (IPM) has proved to be much more effective in reducing
damage to the coffee trees and crops than frequent applications of
insecticides (Bardner, 1985). IPM includes early warning systems
(monitoring of insect populations) in combination with cultural (pruning
and shading), biological (insect traps; introducing insects or microorganisms
parasitic to the pest) and chemical (carefully timed applications
with non-persistent insecticides) methods of control.
2.3.4 Harvesting and yields
In equatorial regions where the harvesting season extends over several
months, selective picking of ripe berries at 7–10 days intervals is common.
Where the harvest season is short, e.g. in Brazil, or the cost of hired labour
high, whole branches are stripped when the majority of the berries are
ripe. Harvesting costs are two to three times higher for selective hand
picking, but quality of the product is usually better (but, see also 3.2.4).
The use of self-propelled mechanical harvesters has found application in
plantation coffee of Brazil and a few other countries (see 3.2.3).
Yields may vary from 200 kg green coffee beans per ha from low-input
smallholder plots to 2 t/ha for arabica and 3.5 t/ha for robusta coffees in
well-managed plantations at conventional plant density and without
shade. Yields of 5 t/ha or higher have been obtained in some closely
40 Espresso Coffee
spaced and unshaded coffee blocks planted with compact-type arabica
cultivars, e.g. in Brazil, Colombia and Kenya.
2.3.5 Agronomic practices in selected countries
There is a great variation in production systems among the coffeeproducing
countries. Comprehensive overviews of all or most important
countries have been presented by Krug and de Poerck (1968), Wrigley
(1988) and the ITC report (2002). The four examples presented here are
Brazil, as the largest producer of natural arabica and leader in
mechanization of coffee cultivation, the Central American region, as
largest producer of washed arabicas, Kenya in East Africa, with a tradition
of producing mild arabicas of the highest quality, and India, as leader in
technology development in arabica and robusta coffees in Asia. See
Chapter 3 for details of the production techniques mentioned below.
2.3.5.1 Brazil
Coffee is grown on some 2.5 million ha (5.5 billion trees) between
latitudes 10 and 24
S, mostly on gently sloping land, often with high-tech
cultivation practices. Brazil is the largest arabica producer in the world,
and its coffee plantations are composed of 73% arabica and 27% robusta
trees. Its share of robusta in the total annual crop has increased to more
than 21% in recent years (ITC, 2002). Domestic consumption accounted
for about 50% of total production of 30.1 million bags in 2000/01. In
recent years new coffee plantings have moved further north up to the states
of Bahia (arabica) and Rondonia (robusta). Recently, incidental severe
droughts have caused more impact on the Brazilian coffee crop than frost.
It is estimated that about 85% of all planted arabica coffee consists of
traditional cultivars, Mundo Novo and Catuai in particular, and the rest
are CLR-resistant cultivars like Tupi, Iapar-95, Icatu and others.
Coffee plantations in Brazil vary from relatively small (5–50 ha) and
medium-sized farms (50–200 ha) to very mechanized large plantations
(200–3000 ha). Plant densities have evolved from 1000–1900 plants/ha,
often with two plants per hole with free growth, to 3500–5700 plants/ha
in hedgerow planting with periodic pruning. The availability of more
compact cultivars has led to the adoption of even higher plant densities in
combination with regular pruning.
Preparation for planting is often entirely mechanized. Tractor-mounted
planting implements have been developed that first draw deep furrows
along the planting lines, then mix the soil with fertilizers and lime and
close the furrows all in one single passage. The bulk of planting material
The plant 41
utilized in Brazil consists of 6-month old seedlings produced in small
plastic bags, but more recently the ‘tubette’ seedlings are becoming
popular since they allow for semi-mechanized planting methods with
consequently 40% reduction on planting costs. The tubette seedlings are
of particular use for arabica cultivars grafted on nematode-resistant
robusta rootstock.
Almost all the coffee in Brazil is grown without shade. Weeds are
controlled with a combination of pre-emergent herbicides and intercropping
with native leguminous plants and monocots. A green cover of
spontaneous species is maintained by mechanized trimming and diluted
solutions of herbicides plus liquid fertilizer. Plantations under full sun
produce more, and so require higher levels of fertilizers. Recommended
fertilizer rates for each 10 bags of green coffee are (in kg/ha): N (100),
P2O5 (30), K2O (120), Mg (20), S (5), B (2) and Mn (3). Fertilization is
split in three to four applications and monitored by leaf analysis: if N is
above 3.2% and K above 3.1%, the remaining N–K dosages are
suspended. Coffee leaf rust is being effectively controlled in susceptible
cultivars by three to four rounds of sprays with copper-based and systemic
fungicides during the main growing season. Coffee berry borer, leaf miners
and in some areas also nematodes are the main coffee pests.
Yields vary from 0.6 t/ha green coffee in low-input coffee farms to 2–3
t/ha in well-maintained plantations. Very high annual yields of 4–7 t/ha
have been reported from technology-driven coffee plantations established
in the last decade in northwest Bahia with short-stature cultivars.
The method of pruning both vertical stems and lateral branches is
becoming very popular among farmers, and special machines have been
developed for these operations.
Harvest season begins in mid-April (at low-altitude farms) and extends
until August (in high lands). Stripping all cherries from branches at once is
the common method of harvesting. To maximize the percentage of ripe
cherries during the single harvesting round, it is common to grow coffee
cultivars with early, medium and late fruit ripening characteristics.
Harvesting is still predominantly a manual operation, accounting for 35–
40% of production costs. Brazil is the world leader in the testing and
development of mechanical implements to reduce costs of coffee harvesting
(see Figure 3.3). Equipment includes hand-held vibrating machines
similar to olive harvesters, tractor-pulled harvesters equipped with one or
two cylinders of vibrating rods to harvest one or both sides of a row of coffee
trees. More advanced harvesting machines include the self-propelled
models equipped with fruit collecting and cleaning systems. Harvesting is
accomplished by driving over the coffee plants and the berries are placed in
bags or directly loaded onto trucks in bulk. These vibrating harvesting
42 Espresso Coffee
machines usually operate at speeds between 0.5–1.0 km/hour. Selfpropelled
grape harvesters from Europe with a system of oscillating arms
are also being tested for ‘selective picking’ of ripe coffee berries. They can
operate at three to four times the speed of current models, but still require
some adaptations to make them suitable for coffee harvesting.
In Brazil, arabica coffee is traditionally harvested by the stripping
method and dry-processed to produce ‘natural’ coffees. Fruits from
stripping harvest include several ripening stages (green, green-ripe, ripe,
overripe and dry). By water flotation, dry and overripe fruits are isolated
for sun drying in separated lots. Certain farmers depulp ripe cherries and
dry the parchment without removing the mucilage. This semi-natural
preparation method is increasing in acceptance, since it reduces the total
patio drying time and offers reduced risk of undesirable fermentation.
Presently, it is estimated that Brazil produces about 3.5 million bags of
pulped natural coffee (see 3.3.3). The Brazilian arabica coffee is
characterized by mild acidity, aromatic notes and heavy body, but
qualitative variations do exist due to different microclimates, varieties
and processing methods (see 3.8 for details on sensorial analysis). Limited
amounts of Brazilian coffee are also processed by the fully wet method.
2.3.5.2 Central America
Central America (Guatemala, El Salvador, Honduras, Nicaragua and
Costa Rica) produced altogether some 13 million bags of washed arabica
and only very little robusta coffee in 2000/01 (ITC, 2002). Coffee is
grown in mountainous regions with volcanic soils at altitudes ranging
from 400–1700 m, but the majority of fields are within 1200–1500m
altitude. In general, the rainy season extends from April/May to October,
but there is a tremendous variation in total annual rainfall (1000–
4500 mm) between regions due to topographical differences. Cultivation
under shade is common in more than 95% of the coffee areas in Central
America, except in Costa Rica, which has 25% of full-sun plantings. The
most common shade trees are Inga spp., Erythrina spp. and Grevillea spp.,
at densities ranging from 100 to 156 trees/ha. While in Costa Rica,
Nicaragua and Honduras most of the coffee is produced by smallholders,
medium to large coffee plantations have an important share of the total
crop in Guatemala and El Salvador.
The main cultivars grown at present are Caturra, Pacas, Catuai and Red
Bourbon. These have largely replaced the traditional Typica or ‘Arabigo’
variety, which was introduced from the Caribbean in the eighteenth
century. The cultivation of compact cultivars like Caturra has stimulated
important technological changes, such as high plant densities, reduced
The plant 43
shading and inputs of fertilizers and pesticides, increasing productivity.
Actually, three coffee production systems can be distinguished in Central
America:
n technical, with compact cultivars (Caturra, Catuai) grown at high
plant densities (5000–7000 plants/ha);
n technical, with tall cultivars (Bourbon lines) at close spacing (3400–
3700 plants/ha);
n traditional, with tall Bourbon cultivars at conventional spacing and
low inputs.
In Costa Rica, the technical farming system with Caturra and Catuai is
most common, whereas in Guatemala and El Salvador, Honduras and
Nicaragua one can see all three systems of cultivation. The adoption rate
of compact growing CLR-resistant cultivars like Catimor and Sarchimor is
still rather modest (about 5%). National average yields of the Central
American countries vary from 800 to 1500 kg/ha of green coffee.
Technical-operation coffee farms can reach up to 3000 kg/ha, whereas
low-input traditional farms produce about 500 kg/ha.
Modern cultivation practices include periodic pruning by rows or by
blocs, while in traditional farming systems selective pruning is often
applied, i.e. plant-by-plant as the need arises. It is also common to bend
orthotropic stems (agobio) to induce multiple vertical branches forming a
typical plant architecture called ‘parra’. The amount and type of fertilizers
applied varies with region and level of production, but is usually given in
NPK dosages of 50–75, 100–150 and 150–300 kg/ha, in two to three split
applications during the rainy season. The economically most important
diseases are coffee leaf rust and ‘Ojo de Gallo’, whereas coffee berry borer
and nematodes are the main pests. In highly affected nematode regions in
Guatemala the control is achieved by grafting arabica cultivars on
resistant robusta rootstock.
Harvesting in low-altitude coffee farms takes place in the period
August–October, at mid-altitude during October–December and at high
altitudes, January–April. Ripe berries are picked in three to five passages
and all coffee is processed by the wet method. Mucilage is mainly removed
by fermentation, but some farms are adopting mechanical removal of
the mucilage after depulping. On-farm wet processing is common in
Guatemala, Honduras and Nicaragua, but in Costa Rica and El Salvador
most harvested cherry is processed in central coffee factories. Wet
parchment coffee is dried on patios under the sun, but mechanical dryers
are used for final drying in mid- to large processing factories. In Central
America green coffee is commercialized by growing altitude and, some-
44 Espresso Coffee
times, location in relation to the Pacific or Atlantic Ocean. For instance,
in Guatemala the type ‘strictly hard bean’ originates from altitudes above
1350 m. In Costa Rica ‘strictly hard bean North’ coffee comes from
altitudes of 1200–1600 m. In Honduras and El Salvador ‘strictly high
grown’ coffee is derived from altitudes above 1200 m.
2.3.5.3 Kenya
Arabica coffee is grown in the highlands east and west of the Great Rift
Valley, just below and above the equator at 1450 to 2000m altitudes.
Kenya produced 1.1 million bags of mild arabica coffee from 150 000 ha in
2000/01, which is almost entirely exported (local consumption 4.5%).
About 60% of total Kenya coffee is produced by smallholders on 75% of
the coffee land area, and the remaining by medium to large plantations.
Coffee production in Kenya increased from 560 000 bags (30 000 ha) in
1963/64 to a peak of 2.2 million bags in 1987/88, but has been declining
since that time due to economic and socio-political factors.
The coffee flowers in March/April and again in November, under
influence of the bimodal rainfall pattern typical of equatorial climates.
Consequently, the harvest season extends over several months, with the
main crop usually maturing in October–December and an early crop in
June–August. Average total rainfall per year in the coffee zone varies from
900mm at low to 2000mm at high altitudes, but the East African climate
is notorious for its recurring cycles of wet and dry years. Larger plantations
are situated in the lower range of the coffee zone, and the water deficit is
compensated by (overhead sprinkler) irrigation to sustain economic yield
levels. Smallholders in the upper altitudes benefit from higher rainfall, but
with no irrigation facilities available their coffee suffers in years of severe
droughts. Coffee is grown without shade trees, except in very high altitude
areas for the protection against wind and extremely low temperatures.
Much of the existing coffee on old plantations is a direct descendant of
the Bourbon-type ‘French Mission’ or ‘Mocha’ coffee plot established at
the St Austin’s Catholic Mission near Nairobi around 1900 with seeds
originating from Yemen. The planting material of farms established after
1950, and all smallholder coffee fields, consists mostly of the cultivars
SL34 and SL28. These two varieties are derived from single plant
selection from French Mission and other Bourbon coffees, respectively,
and growing at high and medium altitudes. The variety K7 is a selection
from the Indian cultivar Kent, and is cultivated in the lowest coffee zones.
The cup quality of SL34, SL28 and also of French Mission is significantly
better than that of K7, but the latter became popular because it has some
resistance to CLR, which is the predominant coffee disease at lower
The plant 45
altitudes. Blue Mountain was recommended in the 1950s to smallholders
in Kissii, a high-altitude and wet coffee zone west of the Great Rift Valley,
on account of its lower susceptibility to CBD than SL34 or SL28, but its
cup quality is rather disappointing according to Kenyan standards. In
1986, the Kenyan Coffee Research Foundation (CRF) released Ruiru 11, a
seed-propagated (F1 hybrid seeds) compact growing and productive
arabica cultivar with high resistance to CBD and also to CLR. Under
good management Ruiru 11 is capable of producing a cup quality similar
to cultivars SL34 or SL28; it is, however, not looked for in top espresso
coffee blends. An estimated 20 000 ha have been planted with Ruiru 11.
Coffee is conventionally planted at a density of about 1320 trees/ha and
maintained as three-stemmed capped (plantations) or free-growth
(smallholders) trees. Plantations with irrigation equipment have often
converted to 2640 trees/ha by doubling within-row density. The diseaseresistant
and compact Ruiru 11 is best planted at 3000–5000 trees/ha,
maintained on one or two uncapped stems per mature tree and after three
full crops clean-stumped to start a new cycle of production. Mulching
(grasses, etc.) is general practice on larger plantations to control weeds
and conserve moisture, but smallholders are often unable to follow
recommended practices because the little crop residues available on the
farm may be required to feed livestock. During recent years of low coffee
prices, regular fertilizer applications are mainly restricted to coffee
plantations, as smallholders lack the financial resources for external
inputs. This also applies to chemical control of diseases (in CBD- and
CLR-susceptible coffee cultivars) and pests. Yields of small coffee farms
have declined considerably and are now often not more than 300–400
kg/ha. Well-maintained large coffee plantations produce 1–2.5 t/ha per
year. Yields of 3 t/ha and more have been obtained in high-density
coffee blocks with Ruiru 11.
All Kenyan coffee is harvested by several rounds of selective picking of
ripe berries and processed according to the wet method (see 3.3.2). A
small fraction, consisting of overripe and black cherry, is dry-processed
and sold as ‘mbuni’ on the local trade. Plantations operate their own
coffee factories, while smallholders bring harvested cherry to cooperative
factories for depulping, fermenting, washing and sun drying the parchment
coffee, usually done on raised tables with wire-mesh tops.
Some factories, mainly on large plantations, also have mechanical dryers
(see 3.4.3) to speed up the drying process during wet weather. Coffee
processing stations are situated near rivers, as large quantities of water are
required for the wet process. Regulations are now in place to prevent coffee
pulp and dirty wastewater from polluting the rivers. Dry parchment is
delivered to one of the four existing coffee mills for curing, which includes
46 Espresso Coffee
hulling, cleaning and grading by size and density. Cup quality of each lot of
green coffee is then determined by liquoring before being offered for sale at
the weekly auction in Nairobi. There are seven grades of green coffee
(from AA to T), and cup classification goes from excellent mild arabica
with high notes of pointed acidity, rich flavour and light body (class 1) and
FAQ (class 4) to a low cup type (class 7). In addition, there are another
three under-grades, including ‘mbuni’, to cup class 10. A rare Kenyan AA
coffee of class 1–2 is unmatched in quality on the European coffee market,
except occasionally by mild arabica coffee (Bourbon cultivar N39) from
the higher slopes of Mount Kilimanjaro in Tanzania.
2.3.5.4 India
Coffee is cultivated mostly in the mountain ranges in the states of
Karnataka, Kerala and Tamil Nadu in southern India, between latitudes 9
and 14
N, robusta at 400–1000m and arabica areas at 700–1600m
altitudes. Small pockets of coffee cultivation also exist in east Andhra
Pradesh, Orissa (18
N) and seven northeastern states (27
N). Arabica
and robusta coffees are generally grown under shade. The upper tier
consists of natural forest trees (Ficus, Artocarpus, Terminalia and other
species) or of planted Grevillea robusta, whereas the lower tier has Erythrina
indica and orange trees. Coffee flowering occurs in March/April at the start
of the main rainy season. Arabica is harvested during the period November
to February and robusta from January to March. Harvesting is entirely
manual, by picking ripe berries in several harvesting rounds.
Coffee production increased from 1.1 million bags (73% arabica) from
92 000 ha in 1960/61 to almost 5 million bags (48% arabica) from
310 000 ha in 2000/01. The national average yield is, therefore, almost
1.0 t green coffee/ha. Robusta coffee generally produces 10–15% more
than arabica plants and large plantations yield about 25% more than
small farms. Smallholders cover about two-third of the coffee area and
contribute with 60% of the total annual coffee crop, and the remaining
40% comes from plantations. Arabica and robusta coffees are processed
according to the wet (washed) or dry methods, and subsequently cleaned
and graded in coffee curing centres. An estimated 80% of all arabica and
30% of the robusta coffees are washed. Coffee produced in large
plantations is practically all processed by the wet method. About 80%
of all the coffee produced in India is exported.
Resistance to CLR has remained the most important objective for
arabica coffee breeding in India since the release of cultivar Kent in the
1920s. The humid and relatively warm climate appears very favourable to
the pathogen and Kent (resistance to CLR race II) was soon attacked by
The plant 47
CLR. There is no other coffee-producing country in the world where CLR
has developed so many new physiological races, the latest ones apparently
capable of overcoming host resistance of some of the most advanced
cultivars. Kent is still grown at very high altitude, where CLR incidence is
relatively low, but almost 70% of all arabica grown in India at present is
S795, a cultivar released in 1946 with resistance to CLR races II and I only,
but high yielding and producing good quality Indian arabica coffee. CLR
can be controlled fairly effectively by prophylactic sprays (Bordeaux
mixture 0.5%) and by systemic fungicides (Plantvax 0.03% or Bayleton
0.02%), but that adds considerably to production costs. Even Cauvery, a
cultivar selected from Catimor (ex CIFC Portugal), showed CLR infection
within a few years after its first release in 1985 (Srinivasan et al., 2000).
There are now about 22 000 ha planted with Cauvery and it is performing
well at higher altitudes, where leaf rust epidemics are less severe. Among
the other arabica cultivars released by the Central Coffee Research
Institute (CCRI) near Chikmagalur, S2828 is most promising for its complete
resistance to all CLR races in combination with good yield, bean size
and cup quality. This cultivar was developed from an interspecific cross
between S274 (robusta) and Kent made in 1934, followed by two backcrosses
to Kent and intensive pedigree selection over several generations.
The first robusta coffee planted in India was in the Wayanad district of
Kerala State around 1905 and much of the present robusta plantings is
directly derived from those early introductions. Family selection for yield
and bean size resulted in a number of selections, of which eventually S274
and the BR series were released in the 1950s. S274 in particular has
become an important robusta cultivar. Probably inspired by favourable
reports from Java of a spontaneous hybrid between C. congensis and C.
canephora named ‘congusta’, breeders at the CCRI made interspecific
crosses between C. congensis and S274 around 1942. After backcrossing to
S274 and a number of generations of sib-mating and selection, the new
cultivar C x R was first released to some estates in 1973–6 in Kerala, but
further selection for uniformity is being pursued. The C x R cultivar has
characteristics very similar to those reported earlier for Congusta on Java,
such as bolder beans and better cup quality, somewhat lower caffeine
content, more compact growth habit, good yield and adaptation to cooler
climates. It appears to have the possibility of considerably improving the
quality of robusta coffees produced in India. Some 10 000 ha have been
planted so far, new planting as well as conversion of old plantations by
top-working. There is also in development a compact growing version of
the C x R variety, analogous to Caturra in arabica.
In general, the capped single-stem pruning system is recommended in
India for both species. Plant densities applied are 3000 plants/ha for tall
48 Espresso Coffee
arabica cultivars, 4400 plants/ha for Catimor arabica and 1100 plants/ha
for robusta plantations. Regular applications of chemical fertilizers are
common practice in large plantations to support economic yield levels
complemented with farmyard manure or composted organic wastes (e.g.
coffee pulp). Weed control is accomplished by manual slashing or
herbicides. Recently, the Indian coffee sector has started actively to
promote organic coffee farming, in which no chemical fertilizers or
herbicides are used, and with minimum use of pesticides.
Black rot is considered the second most important disease after CLR
and this disease affects arabica as well as robusta. So far, no sources of
resistance have been identified and control depends on a number of
cultural measures (e.g. thinning of shade and regular pruning) and
fungicide spray applications. The most devastating insect pests in India
are the white stem borer and coffee berry borer (since 1990). There are no
sources of genetically controlled host resistances available to these two
pests, excepting that generally the white stem borer does not attack
robusta. The CCRI has developed instead a number of effective IPM
systems, including the use of pheromone traps for white stem borer, simple
traps with alcohol mixture as attractant for coffee berry borer, and
parasitoids and natural predators against coffee berry borer, shot-hole
borer, mealy bugs and scales.
Indian coffees are classified according to a rather complex system of
grading, region of origin and specialty coffees. In addition to the regular
coffee grades (PB, AA, A, AB, B and C) for washed/unwashed arabicas
and robustas, the Indian system also distinguishes grades for ‘blacks/
browns’, removed from the regular grades by electronic and manual colour
sorting (see 3.5.4 and 3.5.5), ‘bits’ (beans smaller than C grade and broken
beans) and ‘bulk’ (ungraded and not sorted for off-types). Coffees are also
called after their region of origin, such as Mysore, Chikmagalur, Coorg
(Kodagu), Malabar, Wayanad, Biligiris, Nilgiris, Bababudan, Shevaroys
and Pulneys. Speciality coffees include the Mysore Nuggets EB (extra bold
washed arabicas), Robusta Kaapi Royale (A-grade washed robustas) and
Monsooned coffees. The latter are prepared from cherry arabica and
robusta coffees of AA and A grades, mainly for export to Scandinavian
countries.
2.3.6 Factors determining green bean quality
The following main factors determine the quality of green coffee:
environment, cultural practices, genotype of the variety/cultivar and
post-harvest handling (processing). The first three factors determine the
The plant 49
potential quality of the green coffee, which can still be easily degraded by
poor processing practices. Post-harvest handling of the cherries is carried
out by the wet, semi-wet, or dry (natural) method of processing. The wet
process favours a light body and acid beverage, whereas semi-wet and dry
methods lead to enhanced body with reduced acidity. The effect of
environmental, agronomic and genetics factors on coffee quality will be
discussed below, while coffee processing and its relation to quality is
described in 3.3 to 3.5.
2.3.6.1 Environment
The influence of altitude and temperature on the quality of coffee has
been well documented. Arabica originated from the highlands of Ethiopia
and its mild and pleasant beverage is best preserved under similar growing
conditions. High altitude is critical for cultivation of arabica near the
equator. Moderate temperatures will favour a slow and uniform maturation
process of coffee berries and especially the wide amplitude between
day and night temperatures (thermoperiod) will increase flavours and
aromatic precursors in the beans. Coffee fruits developed at higher
altitudes produce more mucilage and they are richer in sugars and other
soluble solids.
The minimum annual rainfall for arabica production is about 1200mm
per year and the maximum should not exceed 2000–2400 mm. Coffee
plants grow and yield better if exposed to alternated cycles of wet and dry
seasons, and, moreover, a period of water deficit is important to
synchronize flower bud differentiation. Areas with excess precipitations,
especially during crop maturation, have a tendency to produce lower
quality coffee due to irregular cherry ripening and poor conditions for
drying the crop after harvesting. High air humidity may cause beans with
off-flavours like fermented and hardish, phenolic, medicinal and musty
‘rioy’ notes, especially in dry-processed (natural) coffees (see 3.7.2 for a
definition of these terms). Fermentation notes can be avoided if fruits are
harvested at the ripe cherry stage and wet-processed immediately. The
rioy taste seems to be associated with a biological transformation of 2,4,6-
trichlorophenol to 2,4,6-trichloroanisole, which is facilitated by excess of
moisture. In years of excessively long dry seasons, due to changes in
climate (e.g. caused by El Nin˜ o), shoot dieback and premature ripening of
the berries will result in green beans producing liquor with immature and
astringent notes. For example, a fishy off-flavour in immature beans is
attributed to 4-heptenal, an oxidation product of linolenic acid (Full
et al., 1999).
50 Espresso Coffee
2.3.6.2 Cultural practices
The correct amount of nutrients, the equilibrium of elements and the
application of the right fertilizers are of prime importance for production
of high-quality coffee beans. Among the essential elements, nitrogen
potassium, calcium, zinc and boron are considered the most important.
The endogenous level of potassium will influence total sugar and citric
acid content and is a key element during bean filling. Potassium sulphate
is a better source than potassium chloride. Nitrogen is important for
amino acid and protein build-up and influences caffeine content. Nfertilization
increases the total nitrogen in beans, with a weak negative
correlation to cup quality (Amorim et al., 1973). Calcium is an essential
element for cell wall formation (Ca-pectate) providing more compact
beans and improving the resistance to pathogen attacks. Zinc influences
protein and carbohydrate metabolism, and also affects the synthesis of
auxins, which promote cell elongation during fruit formation in the ‘soft
green bean stage’ (Ferreira and Cruz, 1988). Boron influences flowering
and fruit set, consequently affecting yield.
The effective control of pests and diseases is essential for the
production of quality coffee. The coffee berry borer is a real threat to
green bean quality. Perforated beans lose weight and infected fruits are
susceptible to attack by air-borne fungi facilitating undesirable fermentations,
degrading the final quality of the whole lot. Several off-flavours
have been associated with uncontrolled fermentation in coffee, as
illustrated in the normal versus elevated values for the following
compounds: ethyl 2-methylbutyrate (2.4 in normal vs 37 mg/kg in
fermented beans), ethyl 3-methylbutyrate (22 vs 345 mg/kg) and cyclohexanoic
acid ethyl ester (Bade-Wegner et al., 1997). The infestation of
coffee berries by the fruit fly (Ceratitis capitata) is commonly associated
with a fungus infection (Fusarium concolor), which gives a pink colour to
the bean and strongly affects cup quality (Bergamin, 1963). The presence
of virus, transmitted by mites, also affects negatively the final green bean
quality through precocious fruit ripening, irregular bean formation, and in
several instances the presence of dry and immature fruits (beans with
black colour and pungent/bitter taste at cupping).
2.3.6.3 Genetic aspects
The qualitative differences in cup quality among different arabica
varieties have received special attention during past decade, especially
after the release of new disease-resistant cultivars, such as Catimor,
Sarchimor, Ruiru 11 and Icatu. Comparing cup quality of arabica cultivars
The plant 51
requires rigorous standardization to eliminate confounding effects from
differences in climate, cultural practices and processing methods. New
cultivars are usually compared with traditional varieties renowned for
their excellent liquor characteristics, such as Bourbon, Blue Mountain,
Kent and SL28. Cup quality is evaluated on beverage characteristics like
aroma (flowery, ‘peasy’), taste (bitter, acid, sweet, woody-earthy), flavour
(chocolate, caramel), and body (light, medium, heavy). Data from
analytical tests of the chemical composition of green beans are poorly
correlated with the perceived cup quality as determined organoleptically.
This is an additional handicap for objectively establishing the varietal
differences, unless a panel of expert liquorers is available for judgement.
Recently, catimor and sarchimor progenies of var. Colombia have been
submitted to biochemical evaluations (caffeine, chlorogenic acids, fat,
trigonelline, sucrose) in parallel with cup tasting with respect to a
standard (Caturra): sucrose content and beverage acidity could be
correlated; no significant correlation could, however, be found between
degree of introgression and the biochemical parameters (Bertrand et al.,
unpublished results).
Differences between arabica and robusta coffee are generally very
pronounced and well documented. For instance, the musty smell of
robusta is attributed to the presence of 2-methylsoborneol (MIB)
(Vitzthum et al., 1990), and MIB ranges from 120–430 ng/kg in robusta
versus a level lower than 20 ng/kg in arabica (Bade-Wegner et al., 1993).
This knowledge led to a practical application, whereby steam treatment of
green robusta beans denatures MIB, and so is claimed to allow increased
percentage of robusta in commercial coffee blends. Other major
differences in green bean composition between arabica and robusta coffee
are summarized in Table 2.5.
Arabica has lower levels of caffeine, amino acids and chlorogenic acids
in comparison to robusta, but 60% more total oils. Chlorogenic acids
contribute to astringent notes, so the reduced amounts in arabica favours
its final cup quality (see also 3.11.6). It is known that many aromatic
volatile compounds are dissolved (trapped) in oil droplets and released
during brewing, so the oil fraction may explain some differences in cup
quality between arabica and robusta, particularly in espresso (see Chapters
8 and 9). The influence of the relative composition of the lipid fractions
in coffee on final cup quality is not yet well understood. The major lipid
fractions in arabica include (Fonseca and Gutierrez, 1971) linoleic acid
(C18:1, 47%), palmitic acid (C16:0, 41%), oleic acid (C18:2, 6.4%) and
stearic acid (C18:0, 6%) (see also 3.11.9). Most chemical data in green
arabica coffee are presented by ‘origin of sample’ (Santos, Colombia,
Kenya, etc.) without indicating the specific variety. Analytical data based
52 Espresso Coffee
on variety will provide additional information on interactions of genotype
with environment and type of processing. Sampling over successive crops
helps to understand the year-to-year fluctuations for key compounds, as
exemplified below for % total oils from Brazilian trees growing in the
same farm and under identical cultural practices (Table 2.6).
Besides fluctuations in total oil content, there are genotypic differences
in other non-volatile and volatile compounds, but these differences have
not yet been correlated to particular beverage types. It is thought that
reducing sugars contribute to bitterness upon caramelization during
roasting, so the ideal sugar content and relative fractions must be
determined. The acidity notes depend on the total acid content as well as
The plant 53
Table 2.6 Yearly fluctuation in total oil content of
Brazilian arabica coffees
Cultivar 1989 crop 1990 crop 1991 crop Average
Caturra 15.9 11.5 13.8 13.7
Red Catuai 15.8 13.6 14.3 14.6
Catimor 15.5 13.5 14.1 14.4
Mundo Novo 15.7 13.8 14.4 14.6
Laurina 12.3 13.5 14.9 13.6
Icatu 17.2 15.1 15.5 15.9
Crop Average 15.4 13.5 14.4 14.5
Source: Illy and Viani, 1995
Table 2.5 Main chemical differences between
arabica and robusta green coffees
Compound/fraction Arabica (%) Robusta (%) Reference
Caffeine 1.2 2.4 (> 4) 1
Trigonelline 1.0 0.7 1
Amino-acids 0.5 0.8 1
Chlorogenic acids 7.1 10.3 1
Lipids: total 16 (13 17) 10 (7 11) 1, 2
Oleic acid 6.7 8.2 9.7 14.2 2
Dipertenes: cafestol 0.5–0.9 0.2 1
Kahweol 0.3 1
16-0-methylcafestol 0.07 0.15 1
Sources: 1Illy and Viani, 1995; 2Speer and Kolling-Speer, 2001
on the balance among acid compounds present in green bean (Illy and
Viani, 1995). More than 30 acids are present in green coffee, the most
important being the chlorogenic acids, quinic acid, malic acid, citric acid
and phosphoric acid. Some variability seems to occur in different coffee
genotypes (Table 2.7).
The main acid fractions listed in Table 2.7 should not be considered
absolute since there are fluctuations from year to year, and differences
among varieties (Balzer, 2001). In these data, the arabica-Santos sample
is mainly represented by Mundo Novo and Catuai varieties and the
arabica-Kenya sample is an indication of Bourbon SL28 material. It will
be of interest to know how much of the acid fraction is controlled by
specific varieties, and how much is due to environmental and cultural
practices.
The coffee aroma profile is mainly controlled by genotype (species,
varieties), environmental origin and degree of roasting (see 4.4). Mayer et
al. (1999) made a comparative study among genotypes and found that
variety Typica (from Colombia) had a 52% higher content of a sulphur/
roasty compound (2-furfurylthiol) than an arabica sample from Brazil.
Holscher (1996) found similar data, reporting at least five-fold differences
of 2-furfurylthiol between different samples.
The genetic component of coffee aroma profiles may be more a
quantitative rather than a qualitative effect, i.e. the levels present in
green beans at harvest will be modulated by the interaction genotype/
environment. In other words, once an environment is fixed, it will be
always possible to select a variety that produces greater amounts of
particular aromas under a specific location. With further improvement of
analytical tools, it should become possible to differentiate specific coffee
varieties and origins by their profile of volatile compounds, which would
help tremendously in blending standards and quality control.
54 Espresso Coffee
Table 2.7 Variability in acid content among
genotypes
Genotype/origin
Quinic
acid
Malic
acid
Citric
acid
Phosphoric
acid
Arabica/Santos 5.6 6.1 13.8 1.1
Arabica/Kenya 4.7 6.6 11.6 1.4
Robusta/Togo 3.1 2.5 6.7 1.6
Source: Balzer, 2001
2.4 BIOCHEMICAL ECOLOGY
T.W. Baumann
2.4.1 Introduction
Imagining the squadrons of bacteria, moulds, insects and grazing mammals
(not to speak of human beings and their activities), which attack, feed on
and destroy plants, we must wonder why the landscapes are not bare but
still covered with a lush vegetation. Biochemical ecology (or chemical
ecology) emerged from the discovery of the chemicals involved in the
above-mentioned interactions between plants, animals and microorganisms.
Since Fraenkel’s remarkable article (1959) regarding the raison d’eˆtre
of the so-called secondary plant substances (Hartmann, 1996), scientists
are aware that plants (and to a smaller extent also animals and bacteria)
produce a vast array of substances not only to defend themselves against
pathogens and predators but also to attract organisms for their own
benefit. Today, biochemical ecology is an established field of natural
sciences (for a comprehensive review see Harborne, 2001). It is
conceivable that a better knowledge of such chemically mediated
interactions will stimulate biocontrolled farming and assist also coffee
growers in their fight against pests in the plantations, and thus – in the
long term – will undoubtedly improve coffee quality, to which all, farmers,
manufacturers and scientists are committed (Illy, 1997).
Before passing over specifically to biochemical ecology in coffee, two
well-studied examples will serve to illustrate this rapidly emerging
discipline of science. Many flowering species attract the pollinator, such
as an insect (e.g. bee, butterfly), a bird (e.g. humming bird), or a mammal
(e.g. bat), by odours and/or pigmentation. Additionally and very
frequently, the morphology of the flower complies with the needs of a
specific pollinator, for example by providing a landing platform for bees
and bumblebees or a hanging thin throat for hummingbirds, etc.
Generally, the pollinator is rewarded by nectar and by a more or less
welcome dust of pollen. However, about one-third of all orchids do not
invest energy into nectar production but have evolved flowers simulating
various kinds of rewards. For example, several species mimic by odours,
colours, shape and texture the female of a distinct solitary bee. The
delusion is so accurate that the male tries to copulate with the flower
whereby the entire pollinia are positioned, for example, on his head to
carry them to the place of a subsequent frustrating pseudocopulation,
finally resulting in the pollination of that flower (Schiestl et al., 1999).
And moreover, after pollination the flower produces a volatile substance
The plant 55
usually emitted by non-receptive female insects to inhibit copulation
(Schiestl and Ayasse, 2001). This example nicely illustrates that
biochemistry (odours and pigmentation) of biochemical ecology cannot
be separated from physical factors including morphology, texture,
mechanical strength, nutritional energy and, last but not least, appearance
in space and time.
A second example demonstrates how the plant calls for help when
attacked by a predator. The larvae of the beet army worm, Spodoptera
exigua, are a pest of the maize plant. In a series of brilliantly designed
experiments, it has been shown that the saliva of the larvae contains a
signal compound (perhaps partly originating from the destroyed plant
membranes) that, locally, and later also systemically, induces the formation
of leaf volatiles specifically attracting parasitic wasps (Cotesia marginiventris)
which in turn lay their eggs into the larvae, and thus eliminate the
predating insect (Turlings et al., 1990; Alborn et al., 1997). Additionally,
the volatiles may protect the plant from attack by other herbivores (Kessler
and Baldwin, 2001). Today, various examples of such interactions within –
at least – three levels (tritrophic interaction: plant/predator/parasite) are
known and have been successfully studied by coupling highly sensitive
electrophysiological techniques with GC-MS (for reviews see: Dicke and
Van Loon, 2000; Pichersky and Gershenzon, 2002).
Yet, secondary compounds govern, besides the sophisticated interactions
described above, another key process of plant survival, which is
named chemical defence (reviewed in Edwards, 1992; Harborne, 2001).
From an evolutionary point of view, general strategies for optimal defence
against predation were formulated, for example that plants are expected
to accumulate protective phytochemicals in a tissue or organ in direct
proportion to the risk of predation of that unit (Rhoades, 1979).
Therefore, organs and tissues with a high nutritional value (seeds,
young leaves, pollen) have a particularly high risk of predation.
2.4.2 The main secondary metabolites in coffee
(see also 3.11)
The phytochemical catalogue of coffee is very large (for an overview of
physiologically active substances see Viani, 1988; Baumann and Seitz,
1992; Clarke and Vitzthum, 2001); however, with respect to chemical
ecology, only a few most prominent coffee compounds have been intensely
studied so far, namely caffeine, chlorogenic acids and trigonelline.
Caffeine is a purine alkaloid (PuA). PuA are divided into methylxanthines
(caffeine, theobromine, theophylline etc.) and methylated uric acids
56 Espresso Coffee
(theacrine, liberine etc). Their ecological functions in relation to plant
development will be discussed in detail below. 5-Caffeoylquinic acid (5-
CQA) is the main component among the coffee’s chlorogenic acids. It is a
metabolite of the phenylpropanoid pathway and often induced by biotic
(pathogens, herbivores) and abiotic (UV, temperature, nutrient, light)
stress. Thus, it is thought to exert specific functions in plant protection
such as defending against microbial infection and herbivores, acting like a
screen against harmful UV radiation, and scavenging free radicals and
other oxidative species (reviewed in Grace and Logan, 2000). As will be
outlined below, chlorogenic acid is the ally of caffeine, as it associates with
it in a physico-chemical complex (Sondheimer et al., 1961; Horman and
Viani, 1972; Martin et al., 1987). Lastly, trigonelline is a derivative of
nicotinic acid and influences many events in plant life (reviewed in
Minorsky, 2002): it is a hormone-like regulator of the cell cycle and arrests
the cells in the phase prior to mitosis (G2); it has a not yet fully cleared
function in nodulation of the legume alfalfa by rhizobial symbionts
(interacting individuals of different species); it is believed to be a signal
transmitter in the response to oxidative stress; it may act as an
osmoregulator; and, finally, it has been reported to induce leaf closure in
various species showing sleep movements, the so-called nyctinasty.
Additionally, trigonelline was shown to act as a reserve for the synthesis
of coenzymes (NAD) during early coffee seed germination (Shimizu and
Mazzafera, 2000). To our knowledge no report exists regarding the
chemical defence properties of trigonelline, though its considerable
concentration of around 1% (dry weight) in both seeds and leaves, and
even higher in the youngest internode (>2%), this indicating a transport
from old to young leaves (Kende, 1960).
Clearly, the chemical defence by caffeine and chlorogenic acids works
because of (a) their high concentration in the related coffee organ or
tissue, (b) the generally low body weight of the herbivore (a phytophagous
insect may be deterred after one cautious trial, while a large naive animal,
such as a mammal, may swallow one or several plant organs at a time and
thus be intoxicated, but later will avoid that organ and feed selectively on
the rest of the plant) and (c) the need of the herbivore to ingest a large
quantity of leaves due to their overall low protein content. Very
convincingly, the coffee seed, furnished with comparatively high protein
has, in addition to the chemical protection, a strong mechanical defence:
the endosperm is extremely hard (see 2.4.5) and the inner fruit wall, the
endocarp, called ‘parchment’, is tough.
If we relate chemical defence of the coffee bean to the human being
foraging on it, we can state the following: the coffee drinker’s body
weight, which nota bene does not result from the protein content of the
The plant 57
bean(!), is high compared to the biomass of ground endosperm (roast
coffee) used for the coffee brew. And still, some coffee drinkers are only
pleased with a plant extract smoothed by sugar, cream or milk. The
protecting layers around the bean (see 2.4.3 and 2.4.5) already have been
removed by the lengthy and labour-intense processing of the coffee fruit.
Finally, the target molecule (receptor) of pharmacological action in the
coffee drinker is different from that in, for example, an insect (Table 2.8).
2.4.3 From seed to plantlet
As mentioned in the introduction, biochemical ecology is also a matter of
space and time and therefore has to deal with questions such as ‘how and
to where is coffee dispersed?’, ‘how long does it take for a seed (. coffee
bean) to germinate?’, or ‘what happens during the transitions from a seed
to a whole plant?’. Most of the results cited below were obtained by
studies on arabica but, slightly modified, also apply to robusta.
The hobby gardener may seed a so-called ‘green’ (unroasted) coffee bean
possibly available at the local coffee roasting company. By doing so, he or
58 Espresso Coffee
Table 2.8 Organism-related effects of caffeine (multifunctionality)
Organism Effect
Underlying
mechanism, target References
Bacteria
and fungi
(yeasts)
Bacterio- and fungistatic Inhibits UV dark
repair in DNA
Kihlman, 1977,
McCready et al.,
2000
Fungi Fungistatic, reduces
mycotoxin production
Unknown Buchanan et al.,
1981
Plants Inhibits germination,
reduces growth
Not known; inhibits
the formation of the
cell plate, calcium?
Rizvi et al.,
1981, Gimenez-
Martin et al.,
1968
Molluscs
(snails and
slugs)
Molluscicidal, reduces
heart rate
Calcium release,
increases the duration
of action-potential
plateaus?
Hollingsworth
et al., 2002
Insects Disturbs developmental
processes
Inhibits cyclic AMP
phosphodiesterase
Nathanson,
1984
Mammals Activates CNS, constricts
cerebral blood vessels,
increases lipolysis, positive
inotropic
Binds to adenosine
receptors
Nehlig, 1999
she has to bear in mind two things. First, the bean rapidly loses its
germinating power when harvested and processed, depending on the bean
humidity and on the temperature during storage. Generally, within 3
months after harvest and processing, the germination rate drops to zero,
but is extended to one full year or longer at 18
C, when the bean water
content never falls below 40%, achieved by storing the beans vacuumpacked
in a polyethylene bag (Valio, 1976; Couturon, 1980; Van der
Vossen, 1980). Second, under natural conditions the coffee bean will not
or very rarely get directly in touch with soil, because the seed-dispersing
animals (monkeys, elephants, large birds) attracted by the red, fleshy and
sweet coffee berries will, after having ingested them, excrete or regurgitate
seeds still covered by the hard endocarp, so to speak a parchment coffee, as
it also results technically from wet processing (see 3.3.2). Occasionally, the
entire fruit is dispersed or drops beneath the coffee tree. Then, the berry
will dry and shrink very slowly, looking like dry-processed coffee (see
3.3.1) before hulling. The seeds within the shrunk fruit are even better
protected and lose less water than in the parchment coffee. In any case, in
nature under favourable conditions (humidity) the seed will take up water
and germinate, breaking through the outer shell made up either of the
parchment or the complete husk. Finally, going back to the hobby
gardener, we have to add the following: the naked bean will germinate
earlier (by around 2 weeks) than the bean in parch, but has a higher risk of
being attacked by pests and pathogens in the soil during imbibition
(swelling) and germination. Even though caffeine is described as an
effective agent (Table 2.8) against all kinds of organism (generalists), a few
specialists have overcome the defence mechanism and not only detoxify
but possibly also metabolize the PuA for their own needs. Here, a striking
example is the coffee berry borer, Hypothenemus caffeicola, a serious
worldwide pest, which develops within the caffeine-rich endosperm.
The generalist/specialist rule is pertinent to all secondary compounds so
far studied in this respect. Additionally, one can recognize that these
phytochemicals are multifunctional, meaning that they have not only
multiple functions in the plant producing them but exert also various and
specific effects towards the target organism.
Let us return to germination: Most interestingly, during water uptake
(imbibition) caffeine remains almost completely fixed within the bean,
very likely due to an efficient caffeine barrier at its surface made out of
chlorogenic acids (Dentan, 1985). As will be outlined below, these
phenolic compounds are of crucial importance during the entire life of the
coffee plant, since they prevent autotoxicity by caffeine! Only when the
radicle (the thick primary young root) grows into the substrate, is caffeine
released at high concentrations into the environment (Baumann and
The plant 59
Gabriel, 1984). Obviously, under natural conditions the imbibed bean is
safely encapsulated – there is no need to excrete the defence compound
caffeine – whereas during the later stages of germination about one-third of
the caffeine of the young seedling, now exposed to pathogens and
predators, leaks via the root surface into the substrate, where it may inhibit
the competition by other plant species and prepare, along with other
compounds, the way for specific root colonization by microorganisms.
As nicely illustrated in Figure 2.5 (upper row), the emerging primary root
is the first visible sign of germination. However, dramatic changes occur
even earlier inside the bean: the tiny cotyledons invade the nutrient-rich
storage tissue of the bean, the so-called endosperm, and eventually occupy
the entire cavity within the bean. Simultaneously, the hypocotyl (the
region of the stem beneath the stalks of the cotyledons) stretches and
carries the head above the ground. Finally, the cotyledons unfold and shed
the seed coat. Since during invasion into the endosperm they suck up all its
constituents, cotyledons are, in simplified terms, a blot of the original coffee
bean: they contain and conserve all the caffeine of the endosperm, and
hence are most suitable for a screening to detect and select caffeine mutants
(Baumann et al., 1998). Similarly, the chlorogenic acids are also transported
into the cotyledons where, however, a large fraction is possibly used
for lignin synthesis in order to mechanically stabilize the leaf tissue (Aerts
and Baumann, 1994). Now the seedling is ready to develop into a plantlet.
2.4.4 From bud to leaf
The leaves are arranged in pairs, which alternate along the stem but are
adjusted almost in one plane along the side branches (see also Figure 2.2),
this in order to optimize light harvest. The individual leaf pair is born
from the terminal bud of the stem or of a side branch of first and second
order. In the bud, the leaf primordia (tiny pre-formed installations) are
covered by a resinous layer followed by two tough scale-like bracts
(stipules). The life of the leaf pair starts with its emergence from the bud
with the tiny leaf blades still attached to each other. Thereafter, they
separate and the individual leaf expands considerably to achieve its final
size and shape about 4–5 weeks after emergence. It is still soft, light green
and glossy. During the next 2–3 weeks, the leaf texture gets tough and,
coincidentally, the upper surface of the lamina turns from glossy to dull
dark-green, possibly resulting from a chemical change of the epicuticular
waxy coatings: long chain fatty acids are transformed into the
corresponding alkanes (Stocker, 1976). Now, 50–60 days after emergence,
the leaf is fully developed and optimally gathers solar light energy for the
60 Espresso Coffee
formation of sugars from carbon dioxide and water, or, in other words, the
net photosynthesis has reached its maximum where it remains for a long
time (Frischknecht et al., 1982; Mo¨sli Waldhauser et al., 1997). Under
natural and favourable conditions, the coffee leaf’s lifetime lasts for 10–15
months (Van der Vossen and Browning, 1978). Thereafter, senescence
starts and finally the leaf is shed.
If we throw a glance on the concomitant course of the key secondary
compounds, we find exceptionally high foliar concentrations of PuA and
The plant 61
Figure 2.5 Seed germination and seedling development. (Drawing by Yvonne
Boitel-Baur , Zurich, Switzerland). This illustration shows the various stages from
the bean in parch up to the emergence of the first foliage leaf pair
First row from left to right: Germination, the primary root, and later, the
hypocotyl emerge from the seed. Small lateral roots are formed and the
hypocotyl extends to carry the ’head’ above the ground (c.3 weeks)
Middle row from right to left: The apical hook straightens and the cotyledons (not
visible) completely invade in and dissolve the endosperm (7 weeks)
Lower row left: The cotyledons start unfolding, the remaining unresorbed layer of
the endosperm is shed, and subsequently the cotyledons fully expand (10 weeks).
The first foliage leaves appear later. In the upper left corner, coffee in parchment
(upper row) and ’naked’ beans are shown with one of the latter with the
silverskin partially removed
chlorogenic acids as soon as the very tender, nutrient-rich leaflets have left
the mechanically protective bud, what can easily be explained by their
high risk of predation by, for example, phytophagous insects (Frischknecht
et al., 1986). At this stage, both the enzymes (methyltransferases) involved
in caffeine biosynthesis and the key enzyme of phenylpropane synthesis,
phenylalanine ammonia lyase (PAL), show very high activities (Aerts and
Baumann, 1994; Mo¨sli Waldhauser et al., 1997). The leaf alkaloid
concentration increases almost to 0.1M (as related to the tissue water)
and thus has about 10 times the concentration of an espresso!
The velocity of synthesis of both caffeine and chlorogenic acids
decreases sharply during the subsequent leaf expansion. The relative
caffeine content drops as a consequence of dilution by growth. However,
the absolute amount of caffeine per leaf increases steadily because of low
enzyme activities still persisting throughout the entire period of leaf
expansion (Mo¨sli Waldhauser et al., 1997). The fully developed coffee
leaf has, on a dry weight basis, a caffeine content in the range of the bean.
Amazingly, the chlorogenic acids, even though formed in parallel to
caffeine, still continue to increase during the next six weeks (Kappeler,
1988). Shedding leaves have been reported to be caffeine-free, indicating
that the caffeine nitrogen is re-used by the plant.
The concerted formation of both the alkaloids (mainly caffeine) and
chlorogenic acids (mainly 5-CQA) has a physiological significance:
caffeine easily permeates through all kinds of biological barriers, except a
few installed by the caffeine-containing plants themselves (for example,
the coffee bean’s surface mentioned above). In order to avoid autotoxicity,
caffeine is physico-chemically complexed by 5-CQA, and
compartmented in the cell vacuole (Mo¨ sli Waldhauser and Baumann,
1996). Since chlorogenic acid is encaged in the cells where it was
synthesized, one has to assume that caffeine, due to its hydro- and
lipophilic nature, and to its complexing ability, slowly migrates within the
coffee plant towards the sites of highly accumulated chlorogenic acids. In
other words, caffeine is passively dislocated and, so to speak, collected
within the plant in proportion to the tissue concentration of chlorogenic
acid. Apparently, the coffee plant controls caffeine distribution by the
allocation of chlorogenic acids. This is nicely illustrated by the uneven
distribution of caffeine within the lamina of the coffee leaf: it is highly
accumulated at the margins and sharply decreases in concentration
towards the mid-rib (Wenger and Baumann, unpublished). Needless to
say, the chlorogenic acids show the same distribution pattern. In terms of
biochemical ecology it is important to note that this phytochemical leaf
architecture is significant: the leaf margin, a preferential site of insect
attack, is particularly well furnished with the key defence compounds.
62 Espresso Coffee
Whether phytochemical leaf architecture is genetically controlled and
can be influenced by breeding remains to be investigated.
Thus, caffeine is an ideal defence compound: the organism feeding on
caffeine-containing tissues is unable to hinder it from rapid distribution
within and action on its body. However, from the plant’s view the
problem of autotoxicity had to be solved. The solution was, as already
mentioned, complexation by phenolics: all PuA-containing plants
allocate high concentrations of either chlorogenic acids (coffee, mate´),
catechins (cocoa, cola, guarana´), or both (tea). By the use of suspensioncultured
coffee cells as a model system, it has been shown that up to
77.4% of the caffeine is fixed in the complex at 25
C (Mo¨sli Waldhauser
and Baumann, 1996). Amazingly, in 1964 Sondheimer calculated a
complexation degree of 78% for caffeine in the coffee bean at the same
temperature (1964). Obviously, the concentration achieved by the
remaining fraction (approximately 20–25%) is beyond the autotoxicity
level. A temperature increase lowers the degree of complexation and vice
versa. In this context, several intriguing questions regarding the
interdependence between phenolics and PuA in terms of metabolic
regulation, their final concentrations and organ growth rate are not yet
fully answered but are well considered (e.g. Ky et al., 1999) in the work on
interspecific crosses by the group of Michel Noirot in Montpellier, France
(www.coffee-genomics.com).
2.4.5 From flower to fruit
The time period between flower opening (anthesis) and the fully ripe fruit
is species-specific and varies considerably among the coffee species. It
depends further on the genotype and on climatic and cultural conditions.
The economically important species C. arabica and C. canephora require
6–8 and 9–11 months for maturation, respectively (Guerreiro Filho, 1992).
As might be expected, all flower organs contain the purine alkaloid
caffeine, with highest concentrations in the stamens. Amazingly, the
latter accumulate, besides traces of theobromine, easily detectable
amounts of theophylline, indicating an alternative biosynthetic pathway
in the male part of the flower with theophylline as the direct precursor of
caffeine. In leaves and seeds, caffeine biosynthesis was shown to proceed
via theobromine. In analogy to citrus plants, where the highest caffeine
concentration has been found in the protein-rich pollen (Kretschmar and
Baumann, 1999), we may assume a preferential PuA allocation also to
coffee pollen grains. However, the related analyses have not yet been
done. Bees are, in contrast to many insects, not only amazingly tolerant
The plant 63
against caffeine and other phytochemicals (Detzel and Wink, 1993), but
rather, after caffeine uptake, have an improved performance such as a
boost in ovipostion by the young queen, an enhanced activity of the bees
outside the hive, and an improved defence by bees against hornets at the
hive entrance (reviewed in Kretschmar and Baumann, 1999).
When the blossom falls from the coffee tree the persisting ovary
develops into the young green coffee fruit (Figure 2.6 and Figure 2.7).
Fruits always signify high investment costs and, therefore, to defend them
against predators the coffee plant pursues several linked strategies.
First, the very young and green fruits are not showy but rather
inapparently arranged in clusters in the leaf axil (Figure 2.6). Secondly,
both chlorogenic acids and purine alkaloids are highly concentrated, and,
thirdly, the development of the real endosperm is postponed until
mechanical protection works. This last point is a most remarkable feature
of the coffee fruit development. Within 3–4 months after anthesis the still
green fruit reaches a size suggesting readiness for maturation. When cut
across, two greenish beans, already typically rolled up, can be recognized.
However, the appearances are deceptive: the fruit is far from being mature,
the (generally) two beans are false perisperm beans made up of mother
tissue (Carvalho et al., 1969). At the adaxial pole (towards the fruit stalk)
of each bean one can see the beginnings of endosperm development: a
whitish, very soft tissue (also called liquid endosperm) starts to invade into
and resorb the perisperm bean. Recent studies show that perisperm
metabolites such as sugars and organic acids are most likely acquired by the
endosperm (Rogers et al., 1999b). We may assume that this process is
similar to the invasion of the cotyledons into the endosperm during
germination described above: the metabolites shift from one tissue to the
other, whereby they have to pass through the so-called apoplast, i.e. the
extracellular space between peri- and endosperm during seed development,
and between endosperm and cotyledons during germination. However, the
endosperm is more than a simple blot of the perisperm, since it owns high
biosynthetic activities. In conclusion, and philosophically speaking, in
coffee the way to the next generation is characterized by transitions in
which the metabolites are shuffled around twice.
During this invasion the inner layer (endocarp) of the fruit wall
(pericarp) noticeably and increasingly solidifies and later results in the
parch layer described above (see 2.4.2 and 2.4.3). The mechanical
defence of the endosperm itself is remarkably increased by the formation
of thick cell walls containing, besides cellulose, the so-called hemicelluloses,
i.e. arabinogalactan and galactomannan (Bradbury, 2001).
Hemicelluloses are highly complex polysaccharides (see 3.11.3) primarily
renowned for giving an amazing degree of hardness to palm seeds (cf. date,
64 Espresso Coffee
Phoenix dactylifera; vegetable ivory, Phytelephas macrocarpa). It remains to
be mentioned that the coffee perisperm finally atrophies into the thin
seed coat, the silverskin, that falls off during roasting. Very soon after
anthesis the pericarp contains an absolute amount of caffeine kept
unchanged until ripeness. However, the initially high (>2%) caffeine
The plant 65
Figure 2.6 Coffee plant (C. arabica) flowering and fruiting. (Drawing by Beatrice
Ha¨ sler , Uster, Switzerland). The drawing shows a side branch with flowers at
various stages: wilting and falling off, in fresh bloom, or buds (from the base to the
apex). Above, flowers are illustrated in detail and enlarged. The yellow stamens are
inserted in the throat of the corolla. Similarly, various fruit stages are shown: like the
flowers they are arranged in composite clusters at the leaf axil
concentration drops by dilution to around 0.2% during the further growth
and maturation processes, culminating in the transformation of the fruit
wall (pericarp) into three distinct layers which serve for fruit dispersal: the
tough endocarp protects the seed from digesting enzyme activities in the
gut of the frugivores such as birds or mammals; the fleshy, sugarcontaining
(Urbaneja et al., 1996) middle layer (mesocarp) softened by
enzymes (Golden et al., 1993) acts as a reward, while the vivid coloration
by anthocyanins (Barboza and Ramirez-Martinez, 1991) of the outermost
layer (exocarp) is to attract the dispersing animal.
We should not close this section without relating our thoughts about
biochemical ecology to a practical question of our daily life: how does the
espresso bean get its caffeine? Though numerous publications on caffeine
biosynthesis exist (for a comprehensive review see Ashihara and Crozier,
1999), this problem has never been addressed and therefore we can only
speculate about it. Clearly, the endosperm has a certain biosynthetic
capacity for caffeine. But is this all? Are there contributions of other
sources? The perisperm provides around one-third of the seed caffeine as
66 Espresso Coffee
Figure 2.7 Fruit development (C. arabica)
Above: Within about 4 months the green fruit grows to a considerable size.
Comparatively late, i.e. between stage 5 and 7, the endosperm starts to develop
(2–3 months)
Below: When the fruit turns olive, the endosperm is already hard (4–5 months).
Now the mesocarp gets fleshy and the exocarp partially red (5–6 months). After 6
months the exocarp is bright red and the mesocarp very fleshy. Later (7–8
months), the fruit colour turns to dark (dull) red and the mesocarp starts to dry
out. Finally, the fruit starts to shrink and the exocarp gets dark and darker
estimated from the caffeine content of the perisperm bean (see Figure
10.5 in Sondahl and Baumann, 2001). The leaves are not directly
contributing to seed caffeine, but the pericarp may be a valuable source, as
studies with labelled caffeine have shown (Keller et al., 1972; Sondahl
and Baumann, 2001). Obviously, caffeine migrates from the fruit wall into
the developing seed, most likely due to the high concentration of
chlorogenic acids allocated to the perisperm/endosperm. Unfortunately,
the extent of this caffeine transport is unknown. Conceivably, this
fraction depends on both the fruit developmental time and the
chlorogenic acids allocations, and is correspondingly larger in a slowripening
species with a high ratio of seed to pericarp chlorogenic acids.
Again, synthesis, transport and accumulation of chlorogenic acids
eventually determine where and how much caffeine is to be allocated
in the seed. In conclusion, perisperm and pericarp are certainly important
sources of the seed caffeine, whereas the leaves, the pericarp and perhaps
also the greenish perisperm may provide most of the chlorogenic acids
crucial to gather and firmly fix the caffeine to the coffee bean. However,
the degree of contribution from each side (maternal tissues versus
endosperm and embryo) is not yet known. Additional studies on the
developmental biology of the coffee seed (Marraccini et al., 2001a, 2001b)
as well as reciprocal crosses between coffee species differing in their
caffeine and chlorogenic acids content (see 2.4.4, From bud to leaf) will
cast some light into the espresso’s darkness!
2.5 MOLECULAR GENETICS OF COFFEE
G. Graziosi*
2.5.1 Introduction
Research on the main arabica cultivars has two main complications: one
historical and the other biological. As reported by Berthaud and Charrier
(1988), most of the cultivars were derived from a small number of plants
that survived transport from Yemen to Southern Asia and to Central and
South America via Europe. Consequently, the genetic base of the main
The plant 67
*I am indebted to Maro Sondahl, Philippe Lashermes, Francois Anthony and Alexander de Kochko for
providing samples of beans and leaves on many occasions. I wish to thank Maro Sondahl for critical reading
of the manuscript and Alberto Pallavicini, Barbara de Nardi, Paola Rovelli and Elisa Asquini for preparing
the figures. The results reported here were supported in part by the European Community (Microsatellites:
Contract ERBIC18CT970181 and ICA4-CT-2001-10070) and in part by illycaffe` SpA (ESTs).
cultivars is rather restricted and the overall diversity is poor. To overcome
this problem and to introduce new valuable genes, a number of
interspecific crosses has been carried out; Arabusta, Icatu` and Catimor
are well known hybrids (Capot, 1972; Carvalho, 1988). The biological
complication is brought about by the self-fertilizing capability of C.
arabica, which enhances genetic homogeneity.
The relatively small amount of fundamental research on coffee has
been somewhat expanded by recent technical developments and by a
moderate increase in scientific studies. New momentum has been gained
through the advent of genomics. DNA sequencing techniques are now
commonly available in most laboratories, and the Coffea arabica genome
(see Glossary at the end of this section for technical words) has been
considered for large-scale analysis.
2.5.2 The genome
The genome of C. arabica consists of 44 small chromosomes (2n . 4x),
twice the number of C. canephora chromosomes and all other Coffea species
(Krug and Mendes, 1940; Bouharmount, 1959; Kammacher and Capot,
1972; Charrier, 1978; de Kochko et al., 2001). C. arabica is an allotetraploid
plant formed by the spontaneous fusion between the genome of C.
canephora, as a male parent, and the genome of a plant of the C. eugenioides
group as a female parent (Lashermes et al., 1999). Indeed the total amount
of DNAper nucleus in arabica (2.6 pg, about 2.4 109 base pairs) is twice as
much as any other Coffea (Cros et al., 1995). The interspecific cross
occurred considerably less than one million years ago and therefore this
species should be considered, in evolutionary terms, as recent.
If C. arabica is a relatively young species, then the two ancestral
genomes should have maintained their individuality with little redistribution
of genes among the chromosomes of different origin (similar
chromosomes between these two genomes are said to be homoeologous).
Evidence to this effect has been provided by Lashermes et al. (2000a), who
studied the inheritance of single genes in this allotetraploid organism.
They found a normal disomic inheritance, as in any other diploid species.
Nevertheless, arabica must have at least four copies of each gene, two
derived from the canephora ancestor and two derived from the eugenioides
ancestor. It is also reasonable to assume that the pairs of orthologous loci
(corresponding genes on the homoeologous chromosomes) could be
somewhat different. Thus, the homozygosity generated by the selffertilizing
property of this organism is presumably compensated by the
constitutive heterozygosity brought about by the two ancestral genomes.
68 Espresso Coffee
As a final note, it is important to mention that a genetic map of C.
arabica does not exist. Nevertheless, low-density maps of C. canephora and
of interspecific crosses are available (Paillard et al., 1996; Ky et al., 2000;
Lashermes et al., 2001; Herrera et al., 2002). Apart from the relatively low
level of research mentioned above, the lack of an arabica map is due to a
number of reasons: the complexity of a tetraploid genome of recent origin,
the high level of homozygosity and last, but not least, the low number of
available polymorphisms.
2.5.3 DNA polymorphisms and molecular diversity
DNA polymorphisms are an important tool for studying genomes and
they have allowed for the identification and mapping of expressed genes
in many organisms. Whilst it is difficult to identify genes responsible for a
given trait, polymorphisms associated with an interesting gene can be
easily followed through various generations and crosses to assist breeders
in their introgressive and selective protocols. In addition, in the past few
years the C. arabica genome has undergone screening projects aimed to
identify polymorphisms. Some of these studies and the main techniques
are reported here.
Cros et al. (1993) and Lashermes et al. (1996b) reported the
identification of many restriction fragment length polymorphisms
(RFLPs) in various Coffea species. This approach detects sequence
variations, frequently single base mutations, and involves the digestion
of DNA by a specific restriction enzyme. Unfortunately, this classical
technique requires Southern blots, which are rather complex and costly.
Moreover, these polymorphisms are frequently not very informative
because they do not have more than two alleles. A new generation of
RFLP is required, possibly based on the enzymatic amplification (PCR:
polymerase chain reaction) of expressed genes.
Several publications have reported a random amplified polymorphic
DNA (RAPD) approach (Cros et al., 1993; Lashermes et al., 1993, 1996c;
Orozco-Castillo et al., 1994; Zezlina et al., 1999; Ruas et al., 2000). This
technique is very useful since it allows for the identification of DNA
polymorphisms in almost any organism even when there is no other
information available on the DNA under study. It is based on the
enzymatic amplification of short DNA stretches (100–1000 base pairs)
lying between two inverted repeats. Agwanda et al. (1997) reported the
first case of association between RAPD polymorphisms and the T gene for
resistance to coffee berry disease (CBD) in Coffea.
The plant 69
The AFLP technique (amplified fragment length polymorphism) has
proved to be very informative: many electrophoretic bands (30–90 bands)
can be obtained in a single PCR and the probability of finding a
polymorphic band is relatively high. This technique is based on the
digestion of the DNA by two restriction enzymes, adapters are added at
both ends of the DNA fragments and some of the DNA fragments are PCR
amplified. AFLP has been successfully used for characterizing varieties
(Anthony et al., 2002) as well as in evaluating the introgression of genes in
hybrids (Lashermes et al., 2000b). Nevertheless, AFLP is relatively
complex and it does not allow for the identification of heterozygotes.
Undoubtedly, microsatellites are the most useful source of polymorphisms
and they are widely used in plant, animal and human research.
Microsatellites are DNA sequences repeated in tandem and the number of
repeats can vary in different organism of the same species. Usually, they
are very informative because one locus can show many alleles (high
heterozygosity) and because they display codominance, thus allowing for
the identification of heterozygotes. Moreover, analyses are easily carried
out by PCR and a large number of samples can be analysed at low cost and
in a short time. However, the development of assays for these
polymorphisms is sometimes difficult and expensive. The first description
of microsatellites in C. arabica was reported by Mettulio et al. (1999),
Rovelli et al. (2000) and Combes et al. (2000). Since then, microsatellites
have been used in a variety of studies, as reported by Lashermes et al.
(2000b), Prakash et al. (2001) and Anthony et al. (2002). As expected,
microsatellites can show different alleles as well as individual differences
in C. canephora (Taylor et al., 2002) but they can show some
complications in C. arabica. In the cross reported in Figure 2.8, the
microsatellite GTG52 shows normal disomic inheritance, but it is not
unusual to find individual plants displaying more than two alleles (Figure
2.9). Most probably, the primers used in these experiments recognized
more than one locus, as would be expected for a polyploid organism.
There are other classes of polymorphic sequences, for instance the
Internally Transcribed Sequence (ITS) of the ribosomal region, but their
use is limited to specific purposes since they can display sequence
variations within the same plant (Zezlina et al., 1999).
The general picture that can be drawn from polymorphic DNA studies
is that C. arabica is a peculiar crop with a very low level of heterozygosity.
This is not surprising because of the restricted genetic base and the high
incidence of autofertility. There is little doubt that this low heterozygosity
will hamper both the development of a genetic map and the use of
molecular genetics in breeding programmes until the development of a
reasonable number of informative polymorphisms.
70 Espresso Coffee
The plant 71
Figure 2.8 Genescan analysis of the cross Sarcimor x ET6 with the 9TG52
microsatellite
Line 1: The Sarcimor parent was homozygote for the 219 bp allele
Line 2: The ET6 parent was homozygote for the 235 bp allele
Lines 3 and 4: The two F1 samples presented both alleles (heterozygote) as the
other 15 plants of the same progeny. (Cross performed by F. Anthony at CATIE)
Figure 2.9 Samples as in Figure 2.8 analysed with the 7TG46 microsatellite. The
two F1 samples showed three alleles: 131 bp, 140 bp and 151 bp. Most probably
the primers amplified two loci, one of which was homozygote for the 131 bp allele
in both parents
2.5.4 Expressed genes
Completion of the model plant Arabidopsis thaliana genome sequence
(The Arabidopsis Genome Initiative, 2000) has given new insights into
the complexity of the plant genome. About 25 000 genes are sufficient for
supporting the life cycle of a flowering plant and there is no reason to
believe that the coffee plant requires more genes. This estimate could be
valid for C. canephora but it is certainly an underrepresentation for C.
arabica, which has twice as many chromosomes, and the gene copies on
the homoeologous chromosomes might have significantly diverged in
sequence and function throughout evolution. None the less, the size of
the arabica genome renders it amenable to mass analysis.
The first catalogue of expressed genes was reported by Pallavicini et al.
(2001). They partially sequenced a cDNA library prepared from root
meristem mRNA and obtained about 1200 ESTs (expressed sequence tag)
which eventually were reduced to 901 clusters. Sequence homology
analysis identified a provisional function for about 70% of the clusters
(Figure 2.10). The complete list of clusters can be obtained from the
home page of the Department of Biology, University of Trieste.
Recently, the public DNA sequence databases reported new C. arabica
ESTs provided by M.A. Cristancho and S.R. McCouch (Genebank
Accession Numbers: BQ4488720-BQ449166) but a reference publication
is unavailable.
72 Espresso Coffee
Figure 2.10 Functional classification of 1200 ESTs
The public databases report a total of 712 sequences of C. arabica and
nine sequences of C. canephora, mainly microsatellites or ESTs. The
remaining sequences can be classified as reported in Table 2.9.
Among the enzyme sequences, it is worth mentioning the complete
sequence of N-methyltransferase, one of the enzymes involved in caffeine
metabolism, which was provided by Kretschmar and Baumann
(Genebank Accession Numbers: AF94411-AF94420). Also important
are the studies on sugar metabolism (Marraccini et al., 2001b), which,
together with the aromatic compounds, are relevant for coffee quality.
The most studied gene encoded the 11S storage protein, the most
abundant protein in the coffee bean (Acun˘a et al., 1999; Marraccini et al.,
1999; Rogers et al., 1999a). The authors identified the complete coding
sequence as well as the promoter region by expressing the protein in
transformed tobacco plants.
Notably, a number of nucleotide binding site–leucine rich repeat
(NBS–LRR) sequences, which are believed to confer resistance to a wide
spectrum of pests, have been identified (Noir et al., 2001). This is the first
step in the possible use of these genes in breeding programmes or
transformations.
2.5.5 Introducing new genes
Classical breeding has been reviewed by a number of authors, the article
by Van der Vossen (2001) being the most recent. Unfortunately, the
coffee plant is perennial and breeding programs take 20–30 years. Such a
long breeding time coupled with the peculiar genetic structure of C.
arabica (tetraploidy, low heterozygosity and self-fertility) strongly suggests
the use of an alternative route as genetic transformation.
The plant 73
Table 2.9 Categories of
sequences of C. arabica and
C. canephora other than
microsatellite or EST categories
Category No.
Enzymes 19
Structural proteins 5
Ribosomal DNA 4
Chloroplast 6
Successful transformation experiments have been reported by Leroy et
al. (1997a, 1999, 2000) and Spiral et al. (1999). They introduced the
cryIAc gene of Bacillus thuringiensis into arabica and canephora plants
through a disarmed strain of Agrobacterium tumefaciens. The analysis of 60
transformed plants showed a correlation between expression levels of the
cryIAc gene and the newly acquired resistance to leaf miner Perileucoptera
spp. The same technique has been used by Ribas et al. (2001), who
introduced two sequences for the resistance to the herbicide gluphosinate.
An elegant transformation experiment has been reported by Moisyadi
et al. (1998, 1999). They transformed coffee plants with an antisense
sequence of an enzyme apparently involved in the biosynthetic pathway
of caffeine and they obtained plants producing little or no caffeine.
Even if the introduction of genes by transformation must be regarded as
an important tool for improving coffee quality and production, possible
drawbacks must be taken into account. There is little doubt that the
introduction of a gene conferring resistance to one of the many coffee
plant pests would enhance production and be a considerable help to
farmers. Nevertheless, coffee is a perennial plant and a field of plants
resistant to a specific insect, for instance, eventually would select for an
insect insensitive to the toxin. This problem could be partially overcome
by the introduction of more than one toxin for the same pest. In addition,
an important consideration is the attitude of consumers who might be
unwilling to consume a beverage derived from transformed plants.
2.5.6 Prospects
There is little doubt that in the near future more ESTs will be produced
and more polymorphism assays will be developed. In fact, some public
Brazilian funding agencies have launched a research programme for the
production of 200 000 ESTs. Presumably, a number of ‘important’ genes
will be identified, as those involved in flowering and ripening control, or
those responsible for resistance to diseases and pests. These molecular
tools should allow for the amelioration of the coffee plant through
classical breeding. A good example is the introgression of resistance to the
nematode Meloidogyne exigua reported by Bertrand et al. (2001).
Far more complex is the genetic approach to improving the quality of
the coffee beverage. Taste and aroma are presumably under the control of
a large number of genes and affected by environmental conditions.
Nevertheless there is no possibility of having a good quality coffee
without a good genetic base. To approach this problem we have to wait
for the development of new tools. Genome-wide analysis and microarray
74 Espresso Coffee
analysis will allow the study of gene expression and gene–environment
interactions and could become a diagnostic approach to assess quality.
2.5.7 Glossary
AFLP: Amplified fragment length polymorphism. This technique is
based on the digestion of DNA by two restriction enzymes; adapters
are added at both ends of the DNA fragments and some of the DNA
fragments are polymerase chain reaction amplified (see after).
Alleles: Different variants of the same gene.
Allotetraploid: Organism with four sets of chromosomes, two sets
inherited from one ancestral progenitor of a given species and the
remaining two sets inherited from a different species.
Codominance: Two gene variants (alleles) are codominant when they
are simultaneously expressed and visible.
Disomic inheritance: Normal way of inheriting chromosomes or genes.
EST: Expressed sequence tag. Partial sequence of an expressed gene.
Genome: The totality of all the chromosomes and genes of an organism.
Homoeologous chromosomes: Chromosomes carrying similar genes but
originated from two ancestral plants of different species.
Homozygosity: Uniformity of alleles.
ITS: Internally transcribed spacer. Specific portion of DNA located
between the genes coding for the ribosomal RNA.
Microarray: Hybridization technique to visualize the genome expression.
Microsatellite: DNA sequences repeated in tandem.
NBS LRR: Nucleotide binding site leucine rich repeat. Type of gene
conferring resistance to diseases and pests.
PCR: Polymerase chain reaction. In vitro reaction allowing for the
amplification of specific genes or DNA fragments.
Polyploid organism: Organism that contains more than two sets of
chromosomes.
Primers: Short fragments of DNA allowing for the initiation of the
PCR (see above).
RAPD: Random amplified polymorphic DNA. Enzymatic amplification
of short DNA stretches lying between two inverted repeats.
RFLP: Restriction fragment length polymorphism. Variation of the
recognition site of a restriction enzyme (see below).
Restriction enzyme: Enzyme able to cut DNA at or near specific
sequences.
Southern blots: Chromatographic technique for isolating and identifying
specific DNA fragments.
The plant 75
76 Espresso Coffee
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86 Espresso Coffee
CHAPTER3
The raw bean
S. Bee, C.H.J. Brando, G. Brumen,
N. Carvalhaes, I. Ko¨ lling-Speer, K. Speer,
F. Suggi Liverani, A.A. Teixeira,
R. Teixeira, R.A. Thomaziello, R. Viani
and O.G. Vitzthum
3.1 INTRODUCTION
A.A. Teixeira
As seen in the previous chapter, the genotype, the macro- and
microclimates, as well as agricultural practices, play a fundamental role
in the quality of coffee. One of the main characteristics of quality coffee is
the possession of good organoleptic properties. Ripe cherry coffee from the
Coffea arabica species is the ideal raw material for obtaining fine quality
coffee (Figure 3.1). In order to maintain the bean’s inherent qualities,
special care is required during processing.
3.2 HARVESTING
A.A. Teixeira, C.H.J. Brando, R.A. Thomaziello
and R. Teixeira
Preparation for harvesting should be made in advance in order to avoid
last minute surprises and damage to the quality of the coffee. The cleaning
Figure 3.1 Coffee plantation (left) and cherry coffee on the tree (right)
and maintenance of the entire infrastructure and the checking of
equipment to be used should be accomplished at least one month before
the start of the harvest.
Harvesting should only start after a very careful examination of the
level of maturation, when most of the cherries are ripe, with a minimum
(5%) presence of unripe cherries. Unripe cherries lead to light-green
beans, which, when dried at a temperature above 30
C, become blackgreen;
both types weigh less than a normal bean (Teixeira et al., 1991).
The extended stay of the cherries on the tree or on the ground should be
avoided in order not to increase the quantity of sour and black beans,
which also weigh less than normal beans. Light-green, black-green, sour
and black beans are the defects (see 3.7.2) that have the greatest negative
effect on overall coffee quality, damaging type, appearance, roast, yield
and beverage (Teixeira, 1978).
Harvesting can be accomplished in different ways: by stripping onto the
ground (not recommended) or onto sheets, by selective hand picking or
by mechanical means.
3.2.1 Stripping (Figure 3.2a)
This operation is accomplished by stripping all the cherries from the
branches at the same time. The biggest problem is to evaluate the right
time to perform this operation, therefore the need to collect samples of
100 cherries to determine the correct percentage at each stage of
maturation. At the time of harvest, depending on the number and spacing
between one flowering and the other, one can find on a tree immature,
greenish, ripe, overripe and dry cherries.
In the preparation of natural coffee, cherries are dried whole, that is, the
beans are dried while surrounded by the pulp (mucilage and husk). In this
operation, the homogeneous maturation of the cherries is very important
and, therefore, the percentage of green cherries collected should be as low
as possible. In regions with a dry climate, stripping can begin when the
majority of the cherries are in the overripe or partially dry stage. In this
case, drying will be quite uniform and the risk of undesirable fermentation
very low, in contrast to stripping with a high percentage of ripe cherries.
Harvesting by stripping should be done directly onto sheets placed
under the tree beforehand, to avoid contamination and contact with
cherries that have spontaneously fallen and which may have already
deteriorated (Carvalho et al., 1972). For this reason, stripping onto the
ground must be avoided. In general, the sheets are laid under the coffee
trees, in the same direction as the rows of trees, and normally reach
88 Espresso Coffee
beneath three or four plants. Once the stripping is complete, the cherries
should be collected and screen tossed (‘winnowed’) before being bagged
and taken to the processing infrastructure on the day of harvest.
Mechanical winnowing can be used in the coffee plantation or at the
processing installation.
Cherries that have fallen directly onto the ground before harvesting,
called sweepings (varric¸a˜o in Brazil), and those stripped directly onto the
ground should always be processed separately in all the phases of
preparation and should never be mixed with cherries stripped onto sheets.
3.2.2 Hand picking (Figure 3.2b)
In this operation only ripe cherries are picked, normally in baskets or bags.
This type of harvesting is generally employed in regions with constant
rain, where many flowerings occur throughout the year. Harvesting of the
cherries should be performed as many times as necessary. The labour force
should be sufficient to avoid the fall of cherries and their consequent
deterioration. To avoid undesirable fermentation, the ripe cherries should
be taken to the preparation infrastructure and processed on the day of
harvest.
3.2.3 Mechanical harvesting (Figure 3.3)
This technique is mainly applied in Brazil and Hawaii. The operation can
be done by different systems, all based on the vibration of the branches of
the coffee tree:
n self-propelled machines that remove the cherries from the tree,
collect and winnow them and put the product into bags, hoppers,
carts or trucks;
The raw bean 89
Figure 3.2 Harvesting (a) by stripping and (b) by hand picking
(a) (b)
n mechanical stripping machines, pulled by tractors, that drop the
cherries on the ground;
n portable stripping machines (derric¸adeiras in Brazil) carried by the
picker, using pneumatic or internal combustion power, that drop the
cherries on the ground, whether covered (recommended) or not by
cloths or plastic sheets.
All these systems are being constantly improved in order to increase
selectivity and give priority to the harvest of ripe cherries. An
increasingly common practice in mechanical harvesting is to conduct
the stripping operation at two or three different times, concentrating each
time on the part of the tree with the most ripe cherries.
Self-propelled stripping machines and those pulled by tractors can only
be used on plantations that are flat or slightly undulated, from 10 to 20%
of slope depending on the technology. Portable stripping machines are
compatible with any topography and represent an important alternative
to increase the efficiency of the harvest in mountainous areas.
Mechanical harvesting enables a substantial reduction in the operating
costs that comprise one of the main components of the total cost of coffee
production.
3.2.4 The myth of selective picking
It is widely believed that quality coffee can only be obtained if selective
hand picking is used. This is certainly valid for small plantations, but
becomes a long-standing myth on modern medium-sized to large estates.
Quality coffee can be produced using a variety of harvesting systems,
manual or mechanical. Selective coffee picking – the careful hand picking
of only ripe coffee cherries – is indeed one way to ensure that quality on
the tree is transferred to the cup. However, it is not the only way. In fact,
90 Espresso Coffee
Figure 3.3 Mechanical harvesting
selective picking is nothing more than an indicator that only sound, fresh,
ripe coffee cherries are used as raw material to produce the finest beans
from which a perfect cup is brewed. Sound, fresh, ripe cherries may be
obtained from a variety of picking practices combined with processing
techniques. Top-quality coffee can be produced regardless of the
harvesting technique employed.
When unwanted products are picked, quality must be maintained by
post-harvest separation techniques so that high-quality coffee may still be
produced from the remaining ripe cherries. Coffee growers must therefore
be prepared to cope with harvesting that brings in mixed cherries. Their
ability to separate sound, fresh, ripe cherries from immature, over-ripe,
semi-dry and dry cherries is becoming a crucial factor in the production of
top-quality coffees at a reasonable cost. Modern post-harvest processing
equipment now available can handle such mixed cherries efficiently while
using less water and causing less pollution.
3.3 PROCESSING OF THE HARVEST
A.A. Teixeira, C.H.J. Brando, R.A. Thomaziello and
R. Teixeira
Processing must start on the same day as the harvest to avoid undesirable
fermentation and reduce the risk of mould contamination, starting with
the pulp of the fruit, rich in nutrients and moisture (Figure 3.4). This is a
critical point in the preparation of coffee and can often jeopardize an
entire year of work caring for the plants.
In all three types of processing (natural, pulped natural or washed),
whenever possible, it is important that the cherries pass through washer–
separators. In this equipment, rocks and impurities are eliminated and the
cherries are separated by density: on one side the lighter cherries (floaters
– the dry and over-ripe cherries) and on the other side the heavier ones
The raw bean 91
Figure 3.4 Stereo microscope view of a coffee cherry: (left) entire; (centre) pulp
(mucilage and husk) partially removed; (right) section
(immature and ripe cherries), enabling the separation of cherries with
different humidity levels, facilitating drying and making the lot of coffee
more homogeneous (Figure 3.5).
3.3.1 The natural (dry) process (Figure 3.6)
Harvesting must begin when most of the cherries are at the over-ripe and
dry stages, especially in regions characterized by a dry harvest season. The
harvesting of a high percentage of ripe cherries can cause undesirable
fermentation, especially if the drying process is not begun on the same day
as the harvest (see also 3.11.11.1 for the risk of mould formation).
In the processing of natural coffee, the whole cherries (bean, mucilage
and pulp) are dried on patios or racks under the sun or in mechanical
dryers. By this process, if well conducted, it is possible to produce a good
coffee with ‘body’ and a pleasant ‘aroma’ (see 3.8.8 for an explanation
of these terms), greatly appreciated in the preparation of espresso
coffees.
92 Espresso Coffee
Figure 3.5 Cherries passing through washer–separators
Figure 3.6 Patio drying of natural coffee: (left) just harvested; (right) almost dry
3.3.2 The washed (wet) process (Figure 3.7)
The so-called washed or wet process requires a raw material composed of
only ripe cherries that have been selectively picked or are mechanically
separated in the process itself. After passing through the washer–separators
and before removal of the pulp, the separation of the green immature
cherries from the ripe ones can be performed, using differences in pressure,
in a separator of green cherries. This machine has a specially designed
screen with long slotted holes forming a cylinder containing a rotor that
forces the cherries against the stationary screen and towards the edge(s) of
the cylinder. The soft, ripe cherries pass through the holes of the screen.
The hard, unripe cherries, which cannot pass through the holes, go to the
edge(s) of the cylinder where a counter-weight controls their outflow.
The pulping process consists in the removal of the pulp by a pulper,
followed by the removal of the mucilage from the parchment, which can
be accomplished either mechanically, by the use of chemical products, or
by fermentation. The mucilage that adheres to the parchment, with
thickness of 0.5–2.0 mm, is very slimy and composed of pectins and sugars.
Fermentation to remove it is carried out in tanks at ambient
temperature in the presence of microorganisms for between 12 and 36
hours, depending on local temperature. The mass of coffee can be either
immersed in water (wet fermentation) or not (dry fermentation). The
latter type is increasingly employed since it reduces fermentation time. In
wet fermentation, the water must just cover the coffee mass. This process,
however, takes more time and, if longer than 72 hours, can increase the
formation of fluorescent beans, which easily generate ‘stinker’ beans
during storage (see 3.7.2.5 for a definition of the term ‘stinker’).
Fermentation ends when the parchment loses the slimy feel of the
mucilage. This stage can be recognized by rubbing the beans in one’s
hand: when they no longer slip against each other and produce a
characteristic noise, fermentation is concluded.
The raw bean 93
Figure 3.7 Steps of pulping process
Warm water rich in microorganisms is generally used in order to
decrease fermentation time. The use of chemicals is less common and
more risky, and is applied only in research laboratories. In the
fermentation tanks, minerals and sugars are freed from the seeds (coffee
bean with parchment plus mucilage) and can then become concentrated
if the washing water is not changed regularly. The water can easily be
contaminated by all types of microorganisms, leading to uncontrolled
fermentation and a deterioration in cup quality: in this case a slight
‘onion’ flavour, which can reach a very bad ‘stinker’ taste, and can easily
be detected in cup tasting.
Too high pH levels and the presence of ferric ions above 5 mg/l also
lead to the production of off-flavours (Vincent, 1987). Volatile acidity,
mostly owing to the formation of acetic acid, increases with fermentation,
especially if the process lasts longer than 20 hours. After the conclusion of
the pulping operation, the coffee must be very carefully washed so as to
ensure the removal of any trace of mucilage. Incomplete washing can
cause undesirable fermentation, with detrimental consequences to quality.
Removal of mucilage can be carried out mechanically by friction. In
general the mucilage is not completely removed. There has been a strong
trend in the adoption of mechanical removal of the mucilage after the
advent of modern machines, which consume little water and energy and
substitute the traditional aquapulpas. Modern mucilage remover machines
avoid environmental pollution since they consume and contaminate less
water than wet or dry fermentation.
3.3.3 The pulped natural process
A process called pulped natural (cereja descascado in Brazil), intermediate
between the natural and washed processes, began to be used in Brazil in
the early 1990s. In this process, the cherries are pulped and the beans in
parchment dried while surrounded by the mucilage. Fermentation for the
removal of the mucilage is not used in this process. As early as 1960, an
experiment on good cup quality and increased germinative power
demonstrated the validity of pulping cherries without submitting them
to fermentation (Ferraz et al., 1960). The quality of pulped natural coffee,
when well processed, has been shown to be excellent, with the advantage
of producing coffee with greater body than coffee produced using the wet
process.
In this case, harvesting can be conducted by stripping when the
majority of the cherries are ripe. Processing must take place on the same
day, starting with the washer–separators. The pulpers used for the
94 Espresso Coffee
preparation of the pulped natural coffees are equipped with separators of
green (immature) cherries, to separate by pressure immature cherries from
mature ones. In the preparation of pulped natural coffees, the pulp and
remains of mucilage must be disposed of in a location far from the
preparation area. After initial decomposition, it can be used on the farm
as a fertilizer. The immature cherries must be dried separately and coffee
in parchment with unremoved mucilage must be immediately put to dry.
3.3.4 Environmental impact
The processing of the harvest can produce liquid and solid residues that
are aggressive to the environment in case they are not correctly disposed
of or treated.
The natural process is the only one that does not attack the
environment, since it does not require the use of water and does not
produce solid and liquid residues rich in organic substances. Even when
light and heavy cherries are separated by water, contamination is low
(only suspended solids) or inexistent, because the contact between water
and cherries is rapid and the whole cherry does not lose contaminating
substances in the water. The natural process is, therefore, environmentally
friendly by definition.
The pulped natural process produces solid residue (the pulp) and
contaminates the water that enters into contact with the pulp and the
parchment. Finally, the washed (wet) process is the most pollutant of all,
since it requires, in addition to pulping, the removal of the mucilage,
which is extremely rich in organic substances and corresponds to about
half of the polluting load of the process. The washed process is particularly
aggressive when the removal of the mucilage occurs by natural
fermentation, which requires great volumes of water that consequently
become contaminated.
The washed process is, therefore, attracting increasingly great attention
among environmentalists whose pressure is provoking changes to protect
the environment. The pulped natural process was conceived as a less
polluting alternative since it does not require fermentation and was
developed at a time when a greater concern for the environment already
existed.
The tendency in wet processing is to use as little water as possible,
transporting the products in a dry state and not in water channels, using
modern machines that consume less water, substituting fermentation by
mechanical removal of the mucilage and re-circulating used water after
The raw bean 95
the removal of solids. The residual waters can be used for irrigation,
infiltrated into the ground, or be treated before disposal in waterways.
The major recent changes to make coffee mills more environmentally
responsible, which are still ongoing, can be grouped in three areas:
n design or layout of the mills;
n water-saving equipment;
n recycling and safe disposal of wastewater.
Old coffee mills had many channels to convey coffee – and water that
became contaminated in the process – from one machine to another.
Modern wet mill design brings machines close together to enable gravity
feeding without water (dry feeding) when slope is available. If slope is
unavailable dry conveyors are used (e.g., bucket elevators, inclined
conveyors, etc. that convey coffee without water) instead of pumps that
can only move coffee that is mixed with water. New water-saving washer–
separators, replacing water-intensive siphon tanks, pulpers and mechanical
mucilage removers, consume much less water than conventional
machines; this is particularly the case of mucilage removers used to
replace traditional fermentation (the most water-intensive stage of the
process and the most contaminating one). Finally, recycling of liquid and
solid wastes produced by wet milling is becoming a required practice, as is
the safe disposal of such water in a manner that does not harm the
environment. Since wastewater treatment and disposal is cheaper and
more convenient for smaller volumes with high pollution loads than for
large volume with smaller pollution loads, the use of as little water as
possible becomes a very critical issue.
Attempts to make the pulping process totally dry have been frustrated
by a fall in the quality of the beans and have subsequently been
abandoned.
3.4 DRYING
A.A. Teixeira, C.H.J. Brando, R.A. Thomaziello and
R. Teixeira
The more homogeneous the mass of coffee, the better and more uniform
will be the drying process. In green immature cherries, humidity reaches
70%, while in mature cherries it varies from 50 to 70%, in over-ripe
cherries from 35 to 50% and in dry cherries from 16 to 30% (Tosello,
1946; Rigitano et al., 1963).
96 Espresso Coffee
The drying of the coffee should be slow. The withdrawal of the bound
water of the bean is difficult and, therefore, a drawn-out process. The
slower the drying operation, including periods of rest, the more
homogeneous will be the final product.
The drying can be conducted under the sun only, on patios or
suspended tables; in mechanical dryers only; or initiated under the sun
and concluded in mechanical dryers.
3.4.1 Patio drying of natural coffee
It is important that natural coffees, normally harvested by stripping,
consisting of immature, greenish, mature, over-ripe and partially dry
cherries, be dried separately. Therefore, the floaters (over-ripe and
partially dry cherries) must be separated from the immature and mature
cherries to form groups of similar humidity to achieve a uniform product,
of good appearance and homogeneous dryness.
The patio area must be well calculated and large enough to avoid the
coffee remaining in thick layers, especially at the beginning of the drying
process, and so damaging its final quality. The drying patios must be
constructed in areas without accumulations of cold air, with great
exposure to sunlight, with a slope between 0.5 to 1.5% to facilitate water
drainage and be coated with bricks, tiles, concrete or even asphalt (see
Figure 3.6).
At the beginning of the drying process, the coffee must be spread in
thin layers with a height of 2–3 cm, and be constantly turned, always
observing the direction of the sun. If the sun is in front of the worker his
shadow should be behind him or if the sun is behind the worker his
shadow should be in front of him.The volume or kg/m2 of coffee required
to obtain such a thickness varies with the stage of maturation and the
percentage of humidity: unripe, ripe, over-ripe and dry cherries (natural
process) or coffee in parchment (wet process). An indicative correlation
indicates that a layer of 5–6 cm of thickness takes 30–40 kg of fruits per m2
of yard: 2–3 kg of cherry coffee (at 60–65% moisture) or 6–9 kg of
parchment coffee per m2 can be dried per day (Matiello et al., 2002).
The coffee must be constantly turned, 15–17 times per day, in both
directions, to speed up the removal of the easily eliminated external water
and to avoid the appearance of fermented or mouldy beans, easily
identified during classification and rejected by the market.
Every evening during the initial days of drying the coffee should be
heaped in thin rows, of 5–10 cm height, in the direction of the patio’s
The raw bean 97
slope. With the passing of the days, the rows can be thickened until the
coffee reaches a stage of semi-dryness, or 20–30% of humidity. From now
on, every evening, the coffee should be heaped and covered with cotton
or waxed fabric that will facilitate the equalization and uniformity of
drying.
3.4.2 Patio drying of pulped natural and washed
coffees
Fully washed parchment coffee (parchment devoid of mucilage) is dried in
drying patios or suspended tables (Figure 3.8). In the initial phase, the
layer should be very thin, 2–3 cm, with constant turning. Only after skin
drying (the elimination of the external water) can the coffee be dried in
thicker layers. Once it is half dry, the coffee can be heaped and covered
during the night.
It has been observed that the light of the sun, especially radiation
between 400 and 480 nm, is important for the maintenance of coffee
quality, improving aroma and acidity, and reducing mould and bacterial
load, above all in the final phase of the intermediate stage of drying, when
the beans have reached a moisture content below 40% and a dark
translucid aspect (Northmore, 1969).
The care of pulped natural coffees must be greater than that of washed
coffees due to the presence of the (sticky) mucilage. The layers should be
2–3 cm high and constantly turned until the coffee is skin-dry, that is, it
has lost all external humidity. If this drying operation is well performed, it
can be concluded in a single day. Later, the operation will be the same as
the one for coffee in parchment without the mucilage, in patios or
mechanical dryers.
98 Espresso Coffee
Figure 3.8 Sun drying of pulped natural or washed coffee: (left) patio drying;
(right) suspended table drying
3.4.3 Mechanical drying of coffee (Figure 3.9)
Some types of dryer can directly receive the coffee parchment without
mucilage, while others require that the coffee be pre-dried in the sun.
Drying can be accomplished solely by mechanical means in regions with
adverse climate, but normally mechanical drying is used after partial
drying in patios, when the coffee attains a moisture level between 20 and
30%. The dryers should be equipped with heat exchangers (indirect fire
furnaces or steam or water radiators) or gas burner to avoid direct contact
between combustion fumes and coffee and the smell of smoke in coffee.
The use of coffee husks or parchment as fuel can reduce the cost of drying
and the cutting down of trees. Firewood should always be dry, to enable a
higher yield, and to avoid smoke and pollution of the environment.
Mechanical dryers should be loaded with beans of the same moisture to
improve uniformity of drying. All types and brands of dryers are designed
to work with a full load and to avoid heat loss, increase in drying time and
unnecessary energy and labour expenses. The temperature in the coffee
mass should not exceed 40–45
C, to avoid death of the embryo, and
unpleasant smells. When immature cherries are present, the temperature
should not exceed 30
C to avoid their transformation into dark-green and
black-green defects, of very low quality (Teixeira et al., 1982). The 12%
humidity level is reached after the coffee is removed from the dryer,
which it must leave with a slightly higher (around 1%) humidity.
3.4.4 Comparison between natural and washed
coffee
The biggest difference between the chemical composition of washed and
natural coffee is in the soluble solids content, which is higher in the case of
natural coffee. Pulped natural coffee presents an intermediate position.
The raw bean 99
Figure 3.9 Mechanical drying of coffee
The difference can be explained by osmotic phenomena. During cherry
drying a gradient of concentration is established between the mucilage
(mesocarp) and the bean (endosperm) with a migration to the bean of
some sugars present in the mucilage. On the other hand, in the wet process,
there occurs a loss of 0.7% of soluble solids during fermentation and 0.7%
during washing, by the diffusion to the water with the mucilage of aqueous
solids from the internal part of the bean (Wootton, 1971). Sugars consist of
one-third of the total loss of soluble solids during fermentation with a
reduction ranging between 0.34 and 0.51%. Studies show that soluble
solids in arabica varieties cultivated in Campinas, Brazil, ranged from 26.7
to 30.5%; the highest values were measured with the Mokka variety. Dryprocessed
samples yielded 0.4–0.9% more solids than wet-processed ones,
where leaching of water-soluble matter occurs during fermentation (Table
3.1) (Carvalho, 1988). The soluble solid content measured by the
Navellier method (Navellier and Brunin, 1963; Wilbaux, 1967) ranged
from 26.1 to 30.6%, with an average of 28.3% in robusta, while in arabica
coffee it ranged from 23.8 to 27.3%, with an average of 25.6%. In Icatu, an
interspecies hybrid between arabica and robusta coffee, this index reached
levels of 26.8–28.6% (Moraes et al., 1973). The greater amount of soluble
solids present in natural coffees is undoubtedly responsible for their
increased body in comparison with washed ones (Table 3.1).
The mineral content is slightly reduced in wet-processed beans
compared to dry-processed ones (Table 3.2).
The natural process does not require high volumes of water and is less
polluting. However, in the wet process, about 60% of the cherry (pulp)
volume is removed, significantly reducing the area and the time required
for drying. In the wet process, where the pulp and mucilage are removed,
the risk of undesirable fermentation is significantly reduced. A further
advantage that must be considered is the possibility of obtaining a product
with a more homogeneous appearance and drying in the cases of pulped
natural and washed coffees. The body produced during the process of
100 Espresso Coffee
Table 3.1 Soluble solids in washed and
natural arabica coffees (% dry matter)
Variety Washed Natural Difference
Caturra 26.71 27.12 +0.41
Mundo Novo 27.91 28.33 +0.42
Yellow Bourbon 27.81 27.85 +0.04
Maragogype 27.90 28.84 +0.94
Mokka 29.69 30.47 +0.83
drying in natural coffees is, however, basic to the sensorial quality of an
espresso coffee, from which derives the recommendation for their use in
this type of preparation. Pulped natural coffees present a body
intermediate between the washed ones, with low body, and the natural
ones, with good body. These coffees are also strongly appreciated in
blends for espresso coffee.
3.5 FINAL PROCESSING FOR EXPORT AND
ROASTING
C.H.J. Brando and A.A. Teixeira
Dry parchment coffee or dry cherry coffee must be stored at moisture
levels below 12% in order to avoid the development of musty, earthy or
fermented flavours. Dry coffee can be stored in bulk (silos) or in bags in a
dry atmosphere, preferably at mild temperatures and in the dark, before
final processing for export or roasting. Dry cherry and parchment coffee is
stored in Brazil in large wooden cells, the ‘tulhas’, for up to several weeks
to ‘mature’, i.e., to equilibrate the moisture before final processing
(Figure 3.10).
The raw bean 101
Figure 3.10 Tulhas for maturation of coffee used in Brazil: (left) empty; (right) full
Table 3.2 Mineral content of green
coffee (% dry matter)
Type of coffee Minerals Potassium
Dry-processed robusta 4.14–4.39 1.84–2.00
Dry-processed arabica 4.11–4.27 1.77–1.88
Wet-processed arabica 3.58–3.95 1.63–1.70
In areas where coffee is dry-processed, it is common for the farmer to
deliver green (hulled) coffee to a mill. There it is submitted to rigorous
processing which may include size grading, density separation and colour
sorting. Small farmers may deliver semi-finished product in the form of
dry cherries to cooperatives for further processing.
In areas where coffee is wet-processed, parchment coffee is delivered to
a mill. Hulling and further processing of washed or pulped natural coffees
should only take place shortly before coffee is exported.
3.5.1 Cleaning
Final dry coffee processing starts with the cleaning of the parchment or
cherries, performed in two stages: pre-cleaning and destoning. Precleaning
is accomplished by the suction of light impurities and dust
combined with the sieving of impurities larger or smaller than coffee.
Destoning employs flotation of coffee to separate it from the heavier
stones. A plate or rotary magnet may be used at either stage to separate
iron particles.
3.5.2 Hulling and polishing
Hulling is the process by which the outer shell (husk) of parchment or dry
cherry coffee is removed. Several systems are used depending on the
product and on local practices.
Parchment coffee is often hulled by friction as it is forced to travel in
the space between a screw or multi-facet rotor and its static case. These
machines often remove the parchment husk as well as the ‘silverskin’ (the
perisperm) in a process that is also called polishing or hulling-polishing.
In either case, heat results from the friction and care must be taken to
avoid overheating, which may have negative impact on quality.
Arabica cherry coffee is usually hulled in machines with rotating
blades. These ‘beaters’ force coffee through the openings of a cylindrical
case that encloses the rotor. Since little or no heat is generated in the
process, the silverskin is not removed. Friction hullers for cherry coffee are
used primarily for robusta coffee. They have a rotating cylinder with cleats
and a static adjustable blade. In this case heat is developed and some
polishing may take place.
Removal of silverskin (polishing) is commonly required for washed
coffees but seldom for natural coffees. The cross-beater huller used for
cherry coffee may be also used for parchment coffee if polishing is not
required.
102 Espresso Coffee
All types of hullers include an air column, the ‘catador’ that separates
parchment or cherry husk from the hulled green coffee beans. A fan blows
or aspirates husk and discharges it away from the machine, outside the
building or into a husk cyclone and/or silo. Some hullers have a built-in
larger catador, or may be coupled to one, that separates hulled coffee in
different density fractions, e.g., the ‘heavier’, sound beans and the
‘lighter’, often defective beans. A built-in repassing device is often found
in some types of hullers, especially those for cherry coffee.
The proportion between husk and green coffee is around 20% for
parchment coffee and 50% for cherry coffee.
3.5.3 Grading
Green coffee may be separated by size not only for marketing purposes but
also to enable better density and colour separation. Machines that grade
coffee by size, i.e., coffee graders, can also be used to separate coffee beans
by shape. Flat (regular) beans are the ones with one flat and one concave
surface. Round beans from cherries holding just one seed, called
‘peaberries’, ‘mocas’ or ‘caracoles’, occur in much smaller proportions.
Coffee graders use sieves of different sizes and shapes to separate green
beans according to size and shape. Though most countries rely on sieves
with holes measured in multiples of 1=64 s of one inch (e.g., beans size 18
are retained by a sieve 18=64 in.), millimetres are also used. Very large
beans are usually retained by sieve 19 (7.54 mm) and medium to large
beans by sieve 16 (6.35 mm). Peaberries are separated by slotted (oblong)
screens of several sizes (Figure 3.11).
3.5.4 Mechanical sorting
Densimetric separation aims at removing defects associated with less
dense beans, such as malformed beans, insect-damaged beans, fermented
The raw bean 103
Figure 3.11 Cleaning and grading of coffee (courtesy of Sortex Ltd)
beans, some types of black beans, etc. Traditional catadors, that remove
such defects by airflow, are being increasingly replaced by densimetric
tables, also called gravity separators, using flotation for a more efficient
separation.
Air flotation created by powerful fans located below the deck of a
gravity separator causes the light, defective beans to float whereas the
heavier sound beans lie at the bottom, in contact with the deck.
Vibration causes these two fractions to leave the deck separately.
3.5.5 Electronic sorting
S. Bee and F. Suggi Liverani
Differences in bean colour are usually associated with defects (Illy et al.,
1982; Clarke, 1988). This allows the distinguishing of good beans from
defective ones. Coffee sorting by hand is still practised in regions where
labour rates remain low. However, when consumers began demanding
increased quality, cost and complexity of this labour increased.
Automated techniques were introduced since machines can maintain
greater levels of consistency than hand sorting (Anon., 1987), thus
providing a premium quality product at increased margins.
The widespread use of natural coffees in espresso blends makes
electronic sorting a prerequisite for the production of high-quality blends.
The size, cost and complexity of electronic sorting machines varies,
depending on the output and on the complexity of optical measurement
and processing unit. Colour sorters (Vincent, 1987) generally consist of
four principal subsystems (Figure 3.12): feeding system, optical system,
ejection devices and processing unit.
3.5.5.1 Equipment subsystems
Feeding system: In the feeding system, bulk coffee is fed from a vibrating
hopper onto a flat, or channelled, gravity chute. This method separates
the product into a uniform ‘curtain’, or monolayer. The feeding systems
commonly employed are: inclined gravity chute, a flat belt, inclined belt,
contra-rotating rollers and a narrow grooved belt.
Optical system: The optical system measures the magnitude of light
reflected from each single bean. Two or three cameras are used to view
the product from different angles as it leaves the end of the chute,
increasing the defect detection efficiency.
Ejection devices: The ejection process typically takes place while the
product is in free fall. Accepted particles fall along their normal trajectory
while rejected ones are deflected into a receptacle with a blast of
104 Espresso Coffee
compressed air from a high-speed solenoid or piezoelectric valve,
connected to a nozzle. Pneumatic ejector valves must have rapid action
(on/off time of less than 0.25 msec, a typical duty cycle of 150–300 Hz,
firing a pulse of air for 1–3 msec), reliability, a long lifetime (at least a
billion cycles or more) and mechanical strength.
Processing unit: The processing unit manages the control of the
machine and classifies particles as either ‘acceptable’ or ‘rejected’ on the
basis of colour, or both colour and shape. The electronic processing
systems in sorting machines have progressed from the simple analogue
circuits of the early machines to the advanced digital microprocessorbased
ones, as in the present generation of machines. Most of the setting
up of the sorting parameters can be done by the machine itself including,
in some cases, the ability of the machine to ‘learn’ the difference between
a good and a bad product, this is usually represented as a two-dimensional
The raw bean 105
Figure 3.12 Schematic layout of an electronic sorting machine (courtesy of Sortex
Ltd)
reflectivity ‘colour map’ (Figure 3.13), where the boundary curve is the
contour, outlining the acceptable product. A sophisticated control system
will track the average colour of the product so that, even though the
average product colour may change with time, the machine will continue
to remove only the predefined abnormal particles (Figure 3.14).
Advanced sorting machines have the capability of providing information
about the product, for example the number of rejects, or information
about any drifts in colour in a certain batch of product.
3.5.5.2 Sorting techniques
Monochromatic sorting: Monochromatic sorting is based on the
measurement of reflectance at a single isolated band of wavelengths.
The removal of beans lighter or darker in hue than the average is a typical
application of this system.
106 Espresso Coffee
Figure 3.13 Bichromatic colour sorting map (courtesy of Sortex Ltd)
Bichromatic sorting: Bichromatic sorting is only used when a simple
monochromatic measurement is not adequate for effective optical sorting.
Usually green and red filters are used for arabica beans and red or near
infrared for robusta beans, allowing the elimination of unripe, waxy,
chipped, insect-damaged or broken beans as well as of white and black
beans (see 3.7) (Figure 3.15).
Trichromatic sorting: Trichromatic sorting enhances bichromatic
sorting, adding the capability to sort beans by size or shape or for the
detection of gross defects such as the presence of foreign material like
glass, stones and insects. In this way, objects of the same colour but
different shapes, or with holes or cracks, can be effectively removed.
UV fluorescence sorting: It has been found that certain non-visible
defects, such as moulds or bacteria, fluoresce when irradiated with long-
The raw bean 107
Figure 3.15 Spectral reflectance curves obtained from green arabica coffee. The
solid lines represent acceptable beans; the dotted lines represent discoloured beans
(courtesy of Sortex Ltd)
Figure 3.14 An example of coffee sorting (courtesy of Sortex Ltd)
wave ultraviolet light (360 nm). This property may be used as a basis for
sorting. This technique was originally developed for removing ‘stinkers’
from green arabica coffee beans, but has found applications in sorting
other materials as well. However, the fluorescence effects can be best
sorted on freshly harvested coffee. In the case of light beans after a long
storage period following the harvest, this technique faces more difficulties.
Infrared sorting: Over the last decade, the wavelength range used by
sorting machines has been extended from the visible into the infrared
region. Here, both water absorption and other chemical effects play an
important part in determining the reflectivity characteristics of beans.
Optical sorting with lasers: The use of lasers is a technique that is still
in its relative infancy. A laser beam is used to illuminate the product and
the magnitude of reflected light is affected by the amount of laser light
that is either scattered from the surface, or diffused within an object.
Since a laser produces narrow beams of coherent light at a single
wavelength, there is no need to use optical band pass filters.
3.5.5.3 Future trends
Computer vision systems are increasingly being used in the food industry,
pushing the development of new devices and software algorithms, offering
many benefits over a conventional colour sorter, as the ability to simultaneously
sort objects on the basis of multi-criteria. Furthermore, improved
computer technology (Suggi Liverani, 1991) has enabled classification of
green coffee lots automatically, using colour mapping together with
specialized algorithm based on fuzzy logic (Suggi Liverani, 1995). This
improved technology has overcome the limitations of man-made classification
by operators who, for all their experience, may be subject to
variations and imprecision in their perception in detecting defects.
3.6 LOGISTICS
N. Carvalhaes, R. Teixeira, A.A. Teixeira and
C.H.J. Brando
3.6.1 Storage
Correct storage is the maintenance of the product for a certain period
while preserving its original characteristics. Storage can serve the interests
of a nation’s economy by compensating for cyclical and other variations of
agricultural production.
108 Espresso Coffee
Raw coffee, when stored under appropriate conditions of ambient
moisture, temperature and humidity, is quite stable and deterioration is
slow (see 3.11.7 for the changes in amino acid content during storage).
The warehouse must be constructed in a well-ventilated area not subject
to accumulations of cold air. A temperature close to 20
C, and a relative
humidity of the air never exceeding 60%, are optimal. There should be as
little natural light as possible and artificial lighting must be controlled,
preferably located in passageways and corridors. The warehouse must have
good ventilation, without draughts, especially from the south, with welllocated
doors. To avoid development of a woody taste and colour
bleaching of the beans (already observed at moisture levels above 12%)
the coffee must not contain more than 11 0.5% humidity when prepared
by the dry process and 12 0.5%, by the wet process, because excess
humidity in the beans facilitates attacks of mould and bacteria (see also
3.11.11.1). The increase in the humidity level of the beans is on average
0.18% for each percentage unit of increase in the relative humidity of the
air. The correct waterproofing of the floor prevents the deterioration of the
lowest bags in the piles. The coffee bags can be placed on wooden pallets,
or be protected by plastic sheets, to prevent direct contact with the floor
and should never be stacked against walls to prevent any type of humidity.
Coffee warehouses should not contain other agricultural goods or
chemicals that can transmit foreign flavours.
General warehouses with flow for commerce and exportation are
substituting the traditional 60.5 kg bags by so-called ‘big bags’ (with
capacity equivalent to 20 bags), which are made of crossed polypropylene
and are odourless and long-lasting. These serve to store coffee, for
transport to mills, to stuff containers in bulk and for delivery to those
industries that are structured for receiving the product in silos. The factors
that stimulate use of the big bag are: economy of space, ease in internal
transport with forklifts, greater speed and improved stock control.
Some of the changes that take place in the early stages of storage, about
30–40 days of warehousing, may improve quality with the disappearance
of the slightly green taste of fresh recently harvested coffee.
During prolonged storage of hulled coffee, colour changes in the bean
from green to white are common. This discoloration presents a serious
problem since, in addition to depreciating the product in terms of
appearance; such beans can present an inferior cup quality, with
corresponding losses for producers and exporters. It has been verified
that beans proceeding from washed coffee are more affected than those
coming from the dried cherry (pod). The first report published on this
subject (Bacchi, 1962) indicated that bruising of the beans, such as
normally caused by mechanical hulling, is the indirect cause of the
The raw bean 109
whitening of the coffee. Among the extrinsic factors studied, the relative
humidity of the air was the most important. The higher this factor is,
especially at levels above 80%, the faster and the more intense is the
discoloration of the beans.
Another experiment carried out to verify the behaviour of stored beans
in different types of packing and storage time disclosed the occurrence,
after the sixth month of storage, of an irreversible whitening and an
increase in the volume of beans stored in burlap, cotton and paper bags,
while beans stored in cans and plastic bags maintained their green colour
and a normal volume. The whitening of the beans of stored coffees was
more influenced by type of packing than by storage time. This bean
discoloration was accompanied by physical (humidity level, level of dry
matter, absolute density and bean weight) and chemical (soluble nitrogen
content) changes and depreciation in cup quality. The alterations in the
physical and chemical properties are attributed to enzymatic degradation
processes by the polyphenoloxidases and proteases present in the coffee
bean, and are possibly stimulated by atmospheric oxygen and variations in
the humidity of the air, according to the results obtained in paper, cotton
and burlap packing (Mello et al., 1980).
Several factors affect whitening, such as temperature and relative
humidity of the air, ambient light and bean humidity. A study of these
variables indicated that, under constant conditions, only at 10
C of
temperature and 52–67% of air relative humidity did the stored coffee
beans not lose their original colour after 192 days. In some conditions,
even maintaining the bean humidity below 13%, discoloration occurred
during the period. The interaction between temperature and relative
humidity at higher levels makes the beans start to lose colour within a few
days from the beginning of storage (Vilela et al., 2000).
The beans stored in cells of a silo or in bags in warehouses are presented
as a porous mass, constituted by the beans and the interstitial, also called
inter-granulate, space. The oxygen present in the inter-granulate space is
used in the beans’ respiratory process. After harvesting, the beans continue
to live and, like all living organisms, to breathe. The breathing process is
accompanied by the decay of the product’s nutritional substances. The rate
of decay of the stored beans is accelerated as temperature increases.
Beans stored in dry conditions, with humidity between 11 and 13%,
maintain a discreet breathing process. However, if the humidity level is
increased, the breathing and, consequently, the deterioration are
considerably accelerated (Puzzi, 1973).
In Bacchi’s type graphs (Figure 3.16) one can observe that the
hygroscopicity curves are of the sigmoid type, whose final portion,
corresponding to high relative humidity, presents a sharp rise.
110 Espresso Coffee
Temperature, storage time and, most important, moisture are the
critical parameters to control for the prevention of spoilage. About 1% of
the moisture is present as bound water, a further 4% is weakly bound and
the remainder is present as free water. Water availability is measured in
terms of activity (aw), i.e., the ratio between the partial pressure of water
in the bean (P) and the partial pressure of pure water (P0) at the same
temperature (aw.P/P0), and ranges between 0 and 1. Figure 3.17 shows
The raw bean 111
Figure 3.16 Isotherm of moisture content
Figure 3.17 Relationship between water activity and main causes of degradation
the dependence of lipid oxidation (1), non-enzymatic browning (2),
hydrolytic reactions (3), ascorbic acid loss (4), enzymatic activity (5) and
the growth of moulds (6), yeasts (7) and bacteria (8) on aw (Labuza et al.,
1972). The most serious damage to green coffee derives from mould
contamination, the most common moulds belonging to the Penicillium and
Aspergillus species. The principal fungal species found in the storage of
green coffee are: Penicillium spp., Aspergillus ochraceus, A. niger, A.
carbonarius, A. fumigatus, A. tamarii, A. flavus, Wallemia sebi and Eurotium
spp.
Humidity, temperature and storage time are dependent variables. The
shorter the storage time, the better tolerated are extreme conditions of
humidity and temperature. Such interdependence can be well described
by the isochronous storage diagram (Figure 3.18). For instance, for a
storage time of 100 days, the relative humidity must be below 75% and
temperature below 30
C.
Relatively minor variations in storage conditions may lead to marked
differences: for example, at 35
C, coffee changes colour after just one
month even when it is stored under the best possible conditions, whereas
at 30
C it is still unaltered after four months. After one year, quality is
generally reduced, and the beans change colour, becoming white–yellow
112 Espresso Coffee
Figure 3.18 Isochronous storage diagram
and fluorescent, their aroma becomes woody and stale while sourness
increases. Such decay is matched by a chemical degradation of the free
amino acids and sugars, and by oxidation of the lipids; it has also been
linked to attack by Streptococci and oxidation (Multon et al., 1973).
Laboratory tests have shown that coffee placed under inert gas
maintains colour and cup quality for years.
Apart from keeping humidity as low as possible in the storage room,
another way to prevent the microbial contamination that damages coffee
could be irradiation with gamma rays before the beginning of storage; this
method, however, has not been implemented, as it may lead to rejection
by consumers. Tests carried out showed that doses ranging between 7.5
and 10 kGy can be used to sterilize coffee in parchment and effectively
reduce moulds, and at the same time delay, even eliminate recontamination,
without producing structural or organoleptic alterations
(Lopez Garay et al., 1987).
Coffees of different qualities must be kept apart in the warehouse;
otherwise defective coffees may rapidly contaminate the good ones. For
example, after three and a half months good coffees stored near stinker
beans pick up the defect. ‘Rio’ coffee (see 3.7.2.6) can also contaminate
sound coffees.
Green coffee at 11–12% moisture is stored in jute or sisal bags (Figure
3.19). These fibres are treated with paraffin and emulsifying agents to
soften and facilitate spinning and weaving into fabrics. The batching oil
content of the bag is normally between 2 and 4%, and, if unclean oil is
used, it can impart a taint to the coffee beans, with loss of cup quality (Grob
et al., 1991a, 1991b). The replacement of mineral oil by neutral vegetable
oils is now common, and the risk of contamination has disappeared.
The density of green beans is about 0.7 gl–1, and a lot of one hundred
60.5 kg bags requires 8.3m3 of warehouse space (Rigitano et al., 1963).
The market now demands current crop coffees and no longer accepts
old crop coffees for blending. There exists, however, a small market
The raw bean 113
Figure 3.19 Storage
segment that purchases such old crop coffees, where price, not quality, is
the most relevant factor. Depending on climatic conditions and warehousing,
the washed coffees and the pulped natural coffees must be
commercialized at most 6 months after processing, because the loss in
quality is greater than that of natural coffees.
3.6.2 Transport
In recent years, with the delivery of merchandise to consuming countries
at the moment of consumption (just-in-time), the operations of transport
in the ports and container terminals have become very efficient and
shorter in duration. The logistical and transportation conditions vary in
the diverse producing origins, from highly developed to rudimentary.
Most of the producing countries have their coffee transported in 20 feet
containers, usually filled with 60.5 kg bags, sometimes in bulk, rarely in
big bags of 1200 kg, while shipment of loose bags in the hold of ships is
disappearing due to the high risk of damage to the load.
For shipment, coffee is normally transferred to new bags, with a
capacity of 60.5 kg, as used in Brazil (in most countries the bag is 60 kg,
while in Colombia it weighs 70 kg). Arabica coffee is still traded
according to the number of defects (see 3.7) and to cup quality.
Robusta coffee is rarely cup tested by the trade, but the exported quality
must contain few defects and foreign materials.
Up to 320 bags can be shipped in 20 feet containers. After arrival
at destination, they are stored at roasting installations for a period of 1 to 3
months. Coffee can be transported in bulk in 20 feet containers that can
hold the equivalent of 360 bags of 60.5 kg. The use of this system is only
possible when the industry at the receiving end has adequate installations.
High-quality (speciality or gourmet) coffee should always be transported
in 60.5 kg bags in containers, for enhanced quality control.
Containers must be well ventilated and equipped with an appropriate
ventilation system, drainage and moisture-collecting system, free from
smells or strange odours.
The critical parameters that need to be taken into consideration for
transport are once again the relative air humidity, the humidity of the
coffee at the time of loading, the changes in the environmental
conditions from the port of loading to the port of discharge and the
duration of the journey. Controls over the loading and unloading
operations are very important. Below follow the most important
conditions in the transportation.
114 Espresso Coffee
n During container shipment, relative humidity should not exceed
70%; in such conditions the coffee travels safely and the risk of
damage is reduced; in case of high relative humidity, the container
must be loaded with a smaller load (maximum of 285 bags) and/or
stowed in well-ventilated areas.
n The shorter the duration of the journey, the lower the risk of
damage.
n The more extreme the climatic conditions at loading and unloading
(for example: usual tropical conditions when loading and European
cold season when unloading), the shorter the duration of the journey
should be (Jouanjan, 1980).
Transport in bulk offers several advantages, such as:
n Enabling the loading of the container with 10–15% more coffee (up
to 360 bags).
n The moisture in the mass of coffee is more constant.
n There is a saving of US$1 to US$1.50 of the cost of each bag,
depending on the origin of the shipment.
n Loading and unloading are simplified.
n General costs (insurance, freight, port costs and others) are reduced.
When a container is exposed to the sun in the daytime, water evaporates
and then condenses at night in cold spots, giving rise to pockets of moist
coffee, which can go mouldy and contaminate the remaining portion of
the lot during unloading. This problem can be avoided by completely
lining the container with special paper that protects the coffee mass in
bulk. The same problem can occur with 60.5 kg bags, but in this case the
outer surface of the contaminated bags would present blackish or whitish
spots that can easily be recognized and separated. It is important to
observe the quality of the container. Before loading, the coffee must be
checked, weighed and a phytosanitary certificate issued. This procedure is
very important because phytosanitary controls in consuming countries are
becoming stricter and more demanding. Some producing countries, such
as Brazil, are adopting the use of baits in the interior of the container to
control insects.
Once unloaded, the coffee is again weighed, the figures compared with
those indicated on the certificate of loading and a phytosanitary
certificate is issued. Any resulting difference should be within the limits
of the humidity of coffee respiration. Moreover, samples are taken to
verify if the lot complies with local import regulations.
Figure 3.20 gives the flow-chart of coffee processing.
The raw bean 115
3.7 DEFECTS
R. Teixeira and A.A. Teixeira
Espresso coffee is rich in lipids, which are excellent aroma carriers,
making espresso an optimal way to increase the aroma of the cup.
Unfortunately, taints coming from defects already present in the green
bean can also be enhanced, so that defects may constitute a major
problem for the quality of an espresso brew, since all aromas, both good
116 Espresso Coffee
Figure 3.20 Flow-chart of coffee processing
and poor, are obvious before drinking, by smelling the foam. Indeed, it
takes about 50 coffee beans to make a cup of espresso coffee, therefore, a
low percentage of defects of the order of 1–2%, common in fair quality
lots on the market, implies the presence of at least one defective bean
among those used to prepare the cup. Furthermore, volatile substances
contributing to the defect are often perceived at extremely low levels,
masking the pleasant aroma of the sound beans (Table 3.3). For this
reason, a brew, still acceptable in spite of defects when prepared by
another technique, can become undrinkable with the espresso system.
The dimension of the problem has led the coffee scientific community
to devote much effort to the characterization of defects from a chemical,
physical and morphological point of view, to the definition of their causes
and to the development of suitable techniques that can prevent the
appearance of defects. This section will provide a detailed characterization
of defects and then discuss means of eliminating them. In addition to
the defects that lead to loss of organoleptic quality of an espresso, the
physical and morphological defects that hamper good roasting and proper
cup preparation will also be discussed.
3.7.1 Characterization of defects
Coffee beans are considered normal when they produce a drink capable of
satisfying the consumer. Common market usage is that coffee beans
without defects make a sound lot. A defect is anything that diverges from
a normal bean inside the lot and that can be produced in the field or
during the harvest, processing, transport and storage. In commercial use,
it is defined as the number of defective beans and foreign matter present
in specific green coffee samples, normally in 300–500 g, or 1000 beans.
Most producing countries have their own criteria for the classification of
coffee, based on the sum of specific defects, each of which is evaluated in
The raw bean 117
Table 3.3 Threshold level in water of the
most odorous chemicals present in defective
beans
Odorant Odour threshold in water (lgl 1)
2,4,6-trichloroanisole 0.001
2-methylisoborneol 0.0025
geosmin 0.005
Source: R. Viani, 2003
accordance with established commercial criteria. In an effort to cover all
coffee types in a general norm, the International Organization for
Standardization (ISO) has produced in the past Standard 10470-1993 –
Green Coffee – Defect Reference Chart (ISO, 1993), which met with
little utilization. This standard is currently in the final stage of a revision
process that aims at a simple and more precise enforcement. Along with a
reorganized chart of defects, now also shown in photographs, the main
new concepts introduced will be:
n Defects are grouped in five classes, namely
n non-coffee defects (foreign matter) (see 3.7.1.1 and Table 3.4);
n defects of non-bean origin (e.g., husks/hulls) (see 3.7.1.2 and
Table 3.5);
n irregularly formed beans (e.g., ears/shells/broken beans/nipped
beans) (see 3.7.1.3 and Table 3.6);
n beans with an irregular visual appearance, with risk of influencing
cup taste) (see 3.7.1.4 and Table 3.7);
118 Espresso Coffee
Table 3.4 Defects associated with foreign matter
Name
Characteristics/
definition Causes
Brew flavour/
roasting Origin
Stone* Stone of any diameter
found in a green
coffee lot
Inadequate
separation/cleaning
Effect mainly
economic
H/P
Stick* Twig of any diameter
found in a green
coffee lot
Inadequate
separation/cleaning
Non-specific
downgrading
H/P
Clod* Granulated lump of
soil particles
Inadequate
separation/cleaning
Effect mainly
economic
H/P
*Found in WPA/WPR/DPA/DPR.
Abbreviations: WPA, wet-processed arabica; WPR, wet-processed robusta; DPA, dry-processed
arabica; DPR, Dry-processed robusta.
H, Harvest-damaged beans: inadequate crop management (picking cherries before or after
maturation, cherries from the ground, etc.)
P, Process-damaged beans: defects due to imperfect processing operations (pulping, washing,
drying, cleaning, hulling, etc.).
The raw bean 119
Table 3.5 Defects associated with fruit parts
Name
Characteristics/
definition Causes
Brew
flavour/
roasting Origin
Bean in
parchment*
Coffee bean entirely or
partially enclosed in its
parchment (endocarp)
Faulty hulling or husking
of the dry parchment
Non-specific
downgrading
P
Piece of
parchment*
Fragment of dried
endocarp (parchment)
Inadequate separation of
the parchment after
hulling or husking
Non-specific
downgrading
P
Dried
cherry
y
(pod)
Dried cherry of the
coffee tree, comprising
its external envelopes
and one or more beans
Incorrect dehusking,
allowing whole dried
cherries through, not
removed subsequently
Foul flavour P
Husk
fragment
y
Fragment of the dried
external envelope
(pericarp)
Inadequate separation
after dehusking
Foul flavour P
*Found in WPA/WPR/DPA/DPR.
y
Found in DPA/DPR.
P, Process-damaged beans: defects due to imperfect processing operations (pulping, washing,
drying, cleaning, hulling, etc.).
n off-tastes (defects identifiable on cup testing only) (see 3.7.1.5
and Table 3.8).
n Quantifying defects by weight, to allow for exact calculations to be
applied to any contract of purchase of green coffee that may be
negotiated between provider and client.
Defects can modify cup quality leading to unpleasant flavours, the loss of
product due to the presence of foreign matter or the burning of fragments or
small beans in the roaster. Some defects, such as mouldiness, which can
produce toxins, can also affect consumers’ health. Many defects may appear
equally in wet-processed arabica (WPA), wet-processed robusta (WPR),
dry-processed arabica (DPA) and dry-processed robusta (DPR). Other
120 Espresso Coffee
Table 3.6 Defects associated with regularity/integrity of bean shape
Name Characteristics/definition Causes
Brew flavour/
roasting Origin
Shell or ear*;
shell core*
Part of bean originated from the elephant
bean: shell or ear – external part of the
elephant bean presenting a cavity; shell core
– internal part of the elephant bean
Splitting of the elephant bean (growth
defect) generally through handling (dehulling
or dehusking), producing the separation of
the inner and outer parts
Uneven roast;
slight bitterness;
less acidity
F/P
Bean fragment* Fragment of a coffee bean of volume
less than half a bean
Formed mainly during faulty dehulling,
dehusking or pulping operations. Over-drying
making beans easily breakable during
handling
Uneven roast;
bitterness; less
acidity
P
Broken bean* Fragment of a coffee bean of volume
equal to or greater than half a bean
Formed mainly during faulty dehulling,
dehusking or pulping operations.
Over-drying making beans easily breakable
during handling
Uneven roast;
bitterness; less
acidity
P
Insect-damaged bean* Coffee bean damaged internally
or externally by insect attack
Attack on cherries by Hypothenemus
haempei (coffee berry borer)
Increased
bitterness; less
acidity
F
The raw bean 121
Insect-infested
bean*
Coffee bean containing one or more live or
dead insects at any stage of development
Bean attacked by storage pests, generally
Araecerus fasciculatus insect
Loss of flavour;
slight bitterness
S
Pulper-nippedy
pulper-cut beany
Wet-processed coffee bean cut or bruised
during pulping, often with brown or blackish
marks
Faulty adjustment of pulping machine or
feeding with under-ripe cherries or
malformed beans
Non-specific
downgrading to
slightly putrid or
stinker
P
Crushed bean* Crushed beans often partly split and faded
with centre-cut largely open
Treading on the beans during drying.
Hulling or polishing of soft under-dried
beans
Non-specific
downgrading to
rancid, slightly
fermented
P
*Found in WPA/WPR/DPA/DPR.
yFound in WPA/WPR.
Abbreviations: WPA, wet-processed arabica; WPR, wet-processed robusta; DPA, dry-processed arabica; DPR, Dry-processed robusta.
F, Field-damaged bean: defects originating in the field, the coffee tree (genetic problems), the environment (climate, soil, water and nutrient stress), attacks by
pests and diseases.
P, Process-damaged beans: defects due to imperfect processing operations (pulping, washing, drying, cleaning, hulling, etc.).
S, Storage damaged beans: defects due to deficient storage (faulty storage practices and storage pests).
122 Espresso Coffee
Table 3.7 Defects associated with beans irregular in visual appearance (colour and surface texture)
Name Characteristics/definition Causes Brew flavour/roasting Origin
Black bean* Coffee bean whose interior is
partly (partly black bean) or
totally black (endosperm).
Bean from over-ripe cherry, fallen on the ground.
Due to Colletotrichum coffeeanum attack, other
fungi species and pests. Carbohydrate deficiency
bean due to poor cultural practices. Mature cherries
subjected to over-fermentation by moulds/yeasts and
subsequent drying
Slow to roast; beans tend
to be yellowish; loss of
acidity; harsh; ashy flavour
F/P
Black-green bean* Unripe coffee bean often with
a wrinkled surface, with darkgreen
(dark-green bean) to
black-green silverskin colour
High temperatures affecting immature beans,
causing chemical transformation (like fermentation)
with the silverskin becoming dark or black-green
Slow roast; rotten fish
flavour
H/P
Light-green
immature bean*
Unripe coffee bean often with
a wrinkled surface. The bean
has a greenish or metallicgreen
silverskin colour. Cell
walls and internal structure are
not fully developed
Beans from cherries picked before ripening Slow roast;
bitterness; less acidity;
astringent; metallic;
sometimes fermented
H
Sour (ardido)
bean*
Coffee bean deteriorated by
excess fermentation, with a
range of colours: light to dark
brown-reddish, dark brown or
yellowish green internally
(endosperm). In some cases it
has a waxy appearance or a
brown silverskin colour
Bean from immature, mature or overripe cherries
that have been in adverse conditions, becoming
fermented by bacteria or xerophilic moulds, with
embryo death. More frequent in fruits fallen to the
ground, can also appear in fruits on the tree, and
when there is an excessive time between harvesting
and drying or pulping
Sour; fermented; acetic;
fruity; sulphurous; vinegar
flavour
F/H/P
The raw bean 123
Withered bean* Coffee bean wrinkled and light
in weight
Underdeveloped fruit due to genetic and
physiological problems, nutritional lack, drought or
heavily stressed tree
Slight loss of acidity F
Spongy bean* Coffee bean of consistency like
cork that may be verified by
pressure of fingernail. Whitish
in colour
Moisture absorption during storage or transportation
leading to deterioration by enzymatic activity
Roasts rapidly; lack of
acidity; woody
S
Whitish bean* Coffee bean with whitish
colour with tissue of normal
density
Discoloration of surface due to bacteria of the genus
Coccus, during storage or transportation. Associated
with old crop. Faulty drying of the bean
Woody; stale; bitterness
taste
P/S
Mouldy bean* Coffee bean with mould
growth and evidence of attack
by mould visible to the naked
eye
Temperature and humidity conditions favourable to
mould growth
Musty flavour P/S
Frost-damaged
bean*
Coffee bean with spotted
silverskin like a ‘spotted quail
egg’
Frost damaged coffee bean Rancid flavour F
*Found in WPA/WPR/DPA/DPR.
Abbreviations: WPA, wet-processed arabica; WPR, wet-processed robusta; DPA, dry-processed arabica; DPR, Dry-processed robusta.
F, Field damaged bean: defects originating in the field, the coffee tree (genetic problems), the environment (climate, soil, water and nutrient stress), attacks by
pests and diseases.
H, Harvest-damaged beans: inadequate crop management (picking cherries before or after maturation, cherries from the ground, etc.).
P, Process-damaged beans: defects due to imperfect processing operations (pulping, washing, drying, cleaning, hulling, etc.).
S, Storage damaged beans: defects due to deficient storage (faulty storage practices and storage pests).
124 Espresso Coffee
Table 3.8 Defects associated with beans irregular in cup taste after proper roasting and brewing (off-taste coffee)
Name Characteristics/definition Causes
Brew
flavour/
roasting Origin
Foul/
dirty
bean*
The bean presents a normal appearance.
Unpleasant foul flavour is detected in
the cup like earthy, woody, musty or
jute-bag
Unfavourable conditions of temperature, humidity and
time in processing, storage or transportation. Use of
bad quality jute-bag
Musty; earthy,
woody, jutebag
flavours
P/S
Stinker
bean*
The bean presents usually a normal appearance. A
very unpleasant flavour is detected in the cup like
stinker, over-fermented or rotten. Stinker smell when
cut or scratched
Delay in pulping; too long period of fermentation;
wild fermentation in beans trapped in the pulping
machines or tanks. Abrasion of beans during pulping,
losing the superficial protective layer, leaving them
susceptible to microorganism attack. Contamination
with recycled polluted water
Stinker; overfermented,
rotten flavour
F/P/S
Rioy
bean*
The bean presents a normal appearance. Medicinal
smell when cut or scratched.
Overripe cherry left on branch, possibly contaminated
by microorganisms. The drying patio soil is heavily
contaminated by microorganisms and/or TCA
(trichloroanisole)
Medicinal,
phenolic,
flavour of
iodine
F/P/S
*Found in WPA/WPR/DPA/DPR.
Abbreviations: WPA, wet-processed arabica; WPR, wet-processed robusta; DPA, dry-processed arabica; DPR, dry-processed robusta.
F, Field-damaged bean: defects originating in the field, the coffee tree (genetic problems), the environment (climate, soil, water and nutrient stress), attacks by
pests and diseases.
H, Harvest-damaged beans: inadequate crop management (picking cherries before or after maturation, cherries from the ground, etc.).
P, Process-damaged beans: defects due to imperfect processing operations (pulping, washing, drying, cleaning, hulling, etc.).
S, Storage-damaged beans: defects due to deficient storage (faulty storage practices and storage pests).
defects are characteristic of a certain type of processing. The different types
of defects can be divided into categories, which are discussed below.
3.7.1.1 Defects associated with foreign matter
These include defects such as stones, sticks, clods (agglomerations of
earth), metal and foreign matter. They must be removed at an appropriate
stage, for example during the cleaning of green coffee, by sieving,
classifying or by removal of metals.
3.7.1.2 Defects associated with coffee fruit parts
These include defects such as beans in parchment, pieces of parchment,
husk fragments and dried cherries (pods). In general, these are removed by
sieving or by air classifying, leading to a loss of physical volume.
3.7.1.3 Defects associated with regularity/integrity of bean
shape
These include defects like malformed beans (shell/ear and shell core/body
of the elephant), bean fragments, broken beans, insect-damaged beans,
pulper-nipped/pulper-cut beans and crushed beans. They are in general
removed by densimetric sorting and in some cases by optical sorting.
3.7.1.4 Defects associated with beans irregular in visual
appearance (colour and surface texture)
These include defects like black beans, dark- and black-green immature
beans, light-green immature beans, sour beans etc. Some of these can be
removed by hand or by optical sorting techniques.
3.7.1.5 Defects associated with beans irregular in cup taste
after proper roasting and brewing (off-taste coffee)
Defects of sensory concern, to be identified after sample roasting and cup
testing, also entailing a further risk of contamination of other beans. These
include defects like ‘stinker’ beans, foul/dirty beans and ‘rio’ beans. Offtastes
are hard to remove by sorting. They can be identified after cupping a
sample of roast and ground coffee, following proper roasting and brewing.
The raw bean 125
3.7.2 Definition of defects
Many defects have been studied from the physical, chemical, microbiological
and flavour quality points of view.
3.7.2.1 Light-green immature beans
The presence of immature beans, characterized by a light-metallic-green
silverskin colour, is due to the harvest of unripe cherries. The flavour of
the defect, described as an increased bitterness in both arabica and robusta
beans (ISO, 1993), is probably better characterized as metallic and
astringent. The evolution of the phenolic constituents during ripening
(Clifford et al., 1987; Guyot et al., 1988a, 1988b) may give some
indication on the nature of this defect, which has been explained as being
due to an excess of dichlorogenic acids with respect to monochlorogenic
acids, particularly important before the last five weeks of ripening
(Clifford and Ohiokpehai, 1983). This may not be the full explanation
since recent studies have shown that 5-caffeoylquinic acid, the major
component of the family present in coffee, has an astringent taste of its
own (Naish et al., 1993) and so the question remains open. The
microscopic examination of immature beans confirms their incomplete
ripeness (Dentan and Illy, 1985).
The green grassy flavour perceived in some beans has been shown to be
due to incompletely mature beans, which have already reached the size of
ripe beans (Dentan, 1991). These beans appear to be normal, but are still
unripe, even though they are riper than immature beans.
A concentration of light-green immature beans greater than 15%
causes noticeable harm to the brew (Teixeira et al., 1969).
Unripe beans ferment very easily when exposed to temperatures around
40
C, producing dark-green immature beans or, worse, black-green
immature beans (Teixeira et al., 1982). It has been observed that lightgreen
immature beans are sometimes also fermented, which can be
perceived when the beans are scratched or during cupping.
Light-green immature beans are characterized by:
n light-metallic-green silverskin colour;
n metallic and astringent taste, reminiscent of the taste of some amino
acids, perceivable when the proportion of unripe beans exceeds 1% in
espresso;
n vivid silverskin, adhering well to the bean surface;
n cell-walls much thinner than in ripe beans, with a lower cellulose
content;
n absence of saccharose, a constituent of sound ripe beans;
126 Espresso Coffee
n presence of arabinose, absent in ripe beans;
n complete absence of serotonin, formed from tryptophane during
ripening;
n greater than average content of chlorogenic acids (up to 17% in
unripe beans as opposed to 7% in ripe ones);
n reduced lipid content;
n reduced level of oleic acid and increased level of linoleic acid in total
lipids;
n oleic/stearic acids ratio less than 1;
n rather marked minimum at 672 nm in the reflectance spectrum
attributable to the presence of chlorophyll.
3.7.2.2 Black-green/dark-green immature beans
Black-green and dark-green immature beans are formed when unripe
beans (light-green immature beans) are dried at or exposed to an
excessive temperature, above 40
C (Teixeira et al., 1982). They have
black to dark-green silverskin colour and are characterized by astringency
and a taste reminiscent of rotten fish, attributed to high levels of cis-4-
The raw bean 127
Figure 3.21 Arabica beans
Above: Optical microscope images: (left) immature – many cells are still empty,
the cell-walls are thin, the lipids (red) surround proteins (yellow); (right) mature –
the cells are full; the lipids (red) have migrated towards the walls
Below: SEM images: (left) immature; (right) mature
heptenal (Full et al., 1999), which makes this a very serious defect. This
compound is a product of the auto-oxidation of linoleic acid (Grosch,
1998). An experiment was made of removing the silverskin from beans
with these defects and they were classified as sour beans (Teixeira et al.,
1971).
Black-green immature beans can be distinguished under the microscope
(Dentan, 1989) by:
n dark to black-green silverskin colour and wrinkled appearance;
n no microbial attack is apparent, and the content of the cell is
congealed into an undistinguished mass;
n total degradation of proteins in the cells of the surface layers, smaller
in the central area;
n absence of lipids in the peripheral cells, owing to lipase activity;
n very low reflectance; the minimum present in unripe beans is almost
absent;
n density well below that of a healthy bean, which facilitates
densimetric separation.
3.7.2.3 Black beans
Coffee beans whose interior (endosperm) is partially (partially black
beans) or totally black. These beans are more frequently found in overripe
cherries on the tree or fallen on the ground, but can also be
encountered at other phases of maturation (Carvalho et al., 1972) or
during processing. More than 10% of black beans cause noticeable harm
to the brew (Gomes et al., 1967). They have a harsh and ashy flavour.
Black beans can be distinguished by microscopy (Dentan, 1989) by the
following characteristics:
n black beans, where the interior of the bean is also more or less
completely black depending on the severity of the attack, are beans
having undergone a yeast fermentation starting at the epidermis; the
surface of the bean is covered with minute holes surrounded by
mineral micro crystals, left after enzymatic degradation of cellulose.
The more serious the damage, the blacker is the interior of the bean.
When the fermentation has progressed long enough, the black beans
are just a light empty shell, which can be separated densimetrically
from normal beans;
n particularly in Africa, black beans, characterized by more or less
homogeneously black surface, are due to fungal attack by
128 Espresso Coffee
Colletotrichum kahawae (coffeanum); in this case the beans are often
infected by Aspergilli, Penicillia or other moulds;
n easy peeling off of the silverskin.
3.7.2.4 Sour beans
Sour (ardido) beans are deteriorated by excess fermentation, with a sour
taste. They have a range of colours: light to dark reddish-brown, dark
brown or yellowish-green internally (endosperm) and sometimes have a
waxy appearance, typical of dead beans. This defect has different causes. A
study was made removing the silverskin from black-green immature beans
and they were classified as sour beans (Teixeira et al., 1971). Further
studies indicated that sour beans constitute one phase of coffee deterioration,
which ends by reaching a black colour (Gomes et al., 1967; Carvalho
et al., 1972). Microscopic examination indicates that they are ripe beans
probably killed by overheating during processing, or by infection with
xerophilic moulds, such as Aspergilli and Eurotia (Dentan, 1989, 1991).
They are characterized by:
n yellow-green to dark reddish-brown colour, sometimes with waxy
appearance;
n sour and fermented smell and taste.
3.7.2.5 Stinker beans
Stinkers are over-fermented beans, usually with normal appearance but a
rotten smell and flavour. Morphological analyses show that some stinkers
have lost the embryo. Microscopic examination often reveals multiple
contaminations with bacteria and moulds (Dentan, 1991). In one specific
case only one bacterium, Bacillus brevis, was identified. The defect has
been associated with the presence of excessive amounts of dimethylsulphide,
>0.60 mg/kg instead of <0.30 mg/kg found in healthy arabica
coffee, and of dimethyldisulphide, >0.40 instead of 0.15 (Guyot et al.,
1991).
Short chain aliphatic esters and acids, formed during lactic fermentation,
have also been associated with this defect, and a count of lactic acid
bacteria above 106/g is often associated with a fermented, rotten flavour.
A useful indicator of the presence of stinker beans is the development
of red colour by litmus paper suspended above a sample for an hour due to
the presence of acetic acid. This reaction is specific to stinker beans and
does not occur with other defects or with healthy beans, the faster the
reaction, the more serious the defect.
The raw bean 129
Under examination using ultraviolet light, stinker beans show a white
or white-blue fluorescence, probably associated with larger quantities of
free caffeic acid compared with healthy beans. Nevertheless, other defects
and old crop beans are also fluorescent, making it difficult to sort stinker
beans solely by this method.
Stinker beans can contaminate a large batch, even when present at
very low levels, turning healthy beans into stinkers at a lower level of
intensity (Gibson and Butty, 1975). Beans that have become stinkers only
by contact are not fluorescent. A further indicator is headspace
gaschromatography, which shows peaks that do not appear in healthy
beans.
This defect has been attributed to alcoholic fermentation by
Saccaromyces cerevisae.
Possible causes of stinkers are the following:
n too long fermentation;
n wild fermentation of beans trapped in the interstices of pulping
machines, fermentation tanks, etc.;
n wild fermentation with recycled fermentation water, contamination
with polluted water;
n abrasion during pulping, with loss of the superficial protective layer,
which renders the bean easily susceptible to attack by microorganisms;
n contamination of healthy beans by stinkers;
n delay in pulping;
n over-heating during processing: at 55
C, stinkers develop in 4 hours,
whereas at 30
C 4 days are necessary; the most critical processing
steps are the storage of cherries before processing, fermentation, and
the white stage of drying;
n exposure to over-heating in the sun of the heaps of coffee covered by
sheets during drying;
n presence of Ceratitis capitata (as larva) in the cherries.
3.7.2.6 Rioy beans
These beans have a flavour described as medicinal and iodine-like. Rioy
beans were first detected in Brazilian coffee grown in the Rio de Janeiro
area and subsequently in other areas and countries. The substance
responsible for this important defect is 2,4,6-trichloroanisole (TCA),
which in coffee has an odour threshold of 8/ng/l and a flavour threshold of
1–2/ng/l. By direct olfaction it has been described as dusty, musty, earthy,
woody, corky, cereal, iodine-like, phenolic and an oral/retronasal
130 Espresso Coffee
perception of bitterness, burned, rubbery, phenolic, acrid, pungent,
earthy, stale and medicinal (Spadone et al., 1990). Rio-tainted beans
are heavily infested with moulds (Aspergilli, Fusaria, Penicillia, Rhizopus),
and bacteria (Lactobacilli, Streptococci), accompanied by a degradation of
the cell structure (Dentan, 1987). The levels of TCA found in Rioy coffee
are usually well above the threshold. 2,4,6-trichlorophenol (TCP) appears
to be the direct precursor of TCA in coffee, probably by microbial
degradation. The origin of TCP itself is still unclear: some authors
maintain that it derives from the chlorophenols used as fungicides, others
that it is biosynthesized by a yet unknown microorganism. Of the two
hypotheses, the latter is more likely, for only one TCP isomer has been
found in Rioy beans, whereas synthetic products should contain all the
possible isomers (Spadone et al., 1990). Furthermore, moulds with
chloroperoxidase activity are known. Aspergillus fumigatus, found in
Rioy beans but absent in sound beans, could be the source of the defect;
yet, studies carried out on wine, whose corky smell is also due to the
presence of TCA, have shown that Penicillia can synthesize TCA in the
presence of chlorine and methionine. Rioy beans are characterized
(Amorim et al., 1977, Dentan, 1987) as follows (Table 3.9):
n external appearance similar to that of a normal bean;
n full or excessive ripeness; the defect is never present in unripe beans;
n contamination with moulds (Aspergilli, Fusaria, Penicillia, Rhyzopus)
and bacteria (Lactobacilli, Streptococci) degrading the cellular structure;
n adjacent cells no longer attached to one another, and both the
volume and the thickness of the cell-walls well below those of
healthy beans;
n density lower than that of normal beans;
n different microbial populations present inside (Fusaria, Pseudomonas
and yeasts) and outside (Aspergillus versicolor, Wallemia sebi);
n low polyphenoloxidase activity;
The raw bean 131
Table 3.9 Morphological differences between sound and
rioy beans
Sample Bean weight (mg) Density Cell-wall thickness (lm)
Strictly soft 73.03 1.085 6.2
Soft 57.69 1.014 5.6
Rioy 58.21 0.967 5.0
Strong rio flavour 49.24 0.761 4.3
n lower than average content of hydrolysable phenols;
n lower than average content of hydrolysable proteins;
n absence of high and medium molecular weight (150 000 and 64 000
Daltons) proteins, usually present in sound coffees;
n level of low molecular weight proteins higher than in sound coffees;
n presence of some low molecular weight (9000 Daltons) proteins that
usually do not appear in sound coffees;
n UV-VIS reflectance spectrum identical to that of a normal bean.
Figure 3.22(a) shows that the cell-walls have been opened by moulds
forming channels (the white spots are moulds).
The origin of the defect could be due to several factors:
n cherries that are contaminated by moulds and/or specific bacteria
while still on the plant;
n the drying stage after harvesting is too slow;
n the drying patio soil is heavily contaminated by microorganisms and/
or TCA.
3.7.2.7 Whitish beans
The surface discoloration of whitish beans is due to fermentation by
Streptococcus bacteria that in the most severe cases may also reach the
cells under the epidermis (Dentan, 1991). The attack can occur if storage
132 Espresso Coffee
Figure 3.22 (a) Degraded cell structure of a rioy bean; (b) electron microscope
section of a mould-invaded coffee bean
(a) (b)
is too long or in conditions of excessive humidity. White beans are not
very aromatic and give slightly bitter and woody cups. Under ultraviolet
radiation they show a blue fluorescence probably attributable to
chlorogenic acids and caffeic acid, which makes them difficult to sort
out from stinkers because of the fluorescent backgrounds they produce.
3.7.2.8 Mouldy beans
A mould/yeast level above 105/g is always associated with mustiness in
flavour. Geosmin, identified in a heavily rioy and musty tasting sample of
Portorican coffee, is probably the substance responsible for mouldiness in
beans (Spadone et al., 1990).
These beans have a greyish colour, and under microscopic examination
the epidermis, silverskin and central cut appear covered with moulds,
mostly Aspergilli (A. tamarii, A. niger, A. ochraceus and A. Flavus,
essentially). Pseudomonas bacteria living in symbiosis are also visible.
Although the cell-walls are intact, only the peripheral lipids remain inside
(Dentan, 1991). In Figure 3.22b hyphes formed by Aspergillus moulds
are clearly visible in the central split. The presence of moulds or yeasts
(more than 105 units per gram) always produces a putrid smell due to the
presence of geosmin. The cause of contamination can be either inadequate
drying or storage in overly humid and poorly ventilated conditions.
3.7.2.9 Earthy beans
The presence of 2-methylisoborneol, a secondary metabolite of
Actinomycetes, Cyanobacteria and moulds, has been associated with the
earthy flavour of robusta coffee, with a threshold level in water of
1–100 ng/l, and a flavour described as earthy, musty, robusta-like.
The levels present in robusta coffee are at least three times as high as in
arabica (Vitzthum et al., 1990). These data indicate that robusta taste
results, at least partially, from contamination by microorganisms rather
than from specific aroma components.
Defective beans may have colours varying from clear brown to almost
black; the silverskin, but neither the epidermis nor the central cut, is always
infected by various microorganisms, bacteria, yeasts and moulds, Fusarium,
Geotrichum, Eurotium, are often the major populations present, as well as
different Aspergilli (A. flavus, A. fumigatus, A. niger). Although the cell
walls are intact, only the peripheral lipids remain inside (Dentan, 1991).
A patent has been registered for a method of eliminating 2-methylisoborneol
from robusta. By this method, coffee is treated with saturated
The raw bean 133
vapour at 138
C, at a pressure of 3.8–3.9 bar, for 75–90 minutes
(Vitzthum et al., 1990).
3.7.2.10 Peasy beans
This defect, encountered only in Central African arabica coffees, has been
identified as 2-isopropyl-3-methoxypyrazine with a perception threshold of
about 300 ppb in air and 0.1 ppb in water (Becker et al., 1987). The defect
is due to a contamination of the cherry by a bacterium of the
entherobacteriaceae, probably transmitted by Antestiopsis orbitalis ghesquierei
feeding on the cherry (Bouyjou et al., 1993). In the most seriously defective
beans, the concentration of 2-isopropyl-methoxypyrazine can reach 2500
ppb as against 70–90 ppb in non-defective beans (a ratio of 35:1).
Peasy beans have a smell strongly reminiscent of fresh green peas.
Morphologically, peasy beans appear normal, though they show some
fluorescence.
3.8 CLASSIFICATION: PHYSICAL AND
SENSORIAL ANALYSIS
R. Teixeira, A.A. Teixeira and C.H.J. Brando
The classification of coffee, which includes physical and sensorial analysis,
is a very important phase in the commercialization process. Price
quotations, as well as national regulations governing importation into
consuming countries, are established on the basis of such classifications.
In order to discuss the quality and production of fine coffees, there is a
fundamental need for technicians, producers and purchasers to be aware
of the factors that enter into the evaluation of the final item, and to be
able to judge its merits and demerits. Unfortunately, the existence of a
variety of classification systems means that each country adopts a different
classification, requiring equivalency norms for use at the international
level, such as ISO 10470-1993 – Green Coffee – Defect Reference Chart
(ISO, 1993) and ISO 4149-1980 – Green Coffee – Olfactory and Visual
Examination and Determination of Foreign Matter and Defects (ISO,
1980), which are currently being revised.
In Brazil, it is customary to classify by type or defects as well as by cup
quality. In Colombia, coffee is classified by the characteristics of the
plantation, with regard to altitude, by bean size and by region of origin.
Countries of Central America (El Salvador, Honduras, Mexico,
Nicaragua, Guatemala, Costa Rica and Panama) have altitude as the
main criterion, but also take into account appearance, bean size and cup
134 Espresso Coffee
quality. In Africa, the classification varies according to the area of
production. Angola, Cameroon and the Ivory Coast have adopted systems
similar to that of Brazil, which uses defects to establish types. In Kenya
and Tanzania, coffee is classified by ranking in classes, in accordance with
bean size and cup quality, in some of which defects are permissible.
Indonesia classifies its coffees in accordance with the species, method of
preparation, origin and number of defects (Jobin, 1982).
Overall, classification is based on the evaluation of all of the
parameters related to the coffee bean and each one of them must be
examined, separately, by physical methods and by sensory analysis
(Table 3.10).
3.8.1 Classification by species and varieties
Coffee is classified by species and variety of origin. Only two easily
distinguished species are important in economic terms: arabica and robusta.
Arabica fetches a higher price in the commercial market, is more delicate,
demands greater care in its culture, and possesses superior organoleptic
characteristics in comparison with robusta, which is more resistant, has a
lower cost of production, but results in an inferior cup quality.
3.8.2 Classification by screen (size and shape)
Different producing countries use different classifications of bean size. In
Brazil, beans are classified according to size and shape. With regard to
shape, the coffees receive the denomination of chatos or flats (long shape),
and peaberries (round shape), also called mocas or caracoles. The size or
The raw bean 135
Table 3.10 Parameters used in commercial classification
Parameters Description
Species and varieties Arabica /robusta
Classification by screen size Size and shape of the beans
Classification by type (number of defects) Defective beans and foreign matter
Density Specific weight of the beans
Appearance and dryness Bean uniformity
Colour Coloration of the beans
Processing Natural (dry), pulped natural and washed
(wet)
Roast Roast regularity, smell, etc.
Cup quality Characteristic aroma and flavours
screen of the beans is measured by the dimension of the holes in the
screen that holds them back, designated by numbers that, divided by 64,
express the size of the holes in fractions of an inch (see 3.5.3 above).
Screens with round holes are used for chatos (varying from 19 to 10) and
ones with elongated holes are used for peaberries (13 to 8). Some
countries use screens with holes measured in millimetres.
Separation by screens is important, since it allows the selection of beans
by size, separating them into groups adequate for uniform roasting. In
Colombia, coffee is classified as ‘Supremo’, ‘Excelso’ and ‘Pasilla’ (the
latter not exportable); in Kenya the designations used are AA, AB, PB, C,
E, TT, T, UG, which indicate size and shape (Jobin, 1982).
3.8.3 Classification by type (Figure 3.23)
Most producing or importing countries have their own classification
system for defects. Commercial classifications are established in accordance
with the number of visual defects in a certain sample. At the
international level, the ‘New York Coffee and Sugar Exchange’
introduced the concept of black bean equivalent. This system uses the
black bean as a standard of measurement for all defects (for example: 1/2
means 2 defects are equivalent to 1 black bean). Other bodies have
introduced similar scales for counting green coffee defects (Table 3.11).
The International Standardisation Organisation has established several
norms regarding olfactory and visual examinations, determination of
defects and foreign matter (ISO, 1980), determination of the proportion of
beans damaged by insects (ISO, 1985), and the defect reference table
(ISO, 1993), that deal with the majority of defects. Currently, several
norms are being revised, including the defect reference table now in the
draft stage, where each defect is defined and characterized according to its
degree of influence (minimum, medium or maximum) in relation to the
136 Espresso Coffee
Figure 3.23 Classification of coffee: (left) size determination; (right) olfactory
examination
The raw bean 137
Table 3.11 Equivalency ratings of green coffee defects in different countries (ISO, 1993)
Defect USA UK France Brazil Tanzania Indonesia Ethiopia
Brazils Centrals Colombian Liffe 1965 1992 1982 1973
Dried cherry (pod) 1 1 1 1 1 1 1 1 2
Black bean 1 1 1 1 1 1 1 1 2
Semi-black bean – 1/2–1/5 1/2–1/5 – 1/2 – 1/2 1/2 1/2
Sour bean 1 1 1 1/2 1 1/2 1/8 – –
Insect-damaged 1/5–1/10 – – 1/2–1/5 1/10 1/2–1/5 1 1/5–1/10 1/2
Immature bean 1/5 – – 1/5 1/5 1/5 1/2 1/5 1/5
Floater (white light) 1/5 1/5 1/5 1/5 1/3 – – – 1/5
Bean in parchment – – – – 1/2 1/2 1/5 1/2 –
Broken bean
More than half 1/5 1/5 1/5 1/5 – 1/5 1/5 1/5 1/10
Less than half 1/5 1/5 1/5 1/5 1/5 1/5 – 1/5 1/5
Shell 1/3 1/3 1/5 1/5 1/5 1/3 1/5 – –
Husk fragments
Large – – – 1/2 1 1 1 1 3
Medium 1/2 1/2 1/3 – – 1/2 1/2 1/2 1
Small – – – 1/5 – 1/3 – 1/5 1
Parchment fragments
Large – – – 1/2 – – – 1/2 –
Medium 1/2 1/2 1/3 – – – – 1/3 1
Small – – – – 1/3 – – 1/10 –
Twigs
Large 2–3 2–3 2–3 5 2 5 5 5 10
Medium 1 1 1 2 1 2 2 2 5
Small 1/2–1/3 1/3 1/3 1/2 1/3 1 1 1 3
Stones
Large 2–3 2–3 2–3 5 – 5 5 5 10
Medium 1 1 1 2 – 2 2 2 5
Small 1/2–1/3 1/3 1/3 1/2 – 1 1 1 3
Figures in columns give defect values (i.e. 1/2 means: 2 defects equivalent to 1 black bean as the reference).
loss of mass and to organoleptic properties (sensorial concern). A formula
is being introduced that intends to measure the total impact on quality, to
be calculated by multiplying the percentage of defects in a sample by the
coefficient of sensorial concern, according to the values found in the table.
Brazilian type classification, widely used in the international trade,
provides for seven types with decreasing values from 2 to 8 resulting from
the analysis of a 300 g sample of milled coffee, according to the norms
established in the ‘Brazilian Official Classification Table’ (Table 3.12).
Each type has a greater or lesser number of defects consisting of beans that
have been altered in the field, harvesting, processing or storage (e.g.,
physiological or genetic origin and presence of foreign matter).
3.8.4 Appearance and humidity
Appearance and humidity are fundamental factors in the evaluation of
quality, since they serve to predict a good or bad roast and other indications
of the quality of the final product. Appearance is good when the majority of
the beans are perfectly formed, uniform in size, colour and humidity, and
bad when the majority of the beans are not uniform and defective beans are
found. In coffee that has been correctly dried, the humidity level should be
11 0.5% for natural coffees, washed and pulped natural coffees. Incorrect
drying can be identified by spotted or humid beans.
3.8.5 Colour
Colour is always associated with quality. A bluish-green colour is very
desirable in washed coffee, being considered a sign of high quality and
freshness, while a yellowish colour is a sign of old coffee and low quality.
138 Espresso Coffee
Table 3.12 Official Brazilian classification of
green coffee (Classificac¸a˜o Oficial Brasileira – COB)
Grade Number of defects in 300 g of coffee
COB 2 4
COB 3 12
COB 4 26
COB 5 46
COB 6 86
COB 7 160
COB 8 360
Unsuitable for export >380
Factors that contribute to colour variations are: degree of dryness, time of
exposure to light, processing method, storage conditions, bruising,
polishing, etc. The polishing of the bean improves its appearance but
hides defects, due to the removal of the silverskin (Teixeira et al., 1985).
The ISO recommends the following colour classification: blue, greenish,
whitish, yellowish and brownish. The Brazilian classification adopted for
export purposes is: green, greenish, pale, yellow and old (Teixeira et al.,
1970).
3.8.6 Processing
Coffee can be classified according to processing method as natural (dryprocessed),
pulped natural, and washed (wet-processed) coffee. The
processing system can be recognized by the bean colour and by the
appearance of the silverskin. Washed coffees possess a characteristic
shiny, translucent and green-bluish colour. Natural coffees have a semiopaque
colour and a yellowish or even brown skin. Pulped natural coffee
has an intermediate aspect.
3.8.7 Roast
The analysis of the roast is an important step, and an important gauge of
quality. Defects that are not observed in the raw beans come out in the
roast. The immature and sour beans become yellowish while black beans
appear to be burnt. Broken beans, shells and shell core beans, due to their
reduced volume in relation to perfectly formed beans, become darker. In
Brazil, the roast is classified in accordance with appearance and, for
natural coffees, it can be measured as: fine, good, regular and bad
according to the degree of uniformity. The roast of washed and fermented
coffee is considered to be characteristic when the majority of the beans
present a clear and distinct silverskin in the ventral ridge of the bean
(Teixeira and Ferraz, 1963). The roasting of natural coffee presents a
brown silverskin. Pulped natural coffees and those with mechanically
removed mucilage present a roast intermediate between those of natural
and washed coffees.
3.8.8 Cup quality (Figure 3.24)
Even today, greater importance is attached to physical rather than
organoleptic classification. A possible reason is the fact that when these
criteria were introduced well-selected washed arabicas prevailed in the
The raw bean 139
market and such a classification corresponded closely to the quality of the
coffee in the cup. Another reason is that classification systems were
introduced many years ago when knowledge about the subject of cup
quality was limited. The obsolescence of classification systems can
sometimes lead to paradoxical situations, where good coffees are classified
as low grade, due to bean size, while mediocre coffees are well evaluated
and expensive, even if they are poor from an organoleptic point of view.
According to the International Trade Centre (ITC, 2002) the cup or
liquor can be characterized by the following terms:
n ‘Acidy – A desirable flavour that is sharp and pleasing but not biting.
The term ‘acid’ as used by the coffee trade refers to coffee that is
smooth and rich, and has verve, snap and life as against heavy, old
and mellow taste notes.’
n ‘Aroma – Usually, pleasant-smelling substances with the characteristic
odour of coffee.’
n ‘Body – A taste sensation or mouth feeling . . . of a drink
corresponding to a certain consistency or an apparent viscosity . . .
Sought after in most if not all coffees.’
Organoleptic characteristics vary according to the producing country and
this parameter must be considered as specific to each commercial origin.
In general, all washed arabica coffees from Eastern Africa (such as Kenya,
Tanzania and Ethiopia) as well as from Central and South America (such
as Guatemala, Costa Rica and Colombia), are marked by a degree of
acidity and an intense aroma, although each origin has different
characteristics. Natural dry-processed arabica coffees, especially those
from Brazil, are less acid and have a less marked aroma but are endowed
with a richer body ideal for espresso coffee. Finally, robusta coffees are
characterized by a wooden and earthy flavour.
Classification systems that take organoleptic characteristics into
account are used in some producing countries; unfortunately, this type
140 Espresso Coffee
Figure 3.24 Cup tasting
of classification has only a small influence on the price. Brazilian coffees
are classified as to cup quality according to the following ranking (from
best to worst): strictly soft, soft, softish, hard (or hardish), rioy and rio.
Kenyan coffees are graded by size and density into seven grades, divided
into 10 classes by liquoring (fine, fair, fair to good, etc.). Colombia and
Central American countries take altitude into consideration (Jobin,
1982). In addition to this lack of uniformity in classification systems, each
producing country weighs organoleptic defects in a different way, which
may lead to discrepancies in sensorial evaluation between producing and
consuming countries.
3.9 BLENDING
G. Brumen
The blending of coffee is as old as coffee itself. Although the techniques
vary, blending is used to optimize aroma, body and flavour: the goal is to
make a coffee that is higher in cup quality than any of the ingredients
individually, and, extremely important, maintain consistency in the final
roasted product.
Each batch has it own personality in terms of taste, smell, body,
chemical resilience to the hydrolytic action of water, etc., and blending
can complete it and round it up or level it off.
Most espresso blends are based on high quality Brazil arabicas, some
washed, some dry-processed. They often involve some African coffees for
winey acidity or flowery fruitiness, or a high-grown Central American for
a clean acidity. Some roasters add a little robusta to increase body.
Dry-processed coffees are responsible for the attractive ‘crema’ on the cup,
among other mechanical factors in the extraction process (see 8.1.1). Wetprocessed
Central Americans add positive aromatic qualities. Robustas
are used in cheaper blends to increase body and produce more foam.
Besides subjective quality (see Chapter 1), blending also assists in
maintaining objective quality, because the more complex a blend, the
easier it is to maintain constant quality when some ingredients change.
With the exception of a few countries that pay considerable attention
to quality, the majority of producer countries often add up small batches
produced by different growers to form larger ones of a size required by
roasters. Although care is taken so that only batches of equivalent quality
are blended, the result of this deplorable practice is often a quality
downgrading to a level below that of the best fractions.
Coffee history records a number of popular blends that are published
and available for public consumption. Other ‘proprietary’ blends tend to
The raw bean 141
be closely guarded, with the information staying within a company
structure. Proprietary or signature blend leads consumers to equate a
particular coffee profile with a particular brand image. Blending requires
the expert skill of knowing each ingredient coffee, having in mind a clear
cup profile as the goal, and knowing how to achieve it.
Blending may be done before or after roasting. Blending before roasting
is traditionally used by retail and institutional roasters. In this method
coffees with similar characteristics are combined and roasted to the same
development. Generally, professional in-house ‘cuppers’ evaluate the
results of the blend, adjusting components if necessary to satisfy taste
requirements and standards.
n Advantage: Consistency of product.
n Disadvantage: Inability to optimize the character of each coffee.
Blending after roasting is the method traditionally used by many speciality
coffee roasters. The flavour profile development requires that each
individual coffee used in the blend be roasted separately to optimize
flavour. In other words, each coffee will have a different time and
temperature setting. Consequently, the final roast development will be
different for each coffee used in the blend. After roasting, each
component of the blend is individually tasted (cupped), as is the final
blend composition.
n Advantage: Ability to optimize the character of each coffee.
n Disadvantage: Inconsistency of product.
3.10 DECAFFEINATION
O.G. Vitzthum
A major impact compound, as well as the pleasant aroma and flavour
components in roast coffee, is caffeine; it is the physiologically most
active substance in coffee.
Caffeine stimulates the central nervous system, and is responsible for
the vigilant effect of the coffee beverage. However, some people can suffer
from insomnia and restlessness after drinking coffee in the evening.
People who habitually drink coffee may develop a certain tolerance to the
drink, whereas non-coffee drinkers experience a stimulating effect even
from a single cup of coffee (for a discussion of the physiological effects of
caffeine see 10.3.2).
142 Espresso Coffee
It was the German writer J.W. von Goethe who, in 1820, gave some
coffee beans to the chemist Runge (1820) requesting him to isolate the
pharmacologically active compound therein. He found the alkaloid
caffeine, a colourless, slightly bitter-tasting substance. The name was
derived from the botanical name Coffea for coffee.
The idea of reducing or eliminating this physiologically active ingredient
in coffee originated at the end of the nineteenth century. It was after the
frustrating experiments of his chemist Meyer, who tried to extract the
caffeine out of green coffee beans with caffeine-dissolving solvents, that
Bremen coffee merchant L. Roselius had the successful idea of using raw
coffee beans pre-swollen with water for decaffeination with solvents
(Roselius, 1937). He knew that by using a steam treatment the volume of
green coffee could be increased by about 100%. Consequently the solvent
was able to penetrate more easily into the swollen beans and dissolve the
caffeine. Water-immiscible solvents are necessary to avoid the extraction
of the flavour precursor water-soluble components. The hydrophilic
caffeine fortunately dissolves well both in water and in various solvents.
The first commercially decaffeinated coffee was produced in Bremen as
Kaffee HAG at the beginning of the twentieth century. Due to the patent
protection for this invention, many attempts were made in the early 1920s
and 1930s to apply other solvents and processes in search of a legal
circumvention of this technique. Until 1970 two solvents were successfully
in use for the decaffeination of raw coffee beans or aqueous extracts
thereof: methylene chloride or dichloromethane (DCM) and ethylacetate
(EA). A new approach for the decaffeination of coffee was initiated by
Zosel (1971) when, in 1970, he suggested the use of supercritical carbon
dioxide. This activity spurned the development of other processes such as
water extraction and the use of adsorbents or fats and oils.
Nowadays conventional processes such as extraction with DCM and
EA are in use as well as various technologies based on decaffeination with
carbon dioxide or water and certain extraction aids.
3.10.1 Processing
A comprehensive description of decaffeination principles with technical
details has recently been published by W. Heilmann (2001). Here, only
the various decaffeination techniques will be described.
3.10.1.1 Conventional decaffeination
The procedures for decaffeination by the solvents dichloromethane
CH2Cl2 (DCM) and ethylacetate CH3COOC2H5 (EA) are similar,
The raw bean 143
although more coffee solids are lost during EA decaffeination. There are
direct and indirect solvent-based decaffeination processes.
In the direct solvent decaffeination process (Figure 3.25), after
cleaning, green beans are swollen by addition of water or steam into
the extraction vessel. Thereafter solvent is added and decaffeination
started by recycling of the solvent under elevated temperature conditions
(70–100 oC). Processing continues for 8–12 hours, whereby the solvent is
continually replaced by fresh re-distilled material. In a separate vessel the
caffeine-rich solvent is distilled off and led back to the decaffeination
vessel. After a corresponding refining process, the crude caffeine is pure
enough for use in caffeinated beverages. Subsequent to completion of the
decaffeination process, the residual solvents are eliminated by steaming.
The beans are dried and are then ready for roasting. Decaffeinated beans
are usually lighter in colour than the original green raw coffee beans.
In the indirect solvent decaffeination process (Figure 3.26), beans do
not come into direct contact with DCM (Berry and Walter, 1943). A
saturated aqueous extract of green beans, containing all water-soluble
components including caffeine, is decaffeinated by the solvent in a
liquid–liquid extraction process in a separate vessel. The caffeine-free
144 Espresso Coffee
Figure 3.25 Direct solvent decaffeination
extract after decaffeination is returned to the green beans and the process
repeated using a new batch of green beans. Any traces of solvents present
in the returned caffeine-free extract must be removed by subsequent
steaming. After drying, the beans are ready for roasting.
3.10.1.2 Modern decaffeination
A revolution in decaffeination evolved with the invention of caffeine
removal by supercritical carbon dioxide (Figure 3.27) (Martin, 1982;
Lack and Seidlitz, 1993). This process uses the specific solvation power of
gaseous carbon dioxide under elevated pressure conditions for caffeine.
CO2 is in a supercritical physico-chemical state above its critical
temperature of 31 oC and, although still a gas, it has nearly the density
of a liquid if pressures of about 200 atm are applied.
In comparison with conventional solvents, the solubility of caffeine in
supercritical CO2 is low; but multiple recycling of the gas through the
green beans eventually leads to complete caffeine extraction. Caffeine is
removed in every cycle from the gas stream by an effective adsorber such
as active carbon or an ion-exchange resin. The advantage of the process
lies in the fact that CO2 is highly selective in extracting the caffeine out
of the beans so that components that contribute to the aroma-yielding
fraction in coffee are not solubilized.
The raw bean 145
Figure 3.26 Indirect solvent decaffeination
The indirect adsorbent decaffeination process (Figure 3.28) (Fischer
and Kummer, 1979; Blanc and Margolis, 1981) is similar to indirect
solvent decaffeination, except that the solvent is replaced by an extract
that is saturated with the water-soluble compounds of the coffee material.
Such a heated aqueous extract extracts caffeine from the fresh beans;
thereafter the caffeine is selectively removed from the solution in a
separate vessel by a ‘caffeine-selective’ adsorbent such as coated active
carbon or non-ionic microporous resins. These serve to more or less
selectively extract all the caffeine but not the aroma precursor containing
water-soluble substances. This extract, which now contains no caffeine, is
recycled back to the extraction vessel and caffeine is extracted again. The
treated active carbon may be pH-adjusted or coated by carbohydrates to
block the hydrophilic sites on the adsorbent. After use the adsorbents
must be regenerated accordingly.
3.10.1.3 Organoleptic comparison of the different techniques
Although slight differences in taste are noticeable between the same
blend, decaffeinated by MC, supercritical CO2 or water, if the
decaffeination process and the roasting are carried out correctly the
difference in the cup with the corresponding non-decaffeinated blend is
minimal.
146 Espresso Coffee
Figure 3.27 Supercritical CO2 decaffeination
If the process is not carried out under optimal conditions, typical defects
appear at cupping:
n MC-decaffeinated coffee has the ‘cooked’ flavour, associated with
decaffeinated coffee.
n CO2-decaffeinated coffee is flat.
n Water-decaffeinated coffee is thin, due to the loss and/or poor reincorporation
of the soluble solids co-extracted with caffeine (this
defect is often masked by increasing roasting level).
n EA-decaffeinated coffee has always the typical ‘cooked’ flavour
associated with (poorly) decaffeinated coffee.
3.10.2 Perspectives for caffeine elimination
Whereas no major developments are expected for decaffeination
processing in the near future, the application of molecular biology and
biotechnology to coffee may yield new naturally caffeine-free coffee plants
within 5–10 years. The deciphering of sequences of coffee genes is
progressing and biochemical pathways for the formation of caffeine in the
plant are being elucidated (Stiles et al., 2000).
The possibility of cultivating coffee plants without caffeine by
recombinant DNA technology is approaching. Ogawa et al. (2001)
The raw bean 147
Figure 3.28 Indirect adsorbent decaffeination
recently reported on the successful cloning of a corresponding methyltransferase,
which is an important regulator enzyme in the biochemical
pathway of caffeine formation in the plant.
3.10.3 Regulatory aspects
3.10.3.1 Caffeine residue
In European countries the maximum allowed caffeine content in coffee
beans is 0.1%, in soluble coffees, 0.3%. In the USA at least 97%
decaffeination (equivalent to 99.7% caffeine-free coffee) is common in
decaffeinated coffees.
3.10.3.2 Solvent residue
No regulations exist for indirect decaffeination via coated activated
carbon or fats and oils. No restrictions are necessary for ubiquitous liquid
or supercritical carbon dioxide.
The following regulations regarding solvent residues in roasted coffee
beans exist for dichloromethane:
n 2–5 ppm in Europe, 10 ppm in South America. Practical observations
under the provision of good manufacturing practices (GMP) yielded
residues of <1 ppm in European DCM decaffeinated coffees.
n GMP have been requested specifically for EA; in Italy the residues for
EA are restricted to 15 ppm in roasted coffee.
3.10.4 Decaffeination capacity
The approximate (best estimate) decaffeination capacities from 1998 are
given in Table 3.13 (opposite). Consumption of decaffeinated coffees is
highest in Germany, Switzerland and the USA (12–18%).
3.11 RAW BEAN COMPOSITION
I. Ko¨ lling-Speer and K. Speer
The beans of Coffea arabica (arabica coffee) and Coffea canephora (robusta
coffee) vary in mass from 100 mg to over 200 mg, with some evidence of
variation associated with the geographical origin. Both species show
quantitative and qualitative differences in their chemical compositions.
Arabica contains more lipids and trigonelline, while robusta contains
more caffeine and chlorogenic acids. Minor constituents specific to one
148 Espresso Coffee
species have also been identified. The composition of raw coffee beans is
indicated in Table 3.14.
3.11.1 Water content
The water content of green beans, from dry or wet processing, influences the
water activity and stability during storage and varies in Europe from 9 to 13%.
Dry-processed robusta coffees (Vietnam) sometimes show higher values. For
The raw bean 149
Table 3.13 Estimated decaffeination capacity by country
Country Solvent used
Capacity
(tonnes)
Germany DCM; supercritical CO2; liquid CO2; EA 160 000
France DCM; water 45 000
Italy DCM; EA 16 000
Portugal DCM 5 000
Spain EA; water 18 000
Switzerland DCM; EA; water 8 000
Total Europe 252 000
Canada DCM; water 12 000
USA DCM; EA; water; supercritical CO2 120 000
South America DCM; EA; water; fats and oils 50 000
Asia DCM; EA 6 000
World total 440 000
Table 3.14 Composition of raw coffee
beans (% dry matter)
Class Arabica Robusta
Caffeine 1.2 2.2
Lipids 16 10
Chlorogenic acids 6.5 10
Aliphatic acids 1.7 1.6
Total aminoacids 10.3
free 0.5 0.8
Trigonelline 1.0 0.7
Glycosides 0.2 Traces
Minerals 4.2 4.4
potassium 1.7 1.8
Carbohydrates (by difference) 58.9 60.8
storage and transportation the moisture level proposed by the European
Coffee Federation (ITC, 2002, p. 254) is 12.5%. Below 10% the germinating
power decreases and, furthermore, the beans show crack formation. At water
contents >12.5% there is a significant risk of microbiological spoilage. An
international reference method for moisture analysis (ISO, 1983) should be
used to standardize the rapid methods now available.
3.11.2 Ash and minerals
Coffee contains about 4% of mineral constituents (arabicas: 3.6%–4.5%;
robustas 3.6%–4.8%), with potassium amounting to 40% of the total. The
level is slightly higher in dry-processed robusta and arabica than in wetprocessed
arabica (see Table 3.2). The main minerals of raw coffee beans
are given in Table 3.15; fertilizing may change the contents (Amorim
et al., 1973).
Coffee has the highest content of rubidium, which has been analysed in
food (arabica: 25.5–182 mg/kg dry matter; robusta: 6.6–95.2 mg/kg dry
matter). Quijano Rico and Spettel (1975) determined higher content of
copper, strontium und barium in robustas than in arabicas.
3.11.3 Carbohydrates
The total amount of carbohydrates represents about 50% dry base of green
coffee. The composition is complex with a range of different poly-, oligoand
monosaccharides (Table 3.16).
Lu¨llmann and Silwar (1989) analysed the low molecular weight
carbohydrate profile of 20 green coffee samples from 13 different producer
countries. Sucrose content varied from 6.25% to 8.45% for the arabica
samples, and from 0.9% to 4.85% for the robusta samples. Besides sucrose,
150 Espresso Coffee
Table 3.15 Mineral content
of raw coffee (dry base)
Mineral Percent
Potassium 1.63–2.00
Calcium 0.07–0.035
Magnesium 0.16–0.31
Phosphate 0.13–0.22
Sulphate 0.13
small amounts of the monosaccharides fructose, glucose, mannose,
arabinose and rhamnose have been identified, traces of reducing sugars
being higher in robustas.
There was no evidence in green coffees of other simple oligosaccharides
such as raffinose or stachyose. Maltose was determined only in one robusta
sample.
It would be expected that sucrose content would increase with degree
of ripening and this is apparent with defective coffee beans, where for
both immature-black and immature-green Brazilian robusta beans,
sucrose levels were found to be one-third and one-fifth of the level of
normal beans, respectively (Mazzafera, 1999). This was also observed
for Vietnamese robustas, where black beans sorted from a sample
contained 0.9% sucrose; normal beans from the same batch contained
4.0% sucrose.
Glucose content has been negatively correlated with aroma level and
positively with cup sweetness, while fructose content has been negatively
correlated with sweetness (Illy and Viani, 1995). Franz and Maier (1993,
1994) identified inositol tri-, tetra-, penta- and hexaphosphate (phytic
acid). Total inositol phosphate content in robusta tended to be higher
than in arabica, ranging from 0.34 to 0.40% and 0.28 to 0.32%,
respectively. Polyalcohols have also been found in green coffee. Noyes
and Chu (1993) found low levels of mannitol in blends of Brazilian
robustas and arabicas (average content 0.027%).
The chemical structure of green coffee polysaccharides has been
extensively studied by Bradbury and Halliday (1987, 1990) and Fischer et
al. (2000). The soluble fraction is composed of sucrose and of polymers of
galactose, arabinose and mannose (galactomannans and arabinogalac-
The raw bean 151
Table 3.16 Carbohydrates in
raw coffee (% dry base)
Constituent Arabica Robusta
Monosaccharides 0.2–0.5
Sucrose 6–9 3–7
Polysaccharides 43.0–45.0 46.9–48.3
Arabinose 3.4–4.0 3.8–4.1
Mannose 21.3–22.5 21.7–22.4
Glucose 6.7–7.8 7.8–8.7
Galactose 10.4–11.9 12.4–14.0
Rhamnose 0.3
Xylose 0–0.2
tans). The insoluble cell constituents include ‘holocelluloses’, which
contain, in addition to some cellulose, the very hard -1,4-mannan, and a
small amount of ‘hemicellulose’, mainly arabinogalactan. Other polysaccharides
common to the plant kingdom such as starch or pectin are
present, if at all, at only low levels in mature coffee beans. No lignine,
characterized as polymers of xylose, is present in the seed.
3.11.4 Glycosides
A series of diterpene glycosides, the atractylglycosides, was identified in
green and roasted coffee in the 1970s by Spiteller’s group (Ludwig et al.,
1975; Obermann and Spiteller, 1976). The contents in coffee of the
principal components of this compound class, termed KAI, KAII and
KAIII were determined by Maier and Wewetzer (1978) and Aeschbach et
al. (1982). Bradbury and Balzer (1999) have shown that these glycosides
in green coffee actually contain two carboxyl groups at C-4, and
consequently, the compounds were identified as carboxyatractylglycosides.
The carboxyl group is labile, thus explaining why earlier studies
only observed the atractyl forms. Arabica beans contain significantly
more of the glycosides than robusta beans.
3.11.5 Carboxylic acids
Data on acids in green coffee (Table 3.17) came from Kampmann and
Maier (1982), Scholze and Maier (1983, 1984), Engelhardt and Maier
(1984, 1985) and Wo¨hrmann et al. (1997). The total content is similar in
arabica and robusta coffees (average 1.7%) according to Van der Stegen
and Van Duijn (1987).
Quinic acid, the aliphatic acid moiety in the chlorogenic acids, is also
present in the free state. Its content is presumably affected by factors such
as processing, fermentation and age. Free quinic acid content can increase
to 1.5% in old beans. Besides quinic acid, the major acids in green coffee
are malic and citric acids and phosphoric acid. Minor coffee acids were
identified by Ba¨hre (1997) and Ba¨hre and Maier (1999).
The acidity of brewed coffees is an important organoleptic character
and is associated with the best high grown arabicas, such as Kenyans.
3.11.6 Chlorogenic acids
The chlorogenic acids are a group of phenolic acids esterified to quinic
acid (Clifford, 1999; Parliment, 2000). This class of acidic compounds
152 Espresso Coffee
accounts for up to 10% of the weight of green coffee. Mono- and dicaffeoylquinic
acids have been identified in coffee with substitution at the
3-, 4- and 5-position of quinic acid. The phenolic acid fraction of coffee is
composed of caffeoyl-, p-coumaroyl- and feruloyl-acids (Figure 3.29, see
page 154).
In coffee these components are essentially mono- and diesters. The
monoesters decrease during the unripe to semi-ripe stage then increase to
the ripe stage. This is followed by a decrease in the slightly overripe stage,
more important with wet than with dry processing (Castel de Menezes
and Clifford, 1988).
3.11.7 Amino acids, peptides and proteins
Free amino acids are present in raw coffee beans at levels about 1%
(Trautwein, 1987). Post-harvest treatments influence the contents of
individual free amino acids. This has been estimated for eight samples of
arabica by Arnold (1995): The total free amino acids do not show clear
changes after drying at 40
C, but the individual contents change for some
acids, especially glutamic acid, which shows an increase of about 50%,
and aspartic acid, which mainly decreases; the hydrophobic acids (valine,
The raw bean 153
Table 3.17 Contents (g/kg) of quinic, malic, citric
and phosphoric acids of different raw coffees
Species Origin
Quinic
acid
Malic
acid
Citric
acid
Phosphoric
acid
Arabica Santos 5.6 6.1 13.8 1.1
Burundi 5.7 5.1 13.0 1.1
Kenya 4.7 6.6 11.6 1.4
Colombia 5.5
Mocca 4.6 10.5 1.5
Robusta Burundi 3.5 3.8 10.0 1.4
Angola 2.8 9.2 2.2
Togo 3.1 2.5 6.7 1.6
Guinea 3.9
Arabusta Ivory Coast 4.5 9.1 1.7
Excelsa Ivory Coast 5.2 10.9 1.5
Stenophylla Ivory Coast 2.9 10.0 2.2
Liberica Unknown 3.9 10.6 1.8
Source: Kampmann and Maier, 1982; Scholze and Maier, 1983, 1984
phenylalanine, leucine, isoleucine) generally increase. Lipke (1999)
isolated and characterized several coffee peptides.
During storage of the green beans, particularly at elevated temperatures,
there are changes due to proteolysis (e.g. increases in alanine,
isoleucine and tyrosine) and to losses of free amino acids by nonenzymatic
browning reactions (Pokorny et al., 1975).
The proteins consist of a water-soluble (albumin) and a water-insoluble
fraction, present in approximately equal amounts. The majority of the
proteins have molecular weights above 150 000 Daltons. Crude protein
content, calculated from total nitrogen content, must be corrected for
caffeine and ideally also for trigonelline nitrogen. If such corrections are
made the protein content in green coffee is close to 10% with little
quantitative and qualitative differences between species and there is no
significant effect that can be attributed to green bean processing.
Mazzafera (1999) found a higher protein content in mature beans than
in the immature ones.
Several enzymes have been characterized in green coffee: -galactosidase,
malate dehydrogenase, acid phosphatase, peroxidase and more
154 Espresso Coffee
Figure 3.29 Phenolic acids in raw coffee
extensively polyphenol oxidase/tyrosinase/catechol oxidase as indicators
of green coffee bean quality (Clifford, 1985). Polyphenol oxidase is
responsible for the discoloration of defective beans, which is caused by
catalysing the oxidation of the chlorogenic acids.
3.11.8 Non-protein nitrogen
3.11.8.1 Trigonelline
Raw coffee contains trigonelline (arabica: 0.6–1.3%, robusta: 0.3–0.9%).
Dewaxing, decaffeination procedure and steaming (Stennert and Maier,
1994) do not lead to appreciable changes in the trigonelline content
(Figure 3.30).
3.11.8.2 Caffeine and trace purine alkaloids (Figure 3.31)
Caffeine is the major purine in green coffee, where it is probably
associated with the chlorogenic acids by a -electron complex (Horman
and Viani, 1972). The average content of caffeine is 1.2–1.3% in arabica
and 2.2–2.4% in robusta, showing a marked inter-specific difference as
well as high intra-specific variability. Caffeine is absent or present only in
traces in some wild non-commercial species.
In addition to caffeine, coffee also contains the three dimethylxanthines
– paraxanthine, theobromine and theophylline – and other
trace purines, present in larger quantities in robusta – particularly in
immature beans – than in arabica. Kappeler and Baumann (1986) found
that unripe beans have higher contents of theophylline, theobromine,
liberine and theacrine than ripe ones. Weidner and Maier (1999) also
identified paraxanthine and theacrine. Prodolliet et al. (1998) tried to
determine the geographic origin of coffee using the isotopic ratios (C, N,
H) in samples of caffeine extracted from green arabicas and robustas from
16 countries. Both univariate and multivariate analysis allowed neither
the determination of the species nor of the country of origin.
The raw bean 155
Figure 3.30 Trigonelline, a precursor of many aroma compounds
3.11.9 Lipids
The lipid content of green arabica coffee beans averages some 15%, whilst
robusta coffees contain much less, around 10%. Most of the lipids – the
coffee oil – are located in the endosperm; only a small amount, the coffee
wax, is on the outer layer of the bean. The yield of crude lipids depends
not only on bean composition, but also on extraction conditions,
particularly particle size and surface area, choice of solvent and duration
of extraction.
Coffee oil is composed mainly of triglycerides with fatty acids in
proportions similar to those found in common edible vegetable oils. The
relatively large unsaponifiable fraction is rich in diterpenes of the kaurane
family, mainly cafestol, kahweol and 16-O-methylcafestol, which have
received increasing attention in recent years because of their physiological
effects (see 10.3). Furthermore, 16-O-methylcafestol can be used as a
reliable indicator for robusta content in coffee blends. Among the sterols,
that are also part of the unsaponifiable matter, various desmethyl-,
methyl- and dimethylsterols have been identified. The composition of the
lipid fraction of green coffee is given in Table 3.18.
3.11.9.1 Fatty acids
For the most part, the fatty acids are present as glycerol esters in the
triglycerides, some 20% are esterified with diterpenes, and a small
proportion as sterol esters. Folstar et al. (1975) and Speer et al. (1993)
investigated the fatty acids of the triglycerides and the diterpene esters in
detail. The fatty acids in sterol esters were determined by Picard et al.
(1984).
156 Espresso Coffee
Figure 3.31 Purine alkaloids in raw coffee
The presence of free fatty acids (FFA) in coffee has been described by
various authors (Kaufmann and Hamsagar, 1962b; Calzolari and Cerma,
1963; Carisano and Gariboldi, 1964; Wajda and Walczyk, 1978). Speer et
al. (1993) developed a method for the direct determination of free fatty
acids: Nine different free fatty acids were detected, uniformly distributed
in both robusta and arabica coffees. In both species the main fatty acids
are C18:2 and C16. It was also possible to detect large proportions of C18,
C18:1, C20 and C22, whereas there were no more than traces of C14, C18:3
and C24. While the proportion of stearic acid is noticeably smaller than
that of oleic acid in robustas, the percentages of these two acids in arabica
coffees are almost equal. The ratio stearic acid/oleic acid may give a first
indication of robusta content in coffee blends.
3.11.9.2 Diterpenes
The main diterpenes in coffee are pentacyclic diterpene alcohols based on
the kaurane skeleton. Working groups under Bengis and Anderson
(1932), Chakravorty et al. (1943a, 1943b), Wettstein et al. (1945),
Haworth and Johnstone (1957) and Finnegan and Djerassi (1960)
elucidated the structure of two of the coffee diterpenes, namely kahweol
and cafestol. In 1989, 16-O-methylcafestol was isolated from robusta
coffee beans (Speer and Mischnick, 1989; Speer and Mischnick-
Lu¨bbecke, 1989). 16-O-methylkahweol, another O-methyl diterpene,
has recently also been found in robusta coffee beans (Speer et al., 2000).
The structural formulae of these diterpenes are shown in Figure 3.32.
The raw bean 157
Table 3.18 Composition of the lipid
fraction of raw coffee (percentages of
total lipids, average)
Component Percent
Triglycerides 75.2
Esters of diterpene alcohols and fatty acids 18.5
Diterpene alcohols 0.4
Esters of sterols and fatty acids 3.2
Sterols 2.2
Tocopherols 0.04–0.06
Phosphatides 0.1–0.5
Tryptamine derivatives 0.6–1.0
Source: Maier, 1981
Arabica coffee contains cafestol and kahweol. Robusta coffee contains
cafestol, small amounts of kahweol and, additionally, 16-O-methylcafestol
(16-OMC) and traces of 16-O-methylkahweol, which were found
only in robusta coffee beans (Speer and Mischnick-Lu¨bbecke, 1989;
Speer and Montag, 1989; Speer et al., 1991b; Ko¨lling-Speer et al., 2001).
Absence of 16-OMC in arabica coffee beans has been confirmed by
Frega et al. (1994), White (1995) and Trouche et al. (1997). Because of
its stability during the roasting process, 16-OMC can be used to detect
the presence of down to less than 2% robusta in arabica blends (Speer et
al., 1991b).
The diterpenes cafestol, kahweol and 16-OMC are mainly esterified
with various fatty acids. In their free form, they occur only as minor
components in coffee oil (Speer et al., 1991a; Ko¨lling-Speer et al., 1999).
Both free cafestol and kahweol are present in arabica coffees, cafestol
being the major component. In robusta coffee, free cafestol content is
slightly higher than 16-OMC content, and free kahweol is either present
only in traces or absent. Comparison of free diterpenes with total
diterpenes after saponification shows that in arabicas proportions ranged
from 0.7 to 2.5%; in robustas the proportions of free diterpenes were
slightly higher with 1.1 to 3.5%; experiments to verify if the difference is
due to the processing technique or to the species are ongoing.
158 Espresso Coffee
Figure 3.32 Structural formulae of raw coffee diterpenes
Fatty acid esters have been reported (Kaufmann and Hamsagar, 1962a;
Folstar et al., 1975; Folstar, 1985; Pettitt, 1987; Speer, 1991, 1995;
Kurzrock and Speer, 1997a, 1997b): Cafestol, 16-OMC and kahweol
esters with fatty acids such as C14, C16, C18, C18:1, C18:2, C18:3, C20, C22,
C24 were identified, as well as esters with the fatty acid C20:1 and some
odd-numbered fatty acids such as C17, C19, C21 and C23 (Kurzrock and
Speer, 2001a, 2001b). The individual diterpene esters were irregularly
present in the coffee oil. The odd-numbered fatty acid esters were minor
components, whereas the diterpenes esterified with palmitic, linoleic,
oleic, stearic, arachidic, and behenic acid, were present in larger amounts.
Table 3.19 indicates the distribution of the six main esters (summing up
to nearly 98% of the respective diterpenes) for arabica coffee.
The total content of these six cafestol esters in sum ranged between 9.4
and 21.2 g/kg dry weight, corresponding to 5.2 to 11.8 g/kg cafestol in
different arabica coffees, and between 2.2 and 7.6 g/kg dry weight in
robusta, corresponding to 1.2 to 4.2 g/kg cafestol, notably less than in
arabica.
3.11.9.3 Sterols
Coffee contains a number of sterols that are common in other seed oils as
well (Figure 3.33, see page 160). In addition to 4-desmethylsterols, various
4-methyl- and 4,4-dimethylsterols have been identified. Coffee sterols
have been found both in the free (around 40%) and in the esterified form
(around 60%) (Nagasampagi et al., 1971; Itoh et al., 1973a, 1973b;
Tiscornia et al., 1979; Duplatre et al., 1984; Picard et al., 1984; Mariani
The raw bean 159
Table 3.19 Distribution (%) of the diterpene esters in raw
arabica coffee
Cafestol + kahweol1
(n2 = 1)
Cafestol + kahweol2
(n = 1)
Cafestol3
(n = 10)
Kahweol4
(n = 10)
C16 42.5 51.4 40–49 46–50
C18 17.5 9.1 9–11 8–11
C18:1 11.0 7.4 9–15 8–12
C18:2 20.5 26.4 24–30 25–29
C20 6.0 4.6 3–6 3–6
C22 2.5 1.1 0.6–1.2 0.7–1.3
Kahweol esters calculated as cafestol esters.
n = Number of samples analysed.
Sources: 1Kaufmann and Hams, 1962a; 2Folstar, 1985; 3Kurzrock and Speer, 1997a;
4Kurzrock and Speer, 2001a,b
and Fedeli, 1991; Frega et al., 1994). The desmethylsterols represent
90% of the total sterol fraction, which ranges from 1.5% to 2.4% of the
lipids (Picard et al., 1984). Nagasampagi et al. (1971) found higher
proportions (5.4%).
Table 3.20 presents the distribution of the main desmethylsterols in
different robusta and arabica coffee samples. The main sterol is
-sitosterol, with about 50%. 24-methylenecholesterol and 5-avenasterol,
occurring in larger amounts in robusta than in arabica coffee beans,
are suitable for coffee blend studies (Duplatre et al., 1984; Frega et al.,
1994). However, because of their varying natural contents, they can be
only used for determining proportions of robusta in a mixture with arabica
from 20% onward.
3.11.9.4 Tocopherols (Figure 3.34)
The presence of tocopherols in coffee oil was described for the first time
by Folstar et al. (1977). They found concentrations of -tocopherol of
160 Espresso Coffee
Figure 3.33 Sterols of raw coffee beans
89 to 188 mg/kg oil, for - . -tocopherol 252–53 mg/kg oil. The
predominance of -tocopherol is a prominent feature of coffee beans, in
contrast to other vegetables and fruits (Aoyama et al. 1988; Ogawa et al.
1989).
3.11.9.5 Coffee wax
A thin wax layer, corresponding to 2–3% of the total lipids, covers the
surface of green coffee beans. The wax content is generally defined as the
The raw bean 161
Table 3.20 Mean distribution (%)
of desmethylsterols in raw arabica
and robusta coffees (30 samples)
Sterols Arabica Robusta
Cholesterol 0.3 0.2
Campesterol 15.8 16.9
Stigmasterol 21.9 23.1
-Sitosterol 51.6 45.4
5-Avenasterol 2.7 9.1
Campestanol 0.4 0.2
24-Methylenecholesterol 0.2 1.9
Sitostanol 2.0 0.8
7-Stigmastenol 2.2 0.2
7-Avenasterol 1.5 0.4
7-Campesterol 0.6 0.2
5.23-Stigmastadienol 0.3 0.5
5.24-Stigmastadienol 0.1 Traces
Clerosterol 0.5 0.7
Source: Mariani and Fedeli, 1991
Figure 3.34 Tocopherols of raw coffee beans
material obtained from the intact beans by extraction with chlorinated
solvents such as dichloromethane. The first investigations about wax
composition of green arabica coffee beans were done by Wurziger and coworkers
(Dickhaut, 1966; Harms, 1968). They isolated and identified
three carbonic acid 5-hydroxytryptamides (C-5-HT) (Figure 3.35).
Arachidic acid (n . 18), behenic acid (n . 20) and lignoceric acid
(n . 22) are combined with the primary amino group of 5-hydroxytryptamine.
In addition, Folstar et al. (1980) reported the presence of
stearic acid 5-hydroxytryptamide (n . 16), later !-hydroxyarachidic acid
5-hydroxytryptamide (n . 18), and !-hydroxybehenic acid 5-hydroxytryptamide
(n . 20). Arachidic acid and behenic acid 5-hydroxytryptamide
are dominant, but the other amides are only minor constituents.
The content ranges between 500 and 2370 mg/kg in green arabica, and
from 565 to 1120 mg/kg in green robusta (Maier, 1981). Polishing,
dewaxing and decaffeination of the beans leads to a substantial reduction
in C-5-HT (Harms and Wurziger, 1968; Van der Stegen and Noomen,
1977; Hunziker and Miserez, 1979; Folstar et al., 1980). The C-5-HT have
antioxidant properties (Lehmann et al., 1968; Bertholet and Hirsbrunner,
1984).
3.11.10 Volatile constituents
Some 180 volatile substances have been identified in green coffee beans
(Boosfeld and Vitzthum, 1995; Holscher and Steinhart, 1995; Cantergiani
et al., 1999). Arabicas are similar to robustas, but arabicas are
distinguished by a large content of terpenes and fewer aromatic
compounds. The range is affected by green bean processing, and could
be used for evaluating green bean quality. The odour of defective green
beans has been linked with specific components (see 3.7.2).
162 Espresso Coffee
Figure 3.35 Structural formulae of carbonic acid 5-hydroxytryptamides (C-5-HT)
3.11.11 Contaminants
3.11.11.1 Mycotoxins
R. Viani
The occasional occurrence of several mycotoxins that may contaminate
green coffee beans has been known since the early 1970s: they are
sterigmatocystine, aflatoxin B and ochratoxinA(OTA) (Levi et al., 1974).
Sterigmatocystine was only found in extremely contaminated beans,
clearly unfit for consumption, and is of no practical relevance in coffee.
The aflatoxins, aflatoxin B in particular, have sometimes been detected
in green coffee beans, and – in extremely small amounts, apparently of no
practical concern for public health – also in the beverage (Nakajima et al.,
1997).
OTA contamination of coffee has been the subject of intensive
research during the past 15 years, after it was realized, thanks to improved
analytical techniques, that not all the OTA present in green beans was
destroyed during roasting (Tsubouchi et al., 1988). Much information is
available through the three workshops, organized in Nairobi (Anon.,
1997), Helsinki (Van der Stegen and Blanc, 1999) and Trieste (Van der
Stegen et al., 2001) by the Association Scientifique Internationale du Cafe´
(ASIC). A comprehensive review has been published by Bucheli and
Taniwaki (2002).
The mould species, which are known to produce (OTA) in coffee, have
been identified as Aspergillus ochraceus, A. carbonarius and, occasionally,
A. niger (Frank, 2001; Ismayadi and Zaenudin, 2001; Joosten et al., 2001;
Paneer et al., 2001; Pitt et al., 2001). The specific time and place of
contamination have not yet been clearly identified. Both natural and wetprocessed
coffees appear to be at risk of attack by the mould, although
wet-processed coffees are rarely found contaminated by OTA.
The risk of OTA contamination can clearly be minimized by following
good agricultural and good manufacturing practices at the harvest, postharvest
and storage-transportation stages (Teixeira et al., 2001; Viani,
2002):
n Storage of fresh cherries – ripe and overripe cherries, stored over
days piled up or, worse, in plastic bags, often in the sun, are at
increased risk of OTA contamination (Bucheli et al., 2000);
therefore, avoiding storage of fresh cherries before processing is
important not only for coffee organoleptic quality (see 3.2), but also
to reduce the risk of OTA contamination (Figure 3.36).
n Drying – after drying begins, the length of time spent at a water
activity above 0.80 at any time until roasting determines the risk of
The raw bean 163
mould growth and OTA production, which may explain the
increased risk for sun drying with respect to mechanical drying, and
of cherry sun drying with respect to the shorter parchment sun drying.
Organoleptic quality considerations require a regular long drying
process (see 3.3), while at the same time reduction of the risk of OTA
contamination asks for quick drying. The drying process therefore
needs to be optimized to adjust to both requirements (Ismayadi et al.,
2001; McGaw et al., 2001).
n Husks – contamination appears to start within the husk, and, in the
‘appropriate’ moisture conditions, it can reach the bean in 3–4 days;
therefore, freeing green coffee beans from all husk material during
cleaning and grading is important to reduce the OTA load (Pittet et
al., 1996; Blanc et al., 1998; Bucheli et al., 2000).
n Re-wetting – storage in the tropics of green robusta coffee for up to
one year at 70% relative humidity and at temperatures up to 30
C
has not produced any increase in the OTA load (Bucheli et al.,
1998). Transportation trials from the producer countries have
shown that risk of condensation and wetting of coffee occur mainly
during transport overland to the harbour for shipping, and on arrival
at destination in the winter season; this may occur even with
properly dried coffee if too large temperature variations cause
evaporation followed by water condensation on the upper layer of
coffee (Blanc et al., 2001). OTA contamination can also occur
during the decaffeination process (see 3.9), when decaffeinated wet
beans are not properly and rapidly dried (unpublished data).
Avoiding re-wetting at any stage before roasting is therefore very
important (Figure 3.37).
164 Espresso Coffee
Figure 3.36 Storage of fresh cherries must be short!
3.11.11.2 Polycyclic aromatic hydrocarbons (PAH)
I. Ko¨ lling-Speer and K. Speer
Different PAH, e.g. fluoranthene, pyrene, benz[a]anthracene, chrysene,
triphenylene, benzo[b]fluoranthene, benzo[j]fluoranthene, benzo[k]fluoranthene,
benzo[a]fluoranthene, benzo[e]pyrene, benzo[a]pyrene, perylene,
indeno[1,2,3-cd]pyrene, dibenz[ah]anthracene, dibenz[ac]anthracene and
benzo[ghi]perylene were identified and quantified in green and roasted
coffees. The individual PAH can be formed both by the fuel and by the
The raw bean 165
Figure 3.37 Drying coffee – cherry or parchment – must be protected from rewetting
coffee bean under sub-optimal roasting conditions. On comparing the
PAH contents of the unroasted coffee with those of the roasted product it
is seen that it is not so much the roasting process after its optimization but
the PAH content of the raw material which determines the content in
the roasted product (Klein et al., 1993). The contamination of the green
coffee may be due to exhaust gases of cars or to transportation of the
coffee beans in contaminated jute and sisal sacks. Their fibres are treated
before spinning with batching oil. Before bean contamination was
discovered the batching oil commonly consisted of an uncontrolled
mineral oil fraction, which could even be discarded motor oil (Grob et al.,
1991a, 1991b; Moret et al., 1997). The contamination was identified by
analysing the PAH profile (Speer et al., 1990; Moret et al., 1997). Strict
rules have been suscribed and are now adhered by bag manufacturers, who
generally mark by a tag sacks prepared according to GMP.
3.11.11.3 Pesticide residues
I. Ko¨ lling-Speer and K. Speer
Due to the protective layers covering the beans in the fruit (skin, pulp and
parch), the level of pesticides present in green coffee beans is very low.
Cetinkaya (1988) could not detect either organochlorine or organophosphorus
pesticide residues in 50 green coffee samples imported from 11
different countries.
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178 Espresso Coffee
CHAPTER4
Roasting
B. Bonnla¨ nder, R. Eggers,
U.H. Engelhardt and H.G. Maier
4.1 THE PROCESS
R. Eggers
At first sight, roasting of coffee seems to be a well-known and simple
process: It is just the application of heat to raw coffee beans. What is
important is to generate and control the correct temperatures at the right
moment, then stop the process when the aroma has fully developed and
the colour of the coffee is homogeneous throughout the whole bean.
However, on closer inspection, questions arise that have not yet received
an answer: the dependency of the instationary (changing with the time)
temperature distribution in the coffee bean on the parameters governing
the process, such as roast gas temperature, fluid flow conditions and
material properties of the coffee bean. The difficulty in seizing the whole
process comes from the dramatic changes in nearly all the parameters
related to the process: the temperatures, the material properties and
the geometry of the beans. Figure 4.1 demonstrates the meaning of
temperature distribution in a coffee bean during roasting.
The bean has a finite geometry of complicated shape; its inner structure
is heterogeneous; by heat admission its volume swells and the inner
structure changes. Mathematically, temperature becomes a three-dimensional
function of non-steady-state character with unknown moving
boundaries. From a chemical-engineering point of view the roasting
process consists of a combined heat and mass transport superposed by
endothermic and exothermic reactions. Thus the application of heat to the
coffee beans not only generates a temperature field, it also causes inner
pressures and a re-distribution of moisture depending on time and location.
These effects are illustrated in Figure 4.2: Heat energy is admitted to
the surface of the whole green bean, mainly by external hot gas flow, with
additional radiation and contact heat transfer depending on the type of
roaster.
The temperature of the bean surface increases – with heat conduction
into the porous material – due to the temperature gradient. When the
local temperature reaches the evaporation temperature of the bean
moisture, a front of evaporation starts moving towards the centre of the
bean. During this first part of the roasting process the walls of the whole
bean are still relatively firm. Thus the vapour that has been generated
cannot permeate and the pressure build-up makes the bean volume
expand. Evaporation of the bean moisture, being an endothermic
operation, needs latent energy leading to a slowed down kinetic in the
temperature rise within the bean. The swelling and drying result in a
strong decrease in heat conductivity within the section between the
vaporization front and the outer surface of the bean. As a consequence the
temperature gradient is steeper in the dried region of the bean because of
the enhanced resistance to heat transfer. Mechanical and thermal stresses
moving towards the centre of the bean are created, which make the beans
crack or even burst if the superposed stresses overcome the tensile
strength of the bean.
Roasting reactions – browning with formation of flavour compounds at
elevated inner pressures – begin at higher temperatures (T>160
C) –
180 Espresso Coffee
Figure 4.1 Roasting of coffee beans – main aspects
starting at the bean surface and moving towards the inner dry preexpanded
structure of the bean. This second front of moving latent heat is
exothermic. Gaseous reaction products – mainly carbon dioxide – are
generated and entrapped within the cell structure, increasing inner
pressure until they permeate through the walls that are weakened and
partly destructed by the high temperatures as well (Perren et al., 2001).
The roasting process is then a counter-current process with heat transport
inside and mass outside. The phenomenological temperature profile
(Figure 4.2) shows that the distance between the two moving zones of
evaporation and roasting depends on the outer heat transfer, given by the
roasting technique applied, and on the structure of the bean. The latter is
linked with the geographical origin of the coffee to be roasted (Figure 4.3).
In order to achieve a roasting profile as homogeneous as possible, the
process must be precisely controlled, aiming at small temperature
gradients throughout the bean; on the contrary, fast roasting leads to an
overlapping of the evaporation and roasting steps and to an inhomogeneous
profile.
When the desired degree of roast is reached, the beans have to be
cooled down rapidly by water quenching or cold air in order to stop
further changes in colour, flavour and volume. The roasting loss is either
Roasting 181
Figure 4.2 Assumed profiles in the coffee bean during roasting
measured by the dry mass loss (the weight loss based on dry green beans)
or by the total mass loss (moisture plus organic matter loss). Table 4.1
shows the correspondence between roasting degree and dry mass loss
(Clarke, 1987). During roasting the density of the beans decreases to
nearly half its initial value.
The shape of a coffee bean can be defined as half an ellipsoidal body.
Attempts at modelling the temperature field of a coffee bean during
roasting have been made. The influence of the endothermic vaporization
enthalpy as well as of the exothermic reaction enthalpy is shown in
Figure 4.4. In the comparison the temperature increase has been
182 Espresso Coffee
Figure 4.3 Volume expansion for coffees of different origin
Table 4.1 Material data of arabica coffee beans
Coffee Mass Moisture
Roasting
loss
Organic
loss Density Volume Radius* Porosity
(g) (wt%) (wt%) (wt%) (g/ml) (ml) (mm) ( )
Green 0.15 10–12 0 0 1.2–1.4 0.11–0.13 3 <0.1
Medium
to dark
roast
0.13 2–3 15–18 5–8 0.7–0.8 0.16–0.19 3.5 0.5
*Of a sphere with the same volume.
calculated without taking into consideration latent energies. The results,
when latent energies are introduced, indicate a delayed temperature rise
up to 150
C caused by vaporization followed by an accelerated
exothermic temperature increase above 180
C. Additional measurements
of three temperatures – at the surface, between core and surface and in the
core itself – show some remarkable aspects of roasting in a roasting
experiment lasting 600 seconds (Figure 4.5).
Starting at ambient conditions all three temperatures increase with
declining slope until nearly constant temperature has been reached.
During the first period (0–50 sec) of heating up the bean the difference
between core and surface temperatures passes a maximum of at least 70
degrees, whereas the temperature difference between core and half
distance to the surface of the bean slowly increases up to a maximum of
approximately 10 degrees throughout nearly the whole roasting time.
Obviously, the roasting process is much faster in the outer sections of the
bean compared to the core, probably leading to high thermal stresses in
the outer parts of the bean.
A small temperature drop occurs suddenly after 200 seconds’ roasting
time. At this time the inner temperature differences are clearly higher in
comparison to the outer temperature differences. However, the temperature
course drops slightly simultaneously throughout the whole bean.
After this unsteady phenomenon, temperatures increase more rapidly
than before! This could be explained by a pressure built up in the centre
Roasting 183
Figure 4.4 Temperature calculation of a roasting bean
of the bean due to water evaporation starting from the surface towards the
central region, but proceeding with decreasing velocity because the
driving force of heat transfer – the temperature difference to the ambient
– is diminishing all the time. So the pressure inside the bean increases
until the temperature overcomes the vaporization temperature, which is
higher at elevated pressure. Thus the remaining water in the centre of the
bean evaporates spontaneously and an endothermic flash creates the
slight temperature drop. Immediately after flashing the full exothermic
roast reaction is enabled to run to the core of the bean.
In order to roast the bean as homogeneously as possible, a stepwise
process with a slow increase of the temperature of the heating gas seems to
be advantageous. Further, a build-up of pressure in the bean is important
for the generation of sufficient aroma. Thus, temperature control of the
heating gas, permitting not only temperature profiles with moderate
differences, but also a sufficient pressure built up inside the bean, has to be
the objective of roasting.
4.2 ROASTING TECHNIQUES
R. Eggers
Conventional roasting techniques have been described by Sivetz and
Desrosier (1979), Rothfos (1984) and Clarke (1987). Recently, newer
184 Espresso Coffee
Figure 4.5 Experimental temperature profiles in the bean
developments have been summarized by Clarke and Vitzthum (2001).
Documentation is also available from manufacturers of roasting equipment,
e.g. Probat Werke at Emmerich and Neuhaus Neotec at Reinbek,
both in Germany.
From an engineering standpoint, the principles of roasting can be
described from a mechanical, thermal and operational point of view
(Figure 4.6).
In the case of traditional roasters, a tendency towards a larger variety in
coffee products has led to smaller uniform charges and therefore the
demand for large continuous roasters is decreasing in the roast and ground
market. Due to modern control techniques, very consistent products can
be obtained with batch roasters, where the heat input can be varied over
time. Roasters operating under pressure and steam roasters are unavailable
commercially, in spite of ongoing research.
Table 4.2 summarizes the basic principles of modern roasting
technology: a forced convective flow of hot gases passes through a
moving bed of coffee beans. The motion of the beans is either produced
by rotation or by the flow of roasting gases. Hot gas is either produced by
gas or by oil burners.
Due to increasing cost of energy and environmental considerations,
modern roasting equipment usually includes re-circulation of the exhaust
gas, after removal of the solid particles it carries (chaff, dust) by retention
on a cyclone. The gas is either brought back to the burner, or to a thermal
afterburner, operating at temperatures between 400 and 600
C, to burn
Roasting 185
Figure 4.6 Principal roasting techniques
186 Espresso Coffee
Table 4.2 Basic principles in modern roasting technology
Type Principle Characteristics
Rotating
cylinder
Horizontal/vertical
With/without perforated walls
Direct heating by convective flow of
hot gases
Indirect heating by hot drum walls
Batch-operated
Continuously operated by an inner
conveyer
Gas temperatures: 400–550
C
Roasting times: 8.5–20 min
Bowl Direct heating by convective flow of
hot gases
Continuously operated across the gas
stream; rotating
Gas temperatures: 480–550
C
Roasting times: 3–6 min
Fixed
drum
Direct heating by convective flow of
hot gases
Batch operated
Gas temperatures: 400–450
C
Roasting times: 3–6 min
Fluidized
bed
Direct heating by fluidizing gas
Batch operated
Gas temperatures: 240–270
C
Roasting times: 5 min
Spouted
bed
Direct heating by fluidizing gas
Batch operated
Fast roasting:
Gas temperatures: 310–360
C
Roasting times: 1.5–6 min
Slow roasting:
Gas temperatures: 230–275
C
Roasting times: 10–20 min
off both the residual particles from above 1600 to well below 50 mg/Nm3
and the volatile organic matter (aerosols, condensate, etc.) present, as
required by law in some countries. Its residual thermal energy can be used
to preheat incoming fresh air (Illy and Viani, 1995, p. 94).
4.2.1 Energy balance and heat transfer
The investigation of the modes of heat transport from the heating gas to
the surface and inside the bean is rather complex, hence only approximate
calculations are possible. Nevertheless, the heat transfer theory helps one
to understand the roasting process. Assuming spherical coffee beans with
an average and constant diameter of 6mm, the tendency of the
convective heat transfer can be calculated for a given gas temperature.
The heat transfer coefficient is shown versus the Reynolds number Re
in Figure 4.7. In a motionless system (vrel . 0), the heat transfer
coefficient is around 14W/m2K and a minimum fluidization velocity
(e.g. Re . 300) causes -values in the range of 75W/m2K.
The heat transfer to single beans is different from the heat transfer to a
bed of beans. The calculated values are higher than those for single
spheres at the same superficial velocity and the actual roasting process is
expected to show heat transfer values in the region between the two
curves. Two important consequences arise: on the one hand, only a slight
increase in heat transfer is possible at higher velocities; on the other hand,
the improvement of the outer heat transfer is limited due to the heat
transfer resistance of the inner bean structure.
The amount of heat energy Q (KJ) transferred to the bean can be
calculated with the following equation:
Q .
Z
Acs.t. Tg
Tcs.t.
dt .4:1.
where:
Acs(t) surface area of the coffee bean
(increasing during roasting)
Roasting 187
Swirling
bed
Tangential gas inlet
Spiral upward motion of the beans
Direct heat transfer of a moved packed
bed
Gas temperatures: 280
C
Roasting times: 1.5–3 min
Tcs (t) surface temperature of the coffee bean
(increasing during roasting)
convective heat transfer coefficient
Although the packed bed of coffee beans reveals the highest heat
transfer coefficient, the advantage of roasting using fluidizing gases
becomes obvious from equation 4.1. The surface area Acs of the fluidized
beans is in intimate contact with the hot gases and, as a result, the heat
transfer is more effective and homogeneous.
Figure 4.8 shows the balance of energy flow of a roasting section
operated continuously, where:
_M
M mass flow
c heat capacity
T temperature
Index g gas
Index c coffee
Index in input
Index out output
The enthalpies of vaporization, hv, and exothermic reaction, hex,
have to be considered respectively as heat sinks and sources, in balancing
188 Espresso Coffee
Figure 4.7 Heat transfer of coffee beans during roasting
the energy flows. There is a further heat loss _QQloss from the roaster to the
atmosphere.
_M
Mg;in
cg;in
Tg;in
. _MMc;in
cc;in
Tc;in
. hex
hv
. _MMg;out
cg;out
Tg;out
. _MMc;out
Tc;out
. _QQloss
.4:2.
The figures for latent energy are not yet very well known.
By neglecting the difference between _MMg;in and _MMg;out that is due to the
development of roast gases and flavour, and keeping the specific heat of
gas constant, the specific energy demand q.kJ=kg roastcoffee becomes:
q .
_M
Mg
cg Tg;in
Tg;out
. _QQloss
h i
_M
Mc;out
.4:3.
In the case of batch roasting, the temperature of the gas at the outlet of
the roaster increases with time and the specific roasting energy needed
can be calculated by integration:
q .
_M
Mgcg
R
Tg;in
Tg;out.t.
. _QQloss
h i
dt
_M
Mc;out
.4:4.
A comparison of the equation representing the energy demand with
the heat transfer equation indicates that the roasting process to be
energetically optimized must be operated with small temperature
differences and high heat transfer coefficients, meaning low temperatures
and high velocities of the hot gases.
Roasting 189
Figure 4.8 Balance of energy flow
4.2.2 High yield or fast roasting
Although processing at low temperatures has the advantage of a
homogeneous roast in favourable energetic conditions, there is an
alternative for enhanced heat transfer related to equations (4.1) and
(4.4): the increase of the gas temperature Tg and the hot gas to coffee
ratio.
Fast roasting, i.e., providing the thermal energy required for roasting in
a shortened period of time, down to 90 seconds or even less (60 seconds
has been indicated as the shortest roasting time achievable now), has
been made possible by the development of forced convection roasting at
temperatures of 300–400
C. Because of the poor thermal conductivity of
the bean, there is a roasting gradient within the bean (particularly
noticeable at low roast temperatures), bean volume is increased with a
characteristic puffing by 10–15% with respect to conventional roasting,
bulk density is reduced below 300 g/l and extraction yield at brewing is
increased by 20%.
Summarizing, the profile of bean temperature variation during roasting
is an important parameter.
Both bean temperature and roasting time depend strongly on heat
transfer and therefore on the technology applied. The results of the LTLT
(Low Temperature Low Time) and HTST (High Temperature Short
Time) processes integrate into a slightly increasing function. Personal
measurements (Clarke and Vitzthum, 2001, p. 93) are in good agreement
with the literature data. Data on the variation of bean temperature during
roasting have been reported by Da Porto et al. (1991) and by Schenker et
al. (1999). The latter authors roasted 100 g of arabica beans as a fast
fluidized bed in a hot air flow of 0.01885m3/s in two ways: first the socalled
LTLT roasting with a hot gas temperature of 220
C and a roasting
time between 9 and 12 minutes, then HTST roasting at a gas temperature
of 260
C and roasting time between 2.6 and 3 minutes. In the case of
LTLT roasting, the bean temperature and other parameters were given as
a function of roasting time: the temperature rose continuously from 20 to
190
C in 2 minutes; this increase corresponds to 90% of the final increase
(211
C after 14 minutes). The bean temperature data published by Da
Porto et al. (1991) were obtained with a laboratory roaster (HTLT) and
Brazilian Santos coffee. It can be seen that, in comparison with
this traditional laboratory roaster, a fluidized bed roaster produces a
significantly faster increase in bean temperature. However, because of the
poor thermal conductivity of the bean, there is a gradient of roasting level
within the bean, particularly noticeable at low roast; the volume is
increased by 10–15% with respect to conventional roasting with a
190 Espresso Coffee
characteristic puffing of the beans and a bulk density lower than below
300 g/l, and extraction yield at brewing 20% higher.
High yield roasting is not considered optimal for Italian espresso, owing
to the high residual content of chlorogenic acids, which carries into the
cup an astringent sour note (Maier, 1985; Illy and Viani, 1995, p. 96).
4.3 CHANGES PRODUCED BY ROASTING
H. G. Maier
4.3.1 Physical changes
The main transformations occurring in the bean with increasing
temperature are given in Table 4.3, while the most important macroscopic
changes produced by roasting are indicated in Table 4.4 (Pittia et
al., 2001). The roasted coffee bean becomes brown to black, very brittle,
with an increase in volume up to 100% (dark roasting) and a corresponding
decrease of density (300–450 g/l in contrast to 550–700 g/l in
raw beans) (see Figure 4.10), with many macro- and micropores (Schenker
et al., 2000), the cellular structure ruptured, especially upon espresso
roasting. The water content should be near 1%, if no water quench is
applied.
Roasting 191
Figure 4.9 Roasting loss versus bean temperature – different processes
4.3.2 Chemical changes
4.3.2.1 Overall changes
As shown in Table 4.3, during roasting, water (the moisture water of the
green beans and that generated by reactions) and carbon dioxide escape.
They are accompanied by some carbon monoxide and organic volatiles.
Water and carbon dioxide are generated by the very important Maillard
reaction, which leads to the coloured products, the melanoidins, and to
the main part of the organic volatiles. In addition, water and carbon
dioxide are produced by numerous other pyrolytic reactions. Table 4.4
summarizes the macroscopic changes occurring throughout the roasting
operation, and Figure 4.11 gives an idea of the effect on the contents of
the main groups of coffee constituents. The values for roasted coffee are
calculated on green coffee dry basis, in order to make a better comparison
of the changes. The changes within the main groups will be dealt with in
the following paragraphs.
4.3.2.2 Carbohydrates
Of the mono- and disaccharides of green coffee, after roasting only traces
of free sugars remain. Sucrose is supposed to be partially hydrolysed, the
rest being pyrolysed (caramelized). From the reducing sugars or their
192 Espresso Coffee
Table 4.3 Macroscopic changes during roasting
Temperature change within
the bean (
C) Effect
20–130 Liquid-vapour transition of water (bean drying). Colour
fades
130–140 First endothermic maximum. Yellow colouring and
swelling of bean with beginning of non-enzymatic
browning. Roast gases are formed and begin to evaporate
140–160 Complex series of endothermic and exothermic peaks.
Colour changes to light brown. Large increase in bean
volume and micropores. Rests of silverskin are removed.
Bean is very brittle. Some little fissures at the surface
occur. Aroma formation starts
160–190 Roasting reactions move towards the inner dry structure
of the bean (see also Figures 4.2 and 4.4)
190–220 Micro fissures inside the bean. Smoke escapes. Large
volumes of carbon dioxide escape and leave the bean very
porous. Typical flavour of roasted coffee appears
fragmentation products, many volatiles (aroma compounds, volatile acids)
and non-volatiles (melanoidins and their precursors, acids) are formed by
the Maillard reaction and, to a lesser extend, by caramelization. The
Maillard reaction has lower activation energy and is therefore favoured if
reactive nitrogen compounds (amino acids, free amino groups in proteins
and peptides) are present.
Roasting 193
Table 4.4 Macroscopic changes brought about by roasting
Parameter Comment
Colour of bean It fades to whitish-yellow at the beginning of roasting, it darkens
regularly with increasing temperature (see Figure 4.10); natural
arabicas and robustas need a higher final temperature to reach the
same colour as washed arabicas. There is a dark to light colour
gradient from the surface within the bean, particularly noticeable
with low-roasted fast-roasted coffees
Surface of bean Oil sweats to the surface rendering it brilliant, particularly at high
roasting levels
Structure of the
bean
The large volumes of carbon dioxide freed render the bean porous
(see Figure 4.10c)
Brittleness It increases with roasting to a maximum, with an important
modification of the internal texture of the bean, which under the
microscope looks like a volcanic land covered with an amalgam of
the original constituents (see Figure 5.1)
Density Decreases regularly from 550 to 700 g l
1 in raw beans to
300–450 g l
1 in roasted beans (the lowest figures are attained with
fast-roasted coffee)
Hot water
extract
It decreases slightly from green to light-roasted beans, then
increases again slightly with increasing roasting degree (highest with
fast-roasted coffee)
Moisture Both free water, which decreases regularly all through roasting, and
chemically bound water, liberated above 100
C, decrease to below
1% unless water quenching is applied (moisture release is less
efficient with fast-roasted coffee)
Organic losses Destruction of carbohydrates, chlorogenic acids, trigonelline, amino
acids becomes important above 160
C, varying between 1 and 5%
at the lightest commercial roasts, 5–8% for medium-roasted
coffees, to above 12% for dark-roasted ones (lower for fast-roasted
coffee)
Release of CO2 continues for several days
Volatile
constituents
Aroma content reaches a maximum at low roasting, while
destruction becomes more important than generation at medium
roasting losses (aroma generation is higher in fast-roasted coffees,
where a maximum is reached at medium roast)
pH of beverage In washed arabicas it increases from 4.9 for low-roasted coffees to
5.4 for dark-roasted one; higher for dry-processed coffees (lower
values for fast-roasted coffee)
The polysaccharides except cellulose are partially solubilized (Bradbury,
2001; Redgwell et al., 2002). Nevertheless the sum of soluble carbohydrates
is lower in roasted coffee. In espresso, the foam stability is related to
the amount of galactomannan and arabinogalactan and a function of the
degree of roast (Nunes et al., 1997). The carboxyatractyloglycosides are
decarboxylated quantitatively and the so generated atractyloglycosides
are degraded about 50% under ‘normal’ roasting conditions (Bradbury,
2001).
4.3.2.3 Non-volatile lipids
Overall, there are only slight changes in the lipid fraction upon roasting.
The sterols and most of the triglycerides remain unchanged. The level of
194 Espresso Coffee
Figure 4.11 Main families of coffee constituents (average of arabica coffees)
Figure 4.10 Stereo microscope section of a bean: (a) green; (b) toasted to c.70
C;
(c) roasted, showing the porous structure
trans fatty acids increases, especially the contents of C18:2ct and C18:2tc.
The linoleic acid content decreases slightly with roasting temperature.
The diterpenes, cafestol and kahweol, are decomposed to some extent.
Increasing with roasting temperature, dehydrocafestol, dehydrokahweol,
cafestal and kahweal are formed (0.5–2.5% of each educt). 16-Omethylcafestol
(in robusta beans) is less affected. Up to 20% of the
tocopherols and 25–50% of the carbonic acid 5-hydroxytryptamides are
destroyed (Wurziger and Harms, 1969; Wurziger, 1972; Speer and
Ko¨lling-Speer, 2001; Kurt and Speer, 2002).
4.3.2.4 Proteins, peptides and amino acids
Crude protein content (Kjeldahl nitrogen multiplied by 6.25) changes
only very slightly upon roasting, but the nitrogenous components are
thoroughly changed. There is a loss of amino nitrogen of 20–40%, in dark
roasted espresso, about 50% (Thaler and Gaigl, 1963; Macrae, 1985).
Essentially all of the protein in the green coffee is denatured (Macrae,
1985). Some cross-bonds between the amino acid residues of the proteins
are formed (Homma, 2001). Parts of the L-amino acids are isomerized to
form D-amino acids, in commercial blends 7–70%, depending on the
individual acid (Nehring, 1991). The amino acid composition is changed
too: the more stable acids, e.g. glutamic acid, survive while others, e.g.
cysteine or arginine, decrease or are completely destroyed (Macrae, 1985).
In espresso, the foamability depends on the amount of protein in the
infusion and on the degree of roast (Nunes et al., 1997). Of the free amino
acids, only traces are left after roasting. The reaction products are
Maillard products (melanoidins, their precursors and volatiles) and
dioxopiperazines, of which the proline-containing ones are bitter (Ginz,
2001). Some intact amino acid residues are incorporated into the
melanoidins (Maier and Buttle, 1973).
4.3.2.5 Chlorogenic acids
Since chlorogenic acids are the most important group of acids in green
coffee, they will be treated separately. These acids are for the most part
destroyed during roasting (Figure 4.12). In addition, some intact
chlorogenic acids may be bound by melanoidins (Heinrich and Baltes,
1987). It is assumed that the greatest part of chlorogenic acid is
hydrolysed. In addition, isomerization and, to a smaller extent, lactonization
takes place. The 6 lactones of the mono-hydroxycinnamoyl-quinic
acids are assumed to contribute to the bitter taste of roasted coffee (Ginz,
Roasting 195
2001). The 3 di-hydroxycinnamoyl-quinides are supposed to modulate the
effect of caffeine (Figure 4.12) (Martin et al., 2001) (see Chapter 10).
The free quinic acid is also isomerized (5 isomers), forming 4 -lactones
and 3 -lactones (Scholz-Bo¨ttcher et al., 1991; Homma, 2001). A little
part of this acid is decomposed to form simple phenols like hydroquinone.
The greatest part, however, remains unchanged, so that in dark roasts,
quinic acid is the most prevailing of the individual low molecular acids.
Little is known about the fate of caffeic acid. A minor part degrades to
form simple phenols. Two tetra-oxygenated phenylindans have been
found in model roasting of caffeic acid as well as in coffee at levels of
10–15 mg/kg, and neolignans (caffeicins) only in model roastings
(Clifford, 1997). The rest is assumed to be incorporated into the
melanoidins (see 4.5).
4.3.2.6 Other acids
Most of the organic acids present in green coffee behave like the
chlorogenic acids: they are partially decomposed, e.g. citric acid to form
citraconic, itaconic, mesaconic, succinic, glutaric acids and eventually
some minor acids, malic acid to form fumaric and maleic acids (Scholz-
Bo¨ttcher, 1991; Ba¨hre and Maier, 1999). It is likely that esters of these
acids are formed upon roasting, because the acids are liberated on heating
the brew (Maier et al., 1984). Scholz-Bo¨ttcher (1991) detected a fumaric
acid/malic acid ester and another malic acid monoester after roasting of
196 Espresso Coffee
Figure 4.12 Quinides formed during roasting
malic acid. Phosphoric acid is stable, and its content increases by
hydrolysis of the inositol phosphates (Franz and Maier, 1994). A great
number of acids is generated by Maillard reaction or caramelization. The
most prominent are formic and acetic acids. Their contents reach a
maximum at medium roast; upon darker roasting volatilization prevails
over formation. Moreover (in decreasing order of the contents in
espresso), glycolic, lactic, metasaccharinic, threo-and erythro-3-deoxypentonic,
glyceric, 2-furanoic and several minor acids are formed (Ba¨hre
and Maier, 1999).
4.3.2.7 Minerals
With the exception of phosphoric acid (see 4.3.2.6), the mineral content
does not change upon roasting.
4.3.2.8 Alkaloids
Caffeine is stable upon roasting, but a small part is lost by sublimation.
Nevertheless this is often overcompensated by the roasting loss.
Trigonelline is partially decomposed. The remaining content is about
50% in a light roast, traces in a very dark roast (Viani and Horman, 1975;
Stennert and Maier, 1996). The products of model roastings are nicotinic
acid, N-methyl nicotinamide, methyl nicotinate and numerous volatile
nitrogen compounds, e.g. 46% pyridines and 3% pyrroles (Viani and
Horman, 1974). Most are found in coffee, especially nicotinic acid, which
constitutes only 1.5% of the degraded trigonelline, but nevertheless acts
as an important source of vitamin. The trigonelline/nicotinic acid ratio
can be used for a rough estimation of the degree of roast (Stennert and
Maier, 1996).
4.4 VOLATILE AROMA COMPOUNDS
B. Bonnla¨ nder
Green coffee contains about 300 volatiles (Flament, 2001). The content
of some of them, e.g. 3-isobutyl-2-methoxypyrazine, is not changed by
roasting. The content of others, e.g. ethyl-3-methylbutyrate, is diminished,
but most of the volatiles increase upon roasting (Czerny and
Grosch, 2000; Flament, 2001). In addition, about 650 new volatiles have
been identified, bringing to more than 850 the number of volatiles
identified in roasted coffee.
Roasting 197
4.4.1 Generation of roast aroma
Most of the delightful aromatic character of coffee is the result of the
roasting process. Green coffee shows a typical green bell pepper-like
aroma, where isobutylmethoxypyrazine (MIBP) could be identified as the
character impact compound with a ‘peasy’ smell (Vitzthum et al., 1976).
Only part of the more than 300 additional green coffee aroma compounds
identified so far (Flament, 2001) survive the roasting process. The high
temperature (usually 170–230
C for 10–15 min) and the elevated
pressure inside the bean (up to 25 atm) trigger a vast number of chemical
reactions leading to dark colour and more than 1000 volatile and nonvolatile
compounds identified in roasted coffee (Stadler et al., 2002b). In
contrast with green coffee, no single compound but a mixture of approx.
25 very potent compounds represents the significant impression of coffee.
The aroma compounds (approx. 1 g/kg) are concentrated in the coffee oil.
The concentrations of the most potent ones are in the lower ppm (part
per million) or even ppt (part per trillion) range.
4.4.2 Precursors of aroma compounds
The non-volatile constituents play an important role as precursors in the
formation of the volatile compounds in roast coffee. Along with the rising
temperature, and the subsequent drying and swelling of the bean, the
following reactions take place under elevated pressure conditions:
n caramelization and degradation of carbohydrates forming mainly
aldehydes and volatile acids (Yeretzian et al., 2002);
n denaturation of proteins and reaction of free amino acids with
carbohydrates and their degradation products (Maillard reactions);
n production of phenols and taste active compounds from chlorogenic
acids (Pypker and Brouwer, 1970);
n degradation of trigonelline (Stadler et al., 2002a).
According to the different precursor compositions of the two main coffee
species, also a different aroma is observed. The aroma of Coffea canephora
(robusta) is characterized by higher amounts of phenolic compounds
(guaiacol, vinylguaiacol), perceived as harsh earthy notes, originating
from the chlorogenic acids (Vitzthum et al., 1990). Another interesting
class of aroma precursors is that of the glycosides. They can liberate the
bound aroma compounds enzymatically, during post-harvest treatment or
roasting (Weckerle et al., 2002), and are of special interest also in other
foods as aroma storage forms. The common precursor prenylalcohol is a
198 Espresso Coffee
source for sulphur odorants as shown by model reactions with sulphurcontaining
amino acids (Holscher and Steinhart, 1992).
4.4.3 Identification and characterization of aroma
compounds
Pioneer research on coffee aroma was already performed before 1926
when the Nobel Prize winners Reichstein and Staudinger succeeded in
identifying the first aroma compounds like furfurylthiol and the guaiacols
well before the discovery of gas chromatography (GC). After a period of
identifying more and more compounds by GC-mass spectrometry (GCMS)
the current trend in research is to identify the active-smelling ones
by GC-olfactometry (GC-O) in combination with the chemical structural
information obtained from GC-MS. Generally, the procedure requires
extraction and enrichment of the aroma compounds from the natural
material, followed by chromatographic fractionation, and finally the
authentic identification.
4.4.3.1 Extraction procedures
The extraction procedure has to separate the volatile compounds from the
coffee matrix. A very important question is how representative is the
extract, or, in other words, how to avoid artefact formation during
extraction and enrichment (Sarrazin et al., 2000). Several techniques are
widely known in literature, such as simultaneous distillation extraction
(SDE) and high vacuum distillation (HVD) or solvent-assisted flavour
extraction (SAFE, Engel et al., 1999). In addition, the volatile compounds
in the headspace above ground or brewed coffee can be analysed by
headspace (HS) techniques either statically or in dynamic mode. Solid
phase micro extraction (SPME) can be operated without solvent in fully
automated manner either from the brew (IS–SPME) or from the
headspace (HS–SPME). Increasing the amount of stationary phase
dramatically improves the detection limits and is used as stir bar sorptive
extraction (SBSE) in the brew or in the headspace (HSSE), reported by
Bicchi et al. (2002). As the aroma compounds tend to be very volatile,
extraction losses are compensated by adding stable isotope-labelled
reference compounds.
4.4.3.2 Instrumental sensory analysis
GC in combination with olfactometric detection (GC-O) helps to detect
potent odorants without knowing their chemical structure. The effluent
Roasting 199
of a gas chromatograph is split to a conventional detector and a heated
sniffing port, where trained people evaluate and record the sensory
impression of individual compounds (Holscher et al., 1990). Dilution
techniques are used to determine the so-called flavour dilution (FD)
factors. By a stepwise dilution of the aroma extract (1:1 by volume)
followed by GC-O analysis the most important contributors can be
smelled at the highest dilution, thus getting the highest FD factors. This
technique, developed by Ullrich and Grosch (1987), is known as aroma
extract dilution analysis (AEDA).The odour activity value (OAV) can be
expressed as the concentration divided by its threshold only if the
structure and the odour threshold of a substance are known. A method for
the prediction of OAV from FD factors is reported as well as the precision
and the optimal design of AEDA (Ferreira et al., 2001).
4.4.4 Aroma impact compounds in roasted coffee
In arabica coffee the most important contributors to the aroma of roast
and ground coffee are determined by the techniques mentioned above.
3-Mercapto-3-methylbutylformate (MMBF), 2-furfurylthiol, methional,
-damascenone as well as two pyrazines and furanones show the highest
FD factors (Holscher et al., 1992). A comparison of the results of the
major research groups in coffee aroma is given in Table 4.5, together with
the aroma impressions of the substance.
The absolute amounts of the aroma compounds shown in the last
column differ by two orders of magnitude. The potency, especially that
of the sulphur compounds, is demonstrated by their low concentrations.
Because of a very low odour threshold (in air), only 130 mg/kg of MMBF
are sufficient to generate the strongest odour impression, which is more
than 10 000 times over its threshold of 0.003 ng/l of air (OAV of
37 000).
4.4.5 Effects on cup impression
Everybody who has ever had a really bad cup of coffee in direct
comparison to a perfect one knows how big the difference can be. The
perceived quality depends on objective criteria such as quality of green
coffee, but also on subjective or cultural preferences like type of
preparation.
200 Espresso Coffee
Roasting 201
Table 4.5 Aroma impact compounds in roasted coffee powder
Odorant Odour impression Holscher1 (FD) Grosch2 (FD) Schenker3 (FD) Mayer4 (lg/kg)
Methanethiol Putrid, cabbage-like 25 – 4500
2-Methylpropanal Pungent, malty 100 – 24 000
2-Methylbutanal Pungent, fermented 100 16–128 28 600
2,3-Butanedione Buttery 200 – 256–1024 55 700
2,3-Pentanedione Buttery 100 – 4–128 28 300
3-Methyl-2-buten-1-thiol Animal-like, skunky 200 – 64–256 13
2-Methyl-3-furanthiol Roasted meat-like 500 128 32 60
Mercaptopentanone Sweaty, catty 100 – – –
2,3,5-Trimethylpyrazine Roasty, musty 200 64 16–32 6000
2-Furfurylthiol Roasty, coffee-like 500 256 1024 1350
2-Isopropyl-3-methoxy-pyrazine Peasy 100 128 – –
Acetic acid Vinegar-like 100 – – –
Methional Cooked potato 500 128 1024 148
2-Ethyl-3,5-dimethyl-pyrazine Roasty, musty 200 2048 1024 55
(E)-2-Nonenal Fatty 5 64 – 100
2-Vinyl-5-methyl-pyrazine Roasty, musty 200 – 53
3-Mercapto-3-methyl-butylformate Catty, roasted coffee-like 500 2048 1024 130
2-Isobutyl-3-methoxy-pyrazine Paprika-like 500 512 4–64 84
5-Methyl-5H-6,7-dihydro-cyclopentapyrazine Peanut-like 50 – –
2-Phenylacetaldehyde Honey-like 25 64 – 2500
3-Mercapto-3-methylbutanol Soup-like 100 32 – –
2/3-Methylbutanoic acid Sweaty 500 64 1024 25 000
(E)- -Damascenone Honey-like, fruity 500 1024 16–128 258
Guaiacol Phenolic, burnt 200 – 512–1024 3420
4-Ethylguaiacol Clove-like 25 256 – 1780
4-Vinylguaiacol Clove-like 200 512 256 45 100
Vanilline Vanilla-like – 32 – 4050
Peaks sorted by retention time on DB-wax column. Compounds with FD 32 are reported.
Sources: 1Holscher et al. (1990); 2Czerny and Grosch (2000); 3Schenker et al. (2002), range according to roasting isothermal high/low temperature; 4Mayer
and Grosch (2001)
4.4.5.1 Green coffee quality
Correlations of geographical origins to the cup impression can be
recognized by a trained cup taster, but chemical mapping is also possible.
Modern statistical methods (e.g. analysis of variance, ANOVA) help to
map the composition of the aroma against the growing region (Freitas and
Mosca, 1999). Even a mapping against different growing conditions (e.g.
shade or sun) is possible (Bonnlaender, Unpublished results). Next in the
chain of production are the harvest and the post-harvest treatments of the
cherries. Errors in handling can lead to mould growth or other defects (see
2.3.6 and 3.7).
4.4.5.2 Roasting
Older studies compared the composition of the volatiles extracted at
different degrees of roasting (Gretsch et al., 2000). The use of modern fast
analysis tools, such as resonance enhanced multiphoton ionization timeof-
flight mass spectrometry (REMPI/TOFMS), makes online monitoring
of the roasting process possible; a continuous monitoring with sampling
rates up to one per second can be performed showing the evolution
characteristics of specific compounds (Yeretzian et al., 2002). Generally,
most of the aroma compounds are generated at medium roast. Some
aroma compounds degrade at higher temperatures, whereas others like the
guaiacols, furfurylthiol and pyridine, show an increase up to very high
roasts; they contribute particularly to the aroma of dark roasted coffees
(Mayer et al., 1999).
4.4.5.3 Preparation
During the preparation of the beverage the flavour compounds are
extracted from roast and ground coffee as a function of their solubility in
water. Whereas large amounts of 2,3-butandione, 2,3-pentandione and
the furanones get extracted from coffee, the yield of less polar compounds
like furfurylthiol is reduced (Mayer and Grosch, 2001). Figure 4.13 shows
the headspace stir bar sorptive extraction (HSSE) GC-MS analysis of an
espresso versus a drip cup to illustrate differences between the important
Italian beverage espresso and the drip preparation used worldwide.
Addition of milk to the beverage generally reduces the perceived coffee
aroma, increasing the creamy impression. Bucking and Steinhart (2002)
found a characteristic change in sensorial impression, depending on the
type of milk used (skim milk, coffee whitener).
202 Espresso Coffee
4.4.5.4 Staling of coffee (see also 6.1.2 and 6.1.3)
The delicious aroma of fresh roast and ground coffee only lasts for a short
time. Soon after grinding the very fresh, mild, roasty notes diminish and
strong spicy notes appear in the flavour (Mayer and Grosch, 2001). In
particular, the very potent sulphur compounds deteriorate in contact with
air and the ground coffee starts smelling stale after about 10 days. The
structure of the whole bean provides protection to a certain degree by
keeping CO2 gas in its pores. Analytically the ratio of 2-methylfuran to
2-butanone (M/B ratio) can be used as a good freshness indicator, before
lipid oxidation leads to rancid products after several weeks (Arackal and
Lehmann, 1979). Packaging under inert gas (CO2 or N2) with a minimum
of residual oxygen guarantees long shelf life. Freezing also helps, slowing
down staling reactions. After the package has been opened, it is
recommended to store coffee in cool and dry conditions in the dark.
The staling reactions have not been completely explained. Hofmann and
Schieberle (2002) found reduction of sulphur compounds (MMBF,
furfurylthiol) in roast and ground and in liquid coffees by binding to
melanoidins. A radical mechanism is suggested by Blank et al. (2002).
Hydrolysis of chlorogenic quinic lactone leads to liberation in the brew of
the organoleptically undesirable quinic acid.
Roasting 203
Total lon Chromatogram (TIC): HSSE
Total lon Chromatogram (TIC): HSSE
Abundance
800000
600000
400000
200000
0
Abundance
800000
600000
400000
200000
0
Time 8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00
Time 8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00
3-Hexanone
Dimethylsulfide
2-Methyl-2-butenal
1-Methylpyrrole
Pyridine
From stir bar
From stir bar
2-Furfural
2-Furfurylsulfide
2-Furfurylacetate
5-Methylfurfural
2-Furfutylfuran
Furfurylalcohol
2-Acetilfuran
Furfurylmethylether
Methylpyrazine
Dimethylpyrazine
Ethylpyrazine
Figure 4.13 Differences in preparation techniques. HSSE of an espresso (upper
total ion chromatogram) versus a drip coffee (lower chromatogram) analysed by
thermo desorption gas chromatography–mass spectrometry (TDS GC–MS EI)
4.5 MELANOIDINS
U.H. Engelhardt
Melanoidins are coloured (brown) pigments developed during thermal
treatment of foods via the Maillard reaction or by dehydration
(caramelization) reactions of carbohydrates followed by polymerization
(Bradbury, 2001). Basically, melanoidins are formed by interactions
between carbohydrates and compounds, which possess a free amino group,
such as amino acids or peptides. Recently, melanoidins – not only those
from coffee – have attracted lots of interest as regards their occurrence in
foods and the corresponding impact on human health. This is
documented by the fact that the European Community set up a COST
action (COST Action 919: ‘Melanoidins in food and health’) starting in
1999 and ending in 2004. This action includes research on the separation
and characterization of melanoidins and related macromolecules, the
flavour binding, colour, texture and antioxidant properties of melanoidins
and investigation of the physiological effects and fate of melanoidins
(COST, 2002). The foods of relevance specified are coffee, malt, beer,
breakfast cereals and bread.
4.5.1 Chemistry
It can be anticipated that melanoidins are a very heterogeneous group of
compounds as regards molecular mass as well as the chemical and
biological properties. Taking this into account, one may argue that there
is no justification for referring to all those compounds as melanoidins. On
the other hand, is there any alternative? This situation is comparable with
other polymers in foods and beverages, e.g. the thearubigins in tea or
phlobaphenes in cocoa. It is agreed that those are constituents of the
beverage, and contents can be found in tables, but nobody really knows
what the properties and the structures of the compounds are.
What do we know about the formation of melanoidins? Most of the free
mono- and disaccharides are lost during the roasting process; the same is
true for free amino acids (Trautwein, 1987; Bradbury, 2001). They are at
least in part converted into melanoidins. Another part of the sugars
undergoes caramelization reactions, also yielding melanoidin-like pigments
(Bradbury, 2001) or is degraded yielding acidic compounds (Ginz et
al., 2000). A small proportion of the amino acids could be converted into
diketopiperazines (cyclic dehydration products of dipeptides), but,
according to Ginz (2001), it seems to be more likely that the
diketopiperazines are formed via protein degradation. Not much is yet
204 Espresso Coffee
known about the structures of melanoidins. As already mentioned above,
melanoidins are formed during the Maillard reaction along with a variety
of flavour compounds. The Maillard reaction in a food matrix is a
complex process because of the multitude of compounds present.
Consequently, information on the Maillard reaction is best generated
by model experiments with only a limited number of educts present;
which, however, still give rise to a considerable number of volatile and
non-volatile products. Reviews of the Maillard reaction can be found in
Flament (2001, p. 39).
Coffee melanoidins contain phenolic moieties (Heinrich and Baltes,
1987; Homma, 2001), as indicated by experiments using Curie point
pyrolysis high-resolution gas chromatography/mass spectrometry (HRGC/
MS), which gave 97 products, 33 of which were phenols (Heinrich and
Baltes, 1987). The molecular mass of the melanoidin fractions is between
3000 and 100 000 Dalton according to studies using gel chromatography
or HPLC as analytical tools (Homma, 2001). There seems to be more
high molecular weight material in robusta coffee compared to arabica
(Steinhart et al., 1989). Contents of melanoidins in coffee up to 30%
(Bradbury, 2001) or 29.4% (Belitz et al., 2001) are reported in the
literature – in the latter referred to as unknown constituents (colorants,
bitter compounds), while other authors give 15% for brewed coffee and
about 23% for roasted coffee on a dry weight base (Parliment, 2000).
However, it should be stressed that this is just a determination of the
melanoidins by difference and not at all a determination of the
compounds themselves. Regardless what the figures might be, melanoidins
make a relevant contribution in the cup.
Gel filtration chromatography was usually employed to purify the
melanoidin fractions according to the procedure of Maier et al. (1968)
and Maier and Buttle (1973). This fractionation includes extraction with
diethyl ether and adsorption of the water-soluble compounds on
polyamide. The compounds not adsorbed on polyamide were separated
into three fractions using Sephadex G-25. The amount adsorbed gave
two fractions, one of which gave three subfractions. Details can be found
in papers by Maier et al. (1968), Maier (1981) and Macrae (1985). More
recent work usually also employs gel chromatography, but in most cases
without the polyamide step (Steinhart et al., 1989; Nunes and Coimbra,
2001). Basically, coffee is prepared by hot water extraction and defatted
using dichloromethane. The freeze-dried extract is either treated by
ultrafiltration or separated by gel chromatography (Sephadex G-25;
eluent: water) and again freeze-dried (Hofmann et al., 2001; Steinhart et
al., 2001). Hydrolysis was often employed to detect information about the
moieties present in the polymers and sugars, amino acids and phenols.
Roasting 205
An interesting intermediate involved in melanoidin formation has
been recently detected by Hofmann et al. (1999) using EPR and LC-MS.
The compounds are named CROSSPY – (1,4-bis-5-amino-5-carboxy-1-
pentyl)pyrazinium radical cations – which are oxidized yielding diquaternary
pyrazinium ions. Figure 4.14 shows the reaction pathway proposed
by Hofmann and Schieberle (2002), and Hofmann et al. (2001).
Even though the formation of melanoidins is far from being fully
understood, these findings are an important step in gaining knowledge of
206 Espresso Coffee
Figure 4.14 Reaction scheme of melanoidin formation: (a) via CROSSPY radical;
(b) via diquaternary pyrazinium ions
Roasting 207
the mechanisms involved. The amounts of high-molecular mass melanoidins
increased with increasing roasting loss (Ottinger and Hofmann,
2001).
In a study on the interaction of isolated coffee melanoidins and selected
aroma compounds in model experiments Hofmann et al. (2001) detected
a decrease in free furfurylthiol, 3-methyl-2-butene-1-thiol and mercapto-
3-methylbutylformate, which was most significant when the 1500–3000
Da coffee melanoidins were used. In another experiment by Hofmann and
Schieberle (2002) with (2H2)-2-furfurylthiol the spectroscopic data
(2H-NMR, LC-MS) indicated that furfurylthiol reacts with oxidation
products of CROSSPY. The authors proposed that a covalent bond is
formed between these compounds leading to a decrease of the odour
quality of coffee right after brewing (especially as regards the sulphuryroasty
odour quality). Other useful techniques used in melanoidin/
Maillard research are capillary electrophoresis (Borelli et al., 2002; del
Castillo et al., 2002; Re et al., 2002) and matrix-assisted laser desorption
ionization time-of-flight mass spectrometry (MALDI/TOFMS) (Borelli
et al., 2002; Kislinger et al., 2002). Capillary electrophoresis (after a
fractionation by ultrafiltration with a 5000 Da cut-off dialysis membrane)
has been used by Ames et al. (2000) to monitor the development of
coloured compounds in coffee. It can be expected that both techniques
will be used in the characterization of coffee melanoidins.
4.5.2 Physiological effects of melanoidins (see also
Chapter 10)
Maillard reaction products in general may have negative and positive
(among those antioxidant activity) effects (Friedman, 1996). Claims have
been made that the melanoidins do contribute to the antioxidant activity
of coffee brews. Daglia et al. (2000) studied the in vitro antioxidant activity
and the ex vivo protective activity of coffee constituents and stated that the
higher molecular mass fractions had an antioxidant activity while the
lower molecular mass fractions had a protective activity. The antioxidant
properties are related to the degree of roast. Medium roasted coffee had the
most pronounced antioxidative effect (Steinhart et al., 2001). Del Castillo
et al. (2002) used the ABTS . assay to assess the contribution of high and
low molecular compounds to antioxidant activity of coffee brews. The
ABTS test is a spectrophotometric method which is based on the different
colour of 2,20-azino-di-(3-ethylbenzthiazoline-6-sulfonic acid and the
corresponding radical cation, by monitoring at 734 nm the decrease
in ABTS .concentration after addition of the antioxidant; Trolox
(6-hydroxy-2,5,7,8-tetramethylchroman-2-carbon acid) serves as reference.
Again, it was observed that medium-roasted coffee had the highest
antioxidant activity in vitro and that the low molecular mass compounds
had a greater contribution to the antioxidant activity compared to the
high molecular mass compounds. The same group analysed the antioxidant
activity of a coffee model system (consisting of chlorogenic acid, N-acetyl-
1-arginine, sucrose, and cellulose) and found that there was no relationship
between antioxidant activity and colour generation (Charurin et al.,
2002). Morales and Babbel (2002) also used an in vitro system (bleaching of
DMPD .) and found a significant lower antioxidant activity of melanoidins
compared with classical antioxidants such as caffeic or gallic acid. As
with other polymers in foods, there is practically no information available
on the bioavailability of the melanoidins. It seems reasonable to anticipate
that the fractions with different molecular mass also will have a different
bioavailability.
Claims have been made that melanoidins suppress the formation of
n-nitroso amines (Wuerzner et al., 1989). Among other components of
coffee, such as trigonelline, nicotinic and chlorogenic acids, a low
molecular weight coffee melanoidin fraction (1000–3500 Da) contributes
to anti-adhesive properties against Streptococcus mutans adhesive activity
on saliva-coated hydroxyapatite beads, which might contribute to an
anticaries effect (Daglia et al., 2002). As animal data are missing
the authors recommend a careful interpretation of their findings.
Wijewickreme and Kitts (1998) stated a modulation of metal-induced
cytotoxicity by Maillard reaction products from coffee brews.
4.5.3 Summary and outlook
Our knowledge on the chemistry and physiology of coffee melanoidins
and of their impact on the taste of coffee are still fragmentary and far away
from a real structural elucidation. Recently, new information has become
available on one type of melanoidins containing CROSSPY and on their
interaction with sulphur-containing odorants. It might be expected that
our knowledge on melanoidins in general and coffee melanoidins in
particular will increase within the next few years as concentrated efforts
are made in this field of research (e.g. COST action). As soon as
individual structures of melanoidins are known, biological testing of the
compounds becomes possible as well as a more precise evaluation of the
impact of the melanoidins on coffee taste. These results might then
enable roasters to control the formation of melanoidins, which is very
important for the aroma and the overall taste of the coffee beverage.
208 Espresso Coffee
4.6 CONTAMINANTS
H.G. Maier
4.6.1 Mycotoxins (see also 3.11.11.1)
The mycotoxins are largely destroyed upon roasting (Maier, 1991). For
instance, the destruction of aflatoxin B1 in artificially contaminated
green coffee beans during roasting ranges from 90 to 100% (Micco et al.,
1991). Beyond that, in non-mouldy beans, only ochratoxin A has been
found (Maier, 1989). In all the experiments where naturally contaminated
beans, a sampling procedure adapted to mycotoxin inhomogeinity
and roasting conditions within the range of actual practice were
employed, half to almost all of the ochratoxin A disappeared during
roasting (Blanc et al., 1998; Van der Stegen et al., 2001; Viani, 2002).
4.6.2 Polycyclic aromatic hydrocarbons (see also
3.11.11.2)
In most cases, benzo[a]pyrene (BP) has been determined as a leading
substance for all polycyclic aromatic hydrocarbons. After normal roast the
content is often reduced in respect to green coffee. No differences
between roasting by direct firing and by indirect air heating could be
detected. More BP is formed by heat transfer to the beans from contact
with hot surfaces than by transfer from hot gases. The transfer of BP to the
coffee brew depends on the concentration of benzo[a]pyrene in the
roasted coffee and on the water-to-coffee ratio, amounting on average to
approximately 5%, so that the content in coffee brews and extracts is
insignificant (Maier, 1991).
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214 Espresso Coffee
CHAPTER5
Grinding
M. Petracco
The classical representation of coffee usually pictures it in beans, either
green or roasted. But for coffee to be consumed as food, it is necessary to
destroy the beautiful form nature has given the seed by transforming it
into a powder. The operation which effects this conversion is called, in
engineering language ‘comminution’, which means breaking down
particles into smaller fragments, while in coffee industry jargon it is
commonly known as grinding.
Grinding is performed by a grinder, or coffee-mill, and the resulting
product is ground coffee. The term ‘grind’ applies to an important variable
in espresso coffee preparation, as we shall see later: it refers to the degree
of comminution associated with the concept of fineness of ground coffee.
The main objective of grinding is to increase the specific extraction
surface, or rather, to increase the extent of the interface between water
and the solid per unit weight of coffee, so as to facilitate the transfer of
soluble and emulsifiable substances into the brew. Each time a solid body
is broken up, additional surface is generated that comes into contact with
the surrounding environment, in this case the extraction water.
Two apparently contradictory needs must be satisfied to prepare a good
cup of espresso: on the one hand a short percolation time is required,
while, on the other hand, high concentration of soluble solids must be
reached. Both requirements can only be attained if a close contact
between solid particles and extraction water can be achieved. Thus,
espresso percolation needs a plurimodal particle size distribution, where
the finer particles enhance the exposed extraction surface (chemical
need) and the coarser ones allow the water flow (physical need).
5.1 THEORY OF FRACTURE MECHANICS
Before dealing with the various types of devices employed for grinding, it
is useful to give a qualitative notion of the fundamental laws of physics
acting in comminution. To reduce solid blocks into smaller sizes, it is
necessary to produce fractures by applying a set of forces. A quantitative
theoretical analysis of this process would only be possible for very simple
geometric forms; nevertheless, a general outline may be useful to
understand the machine’s application of complex stress configurations
on the material (Fayed and Otten, 1984).
Stresses exerted on an elementary object fall into two categories:
compressive stresses and shear stresses. Both produce strains that may
follow either simple laws, like Hooke’s law in the case of elastic materials
(” . E, where ” stands for strain caused by stress , and E represents the
modulus of elasticity or Young’s modulus), or more complex laws for
materials displaying a plastic or viscous-plastic behaviour, which is not
the case of roasted coffee. A fracture results when the stress exceeds the
limit given by Hooke’s law, that is, when it reaches the so-called ‘breaking
load’ (Pittia and Lerici, 2001).
In reality, the stress is not distributed uniformly inside an object, not
even in the ideal case of an object having a regular form and consisting of
a homogeneous and isotropic material. As a matter of fact, the interior of
an object always displays slight discontinuities, or flaws, like the defects in
the crystal lattice of a mineral, or inhomogeneities in animal or plant
tissues.
Lines of force are concentrated along the rim of discontinuities, and
may easily cause the stresses to reach very high values, which lead to
local micro-fractures that propagate rapidly. In the case of shear stresses,
the relevant flaws tend to lie on the plane of shear, and cause the
formation of a roughly uniform surface of fracture (depending on the
nature of the material), which generally splits the object into two. In the
case of compression forces, the level of stresses is more uniform, and the
flaws may be randomly scattered within the object: a concurrent
formation of fracture lines in various points causes the object to burst,
creating fragments of different random sizes. In an object that does not
possess an elementary form, but has an articulated or compound
geometry, the most stressed points susceptible to failure can be
determined by the use of the design techniques employed in construction
science.
The geometry of a coffee bean is anything but simple, as is often the
case with the works of nature. The coffee bean displays two distinct levels
of complexity:
216 Espresso Coffee
n At a macroscopic level, the bean is made of a layer of tissue folded
together (interposed with a thin layer of a different tissue, the
perisperm, also called ‘silverskin’), forming a solid resembling an
ellipsoid of revolution, as was shown in Figure 4.10c.
n At a microscopic level, the tissue is composed of cells of different
shapes filled with a mass of sugars, proteins and lipids, enclosed by
walls made of polysaccharides (see Figure 3.21b). This content can be
observed only in green coffee, while it apparently disappears after
roasting due to transformation into polymeric material and volatile
compounds (Figure 5.1).
A study of the cellular geometry of roasted coffee, performed by
computer simulation employing the ‘finite element method’, shows that
the application of shear to a reticular structure tends to produce a
comminution into many small particles and a few larger ones of different
size, as shown in Figure 5.2 (Petracco and Marega, 1991).
Thus, two phases of comminution must be taken into consideration in
the design of a grinder:
Grinding 217
Figure 5.1 SEM structure of roasted coffee bean, where the cell walls are no
longer visible, as they are covered by oil
n a crushing phase, where the convolute structure of the brittle roasted
bean is broken down into fragments of approximately one millimetre;
and
n a second phase, properly called grinding, in which such fragments are
submitted to shearing forces.
This grinding process produces a particle distribution that, as proposed in
section 7.5, has a favourable if not indispensable effect on espresso
brewing, though theoretically it does not contribute to maximization of
the surface.
5.2 COFFEE GRINDERS
Roasted coffee possesses a particular feature that excludes from the outset
the use of a major class of grinders widely employed in the mining
industry, naming ball mills. The solubility of coffee in water, even in cold
water, does not allow the use of wet-processing equipment. This is
unfortunate, because wet-processing equipment is most appropriate when
a specific distribution spectrum is sought for, since it operates with
adjusted recycling of material. This only leaves the use of dry-processing
equipment, operating according to two commonly employed operating
principles: ‘impact’ grinding and ‘gap’ grinding.
218 Espresso Coffee
Figure 5.2 Simulation of sub-particle formation
5.2.1 Impact grinding
Impact grinding is accomplished by a series of blades, rotating at high
speed, which exert a shock impulse on the particles encountered in their
trajectory. This effect resembles grinding by compression, with the
difference that, in the case under consideration, the reaction is given by
the inertial force of the particle suddenly removed from its state of
relative rest. This technique is utilized only in small-size grinders in the
home, because of one major defect: it does not easily allow the regulation
of particle size, since the ground product remains between the blades even
after being ground and so undergoes further impacts, making powder
fineness also dependent on the length of the operation.
5.2.2 Gap grinding
Gap grinding is based on the passage of the beans, usually by dropping,
through a gap between moving tools, called ‘cutting tools’. The geometry
of the cutters causes a gradual reduction of the width of the gap during
rotation, forcing the particles to come into contact with both cutters,
which apply a force couple on each particle. According to the shape of
the cutting surfaces, the force exerted may either be of a compressive or of
a shear type. The most common styles of cutter pairs used in the coffee
industry are (Figure 5.3):
n Roller cutters: a couple of rifled cylinders with parallel axes, counterrotating
and juxtaposed so as almost to touch themselves at their
generating lines. Beans are fed into the device by timed dropping, so
as not to obstruct the gap between cutters. These tools are mainly
used at industrial level.
Grinding 219
Figure 5.3 Cutters: (left) roller cutters; (centre) conical cutters; (right) flat cutters
n Conical cutters: a male conical wheel rotating coaxially in a static
truncated-cone-shape cavity. Again, feeding is performed by gravity,
often by rejection, namely a column weight of the material to be
ground. Excellent size versus throughput ratio.
n Flat cutters: a couple of disks with truncated-cone cavity, juxtaposed
so as to almost touch their bases. One of them, usually the lower one,
rotates coaxially. Feeding, by rejection or by forcing with worm screw
system, is accomplished from the upper to the lower part. Ground
coffee powder does not come out by gravity, but by the effect of the
centrifugal force of rotation. These cutters are used both for industrial
and professional grinding.
As already stated, the design of a grinding device must take into
consideration the existence of two phases of comminution: the crushing
of the bean (also called pre-breaking) and the grinding of the fragments to
the desired fineness (also called finishing). These requirements can be
met by two systems:
n A rather inexpensive system widely used in professional (coffee-bar
usage) or home grinders fitted with conical or flat cutters, where
cutters with unequal teeth are used. They are capable of forming gaps
of different size, and of seizing whole beans as well as fragments.
n A system mostly reserved to industrial equipment because of its
manufacturing complexity uses a series of grinding stages, each
optimized for a specific particle size. In this configuration, each stage
may also function as a regulator for the feeding of the following stage:
in this case, rejection feeding is utilized only in the first stage, that is,
in the stage that receives whole beans.
Obviously, a greater complexity of configuration is matched not only by
higher equipment costs, but also by a greater difficulty in controlling
stability. For this reason, industrial grinders are manufactured mainly
according to two basic models:
n A grinder with one or two pairs of toothed rollers, in series, preceded
by another pair of pre-breaking rollers, with capacities between
100 kg/h and several tons per hour.
n A grinder with a pair of flat cutters preceded by a conical prebreaking
stage, used for small to medium throughput (50–500 kg/h) of
ground product.
220 Espresso Coffee
5.2.3 Homogenization step
Besides the cutting tools, grinding equipment usually includes a
homogenization and blending step for the ground product, consisting of
rotating units with blades, worm screws, etc. Once the product has been
ground, it is transported to the packaging section of the plant by means of
conveyor belts, cups or pneumatic conveyors, sometimes under inert
atmosphere.
5.3 METHODS FOR MEASURING GROUND
PRODUCT FINENESS
The objective of grinding is to obtain a distribution of particles suitable
for a specific purpose. This statement is demonstrated by a practical
method applied by espresso bartenders for checking the fineness of
ground coffee before introducing it into the espresso machine: the
operator draws a single portion of coffee powder from the grinder, and uses
it to prepare a cup of coffee. The quantity of liquid percolated in a given
time is used as a measurement unit for regulating the primary variable of
the grinder: the so-called ‘notch’, namely the cutters’ position determining
the shear gap.
This method has the clear advantage of allowing an immediate
feedback, and an intuitive understanding of the cause–effect relationship.
However, it may be prone to significant interference, because the
quantity of beverage in a cup constitutes the final physical variable of
the entire process, from grinding to extraction. As such, it may be
influenced not only by the adjustment of the grinder, but also by other
variables such as the ground coffee’s waiting time before undergoing
extraction, the compacting force exerted on the cake, and other
parameters in adjusting the espresso machine (pressure, temperature,
etc.; see also Chapter 7).
In order to relieve the evaluation of grinding effectiveness from any
percolation tests, it is necessary to find a method that can be applied
directly on the coffee powder. One such example is the simple, classical
tactile examination performed by master grinding-operators for decades:
it consists in assessing the coarseness of ground coffee by rubbing a
pinch of powder between finger tips. However, such a subjective,
individual analysis obviously pertains more to the domain of art than to
that of science. Consequently, a major concern of the coffee industry
is to devise an objective method for measuring ground coffee particle
sizes.
Grinding 221
5.3.1 Sifting methods
One of the earliest methods of analysis adopted (Allen, 1968) consists in
shaking the ground product through a stack of graded-mesh sieves, with
incrementally smaller-sized openings. The net weight retained on each
sieve is recorded on linear, semilogarithmic or logarithmic charts
(Prasher, 1987), showing a curve that characterizes size distribution.
Various standardization proposals have been put forward for the shape of
the sieve as well as for automated or manual shaking procedures (DIN,
1997). Among the many ingenious proposals, some consider sieving aided
by vibrating chains, counter air-current as in DIN Norm 10765–1975, or
solvent counterflow.
The sieving system has not met with much favour in the specific field of
coffee grinding, specially when espresso is concerned, due to two basic
considerations:
n All methods have a poor repeatability, due to a substantial amount of
oil coming up to the powder surface after grinding. The oil acts as an
adhesive on the particles, forming random yet stable aggregates
unable to pass through meshes. Moreover, particles passing through
one sieve may commingle with other aggregates on the following
sieves, thus distorting the objective results of the analysis. This
drawback is more evident in espresso than in other brewing
techniques, because of its darker roasting and finer grinding needs.
n Sieving methods are long and laborious, and yield a characterization
of the ground material which is often inadequate, since it is drawn
from a too-limited series of sieves.
5.3.2 Imaging and sensing zone methods
Another intuitive approach to measuring ground coffee fineness consists
in placing a powder sample on a microscope slide marked with a grid, and
directly determining the size of a significant number of particles. This
method is quite labour-intensive, even if eased nowadays by photographic
microscopy techniques assisted by an automated specimen holder, and by
computerized image analysis that provides histograms by particle size
groups. As with all other micro-determination techniques, this measuring
method also suffers from technical problems in sampling, and from the
issue of representativeness of samples having the size of a few cubic
millimetres, instead of tens or hundreds of grams used in sieving. A further
difficulty lies in spreading out the particles in a thin layer, so as to allow
222 Espresso Coffee
their counting and measuring. Other methods (Svarovski, 1990) propose
dispersing the powder in a solvent in order to determine physical
characteristics such as electric conductivity (sensing zone counter),
sedimentation rate, and density.
5.3.3 Laser diffractometry
Size distribution analyses based on optical principles appear much more
rapid and promising. The most widely used method is laser diffractometry,
suitable for ground coffee particles sized from 5 mm to 2000 mm. This
technique uses the exact parallelism of coherent light emitted by laser
sources in the visible or near-infrared. By means of very high-quality lens
systems, it is possible to concentrate a beam of such light on an extremely
small punctiform target (<0.1 mm) surrounded by concentric annular
sensors. Particles passing through the beam produce a scattering, which
disperses a little amount of light into a conical geometry. The smaller the
particle, the greater the angle of the diffraction cone. The quantity of
light striking each of the sensors displays a trend characterizing the
particle sizes. Computer processing of this information produces a chart
that associates size ranges with the concentration of particles contained in
that range.
A difficulty set by this method lies in sample presentation: if performed
dry, by pneumatic dispersion devices (Illy, 1994), it allows a short analysis
time (a few minutes), which renders this method much more interesting
than the more accurate but time-consuming wet presentation. A
pneumatic dispersor is a tube to which a considerable quantity of coffee
powder (up to 100 g) is continuously fed by vibrating conveyors.
Compressed air is injected in various sites at different angles, exerting
shear forces on particle clusters, breaking them up. The amount of feed
and of air used must be calibrated to obtain the prescribed beam
obscuration. Wet presenters require a much smaller quantity of powder
(less than one gram), to be suspended in a convenient liquid (hexane, hot
water) sometimes with the help of ultrasonication and surfactants. The
suspension is then circulated through a shallow transparent cell placed
across the beam’s path. Typical analysis times range from a couple of
minutes for dry presentation up to 20 minutes for wet presentation.
A picture of the size distribution of a powder sample is given by a
histogram or, when many measure points are used, by a chart showing a
grading curve (Figure 5.4).
The curve is plotted on a diagram in which the axis of the abscissa,
usually on a logarithmic scale, carries the ‘equivalent diameter’ variable:
Grinding 223
this variable can be defined differently according to the technique
adopted, but it is somehow linked with particle size. The percentages of
particles belonging to each size group are plotted on the ordinate axis in
differential (frequency chart) or integral form (cumulative curve). The
percentage may refer to the number of particles, to their volume or mass
or weight, or even to their surface. It is possible to pass from one chart
form to the other by calculation, once the relationship between particle
shape and density factors has been determined. The comparison of ground
coffee batches obtained from different grinding equipments shows
differences that characterize each ground coffee by its own grading
curve, just as fingerprints identify a person.
Several companies sell turnkey computerized analysers, designed for
general powders. They differ by bench length (that determines the
maximum measurable particle diameter) and presentation accessories. An
effort to adapt sample presentation devices to ground coffee is always
needed, due to its sticky character and fouling tendency. Some brands
worth mentioning are Coulter, Malvern, Fritsch and Sympatech.
5.4 PARAMETERS INFLUENCING GRINDING
The unit operation ‘comminution’ involves two types of input variables:
those linked with raw material and those linked with grinding machinery.
224 Espresso Coffee
Figure 5.4 Particle size frequency distributions: (a) percentage at 0.205mm (max.
count); (b) percentage at 1.097mm (max. surface); (c) percentage at 517.2mm
(max. volume)
5.4.1 Variability of the coffee blend
The first type of variable includes, chiefly, the variability in composition
of the coffee to be ground. As explained in Chapter 4 on roasting, espresso
needs a balance of flavours and aromas, obtained from a blend composed
of different origins. Thus the grinder is confronted with beans that not
only come from different botanical varieties and producing countries, but
are also dried and processed by diverse methods. This often results in a
lack of homogeneity in hardness: in addition, hardness is constantly
susceptible to daily changes owing to dissimilarities in the batches, since
it must be remembered that coffee is an agricultural commodity subject to
the laws of climate and of natural evolution.
5.4.2 Roasting degree
Pyrolysis reactions forming volatile gases and aromas produce a considerable
expansion of the bean, depending on final roasting temperature and
rate of roasting (see 4.4). This expansion occurs at the expense of the
elasticity of the cell walls, which become brittle and lose tenacity,
changing from a nearly plastic behaviour in the green beans to an elastic
behaviour, and eventually to a brittle failure in the dark roasted beans. If
coffee is air-cooled after roasting, the high quantity of gas trapped in the
cells may affect the grinding process stability. A way to induce degassing
naturally, performed in industrial practice, is to allow the roasted beans to
rest in a silo for a suitable number of hours.
5.4.3 Moisture of the roasted beans
As seen in 4.1, water quenching may result in a substantial residual
moisture, which increases the bean’s tenacity and hence the grinder’s
consumption of energy with a resulting over-heating of the coffee.
Furthermore, contrary to air-cooling, quenching provokes cell-wall
cracking from the sudden volume contractions resulting from the brutal
temperature drop. This causes a larger degassing of the beans before
grinding and modifies the resistance to comminution of the cell structure.
5.4.4 Cutters’ distance
Machinery-related variables capable of affecting grinding results may
be grouped in control variables and in disturbances. The major control
Grinding 225
variable, closely related to the ‘notch’ of the coffee-bar grinder, is the
mechanical adjustment that allows regulation of the shear gap (or gaps) to
obtain a finer powder. In flat or conical cutters this adjustment consists in
drawing together coaxially the pair of cutting elements. The operation
requires a good degree of precision and low working tolerance, and must
be free from hysteresis; it is, however, easy to perform. In roller cutters,
the two cutting cylinders are drawn together by translating the axis of the
mobile cutter parallel to that of the fixed cutter: this delicate operation
requires complex and costly mechanisms. In both cases, an important role
is played by the mechanism controlling the separation between the
cutters, which places the pair of rotating cutters almost into contact.
5.4.5 Other parameters
Equally important is the adjustment of the connected units, primarily the
feeding and pre-breaking mechanism; such operation has a considerable
effect on hourly throughput and on ground product quality, by
contributing to the avoidance of flooding and overheating. Less
important, and therefore less commonly performed, is the regulation of
the cutters’ rotation speed; such operation could either be performed by
complex and delicate kinematic mechanisms or through more modern but
equally expensive means, such as three-phase power supply frequency
converters.
5.4.6 Disturbances
Disturbances are those hard-to-control variables causing equipment to
perform irregularly, which industrial practice aims to minimize.
A major cause of loss of control in a grinder undoubtedly comes from
energy dissipation during the application of mechanical power to break
down the product. As in all mechanical operations, there is an inevitable
element of friction, which adds to the heat generated by fracturing and
causes differential expansion of metal parts. This is especially critical
where tenths of a millimetre play an important role in controlling particle
size distribution. Even though various systems for minimizing these
harmful effects (thermostatation jacket with circulating liquid, injection
of boiling-point liquid nitrogen between the cutters) have been proposed,
the golden rule of grinding is still to wait for the machine to reach
thermal stability before considering it ‘operational’, that is, before
considering the ground product optimal.
226 Espresso Coffee
As in all machinery, grinders are prone to wear and tear, and need
proper maintenance. The parts that must be kept under control most
carefully are of course the cutting tools, whose blades need periodical
sharpening to recover their original performance. The result of poorly
sharpened tools is an increase in adverse thermal effects, due to the higher
compression-to-shear ratio applied to the coffee particles by blunt cutting
edges. Worn cutters impart to the beverage a ‘burnt’, bitter taste by
overheating the coffee.
Also worth mentioning are the disturbances caused by the electric
power supply, or by foreign matter. Power supply disorders may affect
rotation speed for a few seconds and destabilize a possibly already unstable
equilibrium. Foreign matter that may be present in spite of the utmost
attention and in spite of all the equipment installed for its elimination
(pebble removers, magnets, etc.), may suddenly block the cutters and
even damage them permanently.
5.5 PHYSICO-CHEMICAL MODIFICATIONS DUE
TO GRINDING
The main influence of the grinding process on its feed, the roasted beans,
is of course fragmentation. Its efficacy, often evaluated by the empirical
method of rubbing a pinch of powder between the fingers to feel its
coarseness, should be quantified by analysing the product by means of an
objective measurement method, as seen in 5.3.
Grinding operations, besides the described effect on the physical
structure, also impart to coffee some chemical modifications due to the
temperature reached during grinding. It is quite common reading
temperatures up to 80
C on the ground coffee thermocouple, which
probably correspond to real temperatures of over 100
C. These levels are
well below those reached in the roasting process, and the coffee beans are
exposed to them for a shorter period than during roasting (only a few
seconds, instead of minutes). Nevertheless, such temperatures speed up
Maillard and oxidation reactions, with the detrimental effects on coffee
taste that can be judged by comparative organoleptic tests.
Temperature acts also on a macroscopic phenomenon that is important
for espresso beverage: the release of volatile aroma. When broken down
by grinding, coffee cells release their content of pyrolytic gas formed at
roasting. This gas is mostly composed of CO2 and CO, accompanied by
small amounts of hundreds of volatile chemical species (see 4.4).
Although present only in low concentrations, volatiles are essential in
Grinding 227
bringing forth the typical coffee aroma. Most industrial grinding
equipment is designed to handle the release of gas by conveying it
externally through a filter. Techniques for aroma recovery have been
proposed, such as cryogenic condensation, absorption in oil, or adsorption
on porous material.
However, the unfractured cells still present in the larger-size particles
(over 50 mm) keep their content of high pressure roasting gas, which will
contribute to the formation of ‘crema’, the typical espresso foam (see
8.1.1). Inevitably, the gas will slowly escape during storage through microcracks,
or through the natural porosity of the cell walls. This evolution is
accelerated by high grinding temperature, which increases the pressure
within the cell. While the pros and cons of degassing will be discussed in
6.1.2.1, it is certain that the worst method to degas ground coffee is by
overheating it.
One last macroscopic effect induced by grinding consists in the
appearance of coffee oil on the surface of the powder, resulting from the
release of the lipidic content of so far undamaged cells. As already
explained in 3.11.9 and 4.3.2.3, the major part of the coffee lipid fraction
is composed of triglycerides of palmitic and linoleic acids, and of stearic
and oleic acids in a lower percentage. This blend is very viscous at room
temperature, but starts becoming fluid around 40
C (Table 5.1). At
higher temperatures, fluidity is such that oil can easily flow through micro
cracks and cover the outer surface of the particles with a layer that
regains its sticky, semi-solid state at room temperature. This phenomenon
affects the cohesive behaviour of ground coffee particles when compacting
(see 6.1.3.3), hence influencing the hydraulic resistance of the cake.
Overheating on grinding may thus be responsible for erratic percolation
behaviour when brewing espresso. It must be stressed therefore that
temperature control in grinding equipment is one of the main
requirements to obtain a powder fit for preparing consistently perfect
espresso cups.
228 Espresso Coffee
Table 5.1 Coffee
oil viscosity
T (
C) Viscosity (mPa s)
0 Solid
20 70.6
50 22.4
92 7.8
REFERENCES
Allen T. (1968) Particle Size Measurement. London: Chapman and Hall,
pp. 165, 606.
DIN (1997) DIN-Taschenbuch 133: Partikelmesstechnik Deutsches Institut
fu¨r Normung, Berlin.
Fayed M.E. and Otten L. (1984) Handbook of Powder Science and Technology.
New York: Van Nostrand, pp. 562–606.
Illy E. (1994) System for controlling the grinding of roast coffee. US patent
5,350,123/27.
Petracco M. and Marega G. (1991) Coffee grinding dynamics: a new approach
by computer simulation. Proc. 14th ASIC Coll., pp. 319–330.
Pittia P. and Lerici C.R. (2001) Textural changes of coffee beans as affected
by roasting conditions. Lebensmitt. Wiss. Technol. 34, 168–175.
Prasher C.L. (1987) Crushing and Grinding Process Handbook. Chichester: John
Wiley & Sons, pp. 87–102.
Svarovski L. (1990) Characterization of powders. In M.J. Rhodes (eds),
Principles of Powder Technology. Chichester: John Wiley & Sons,
pp. 51–61.
Grinding 229
CHAPTER6
Storage and
packaging
M.C. Nicoli and O. Savonitti
6.1 PHYSICAL AND CHEMICAL CHANGES OF
ROASTED COFFEE DURING STORAGE
M.C. Nicoli
Roasted coffee is a shelf-stable product. In fact, thanks to the high
temperatures attained in the roasting process and to its low water activity
(aw), no enzymatic and microbial spoilage occurs. However, during storage,
coffee undergoes important chemical and physical changes, which greatly
affect quality and acceptability of the brew. These events are responsible
for staling of roasted coffee but they are also believed to be involved in the
so-called ageing process. The border existing between ageing and staling is
unclear, and the reactions which prevail in ageing with respect to staling
are still unknown. Ageing is a short length of time, just after roasting, in
which coffee is allowed to rest under proper technological conditions for
improving its sensorial properties. Although many of the physical and
chemical changes occurring in roasted coffee during storage are considered
unavoidable, the rate at which they occur mostly depends on some
environmental and processing variables such as oxygen and moisture
availability, exposed surface as well as packaging conditions. The adoption
of proper grinding and packaging conditions can greatly slow down staling
reactions. However, they can be accelerated again, proceeding at a
considerable rate, after the consumer opens the packaging. In fact, home
storage conditions, mainly in terms of oxygen availability, relative
humidity and temperature, are the key factors in determining the socalled
secondary shelf life of the product (Full et al., 2001).
Table 6.1 summarizes the most important physical and chemical events
that take place in roasted coffee during storage.
Solubilization and adsorption represent the main mechanisms through
which aroma compounds are retained into the bean structure. In fact,
volatiles, as well as carbon dioxide, can exist entrapped in the pores,
dissolved in coffee oils and moisture and probably adsorbed into active
sites (Labuza et al., 2001). This represents an important requisite for
obtaining a high quality coffee cup. However, volatile compounds are
easily released in the vapour phase due to diffusion mechanisms. The
latter, together with oxidation reactions, are considered the main causes
of coffee staling. The evolution of carbon dioxide, which is the most
important non-aromatic volatile found in fresh roasted coffee, represents
an additional problem during storage, mainly from a technological point
of view. In fact, in the case of use of flexible pouches, it causes a rapid loss
in the package integrity due to swelling and bursting. The amount of CO2
formed depends on the degree of roasting and can be up to 10 ml/g (SPT),
corresponding to 1–2% of roasted coffee weight (Sivetz and Desrosier,
1979). For this reason, different procedures such as degassing or
pressurization, together with the adoption of suitable packaging solutions,
have been developed in order to allow carbon dioxide to be released,
minimizing aroma losses and oxidations (see 6.2).
Since roasted coffee is a dehydrated product, the major factors
controlling its stability during storage are oxygen concentration (O2),
water activity (aw) – particularly if roasting had been quenched by adding
water – and temperature. Technological operations such as grinding,
degassing and packaging may accelerate the above-mentioned changes
due to the increase in the exposed surface, as well as in oxygen and
moisture availability. In addition, since the pressure in the bean pores
should be greater than atmospheric pressure, the external pressure applied
to the coffee beans represents an additional variable to be considered for
enhancing coffee stability during storage. In fact, as the external pressure
becomes higher than the partial pressure of volatiles present in the beans,
the degassing rate is reduced allowing a larger quantity of volatiles to be
dissolved in the lipid fraction or adsorbed on the active sites (Clarke,
1987a; Labuza et al., 2001).
Storage and packaging 231
Table 6.1 Physical and
chemical changes occurring
in roasted coffee during
storage
Volatile solubilization CO2 release
Volatile adsorption Oxidations
Volatile release Oil migration
6.1.1 The role of Maillard reaction products
on roasted coffee stability during storage
(see also 4.5)
Coffee staling takes its origin from roasting. Maillard reaction and
pyrolysis, which are the main chemical events occurring in coffee during
roasting, transform the beans into a very unstable and reactive system. In
broad terms, the most macroscopic changes occurring in coffee during
roasting and storage can be attributed to the formation of Maillard
reaction products (MRPs) and carbon dioxide. The latter is a reaction
product from both Maillard reaction and pyrolysis (Hodge, 1953;
Yaylayan and Huyghues-Despointes, 1994). The formation of volatiles
and CO2 during roasting causes the expansion of the beans due to the
internal build-up of gases which, along with the high temperatures, allows
internal pores and pockets to be formed. However, the increased CO2
formation in the advanced stages of the roasting process corresponds to a
progressive reduction of the ability of the bean to entrap and retain CO2
and other volatiles, as the bean loses its elasticity and surface cracks are
formed under the push of the increased internal pressure (Massini et al.,
1990). A comparison between the amount of CO2 formed during roasting
and retained into the bean during storage is shown in Figure 6.1.
232 Espresso Coffee
Figure 6.1 Comparison between amount of CO2 produced during roasting and
retained into the coffee beans (data elaborated from Barbera, 1967; Massini et al.,
1990). Values of CO2 retained are referred to the percentage of CO2 present in the
headspace in equilibrium with the sample after 5 minutes’ storage after roasting
It can be observed that, as the bean temperature rises during roasting,
increased amounts of CO2 are formed. However, this corresponds to a
progressive loss of the CO2 retention capacity of the bean. Among the
hundreds of compounds generally considered as MRPs, volatiles and
melanoidins play a key role in determining the aroma and body of the
coffee brew. Also, the development of the Maillard reaction during
roasting is responsible for other important changes affecting coffee
stability during storage. The most important effects exerted by MRPs in
roasted coffee are summarized in Table 6.2.
Many of the low molecular weight MRPs are responsible for the
formation of the typical coffee flavour. It is well known that the aroma
profile of roasted coffee is given by several hundreds of compounds
covering many different chemical classes. The development of the typical
stale flavour of old coffee is the sum of two different actions: the partition
of volatiles in the vapour phase and the development of off-flavours as the
result of their oxidation. It has been observed that methanethiol, Strecker
aldehydes and -dicarbonyls are the most important low-boiling
compounds responsible for the freshness of coffee aroma (Steinhardt
and Holscher, 1991). These molecules, in addition to their high volatility,
are particularly prone to oxidation. However, it must be pointed out that
the aroma perception could also be affected by the ability of melanoidins
to bind specific volatile molecules, as recently observed for some thiol
compounds by Hofmann et al. (2001).
Most of the effects of MRPs on the shelf life of roasted coffee trace their
origin to the ability of these compounds to act as both pro-oxidants and
antioxidants. These contradictory functions may be explained by the
complex and different chemical structures of MRPs. In broad terms, while
melanoidins are generally recognized to be strong antioxidants, uncoloured
compounds, such as those formed during the early and intermediate
stages of Maillard reaction, may exert pro-oxidant activity, being
Storage and packaging 233
Table 6.2 Significance of MRPs in coffee
during storage
Flavour Shelf life
Volatile formation Oxidation reactions:
Degradation of aroma compounds
Lipid oxidation slow-down
Microbial growth inhibition
Volatile binding
in most cases radicals (Namiki, 1990; Manzocco et al., 2001, 2002).
During storage, radical reactions may occur leading to the formation of
non-radical forms as well as to the generation of radical species
chemically different from those initially present (Goodman et al., 2001).
Among melanoidins, recent findings suggest that the fraction of low
molecular weight melanoidins (MW<1000–3000 Da) shows higher antioxidant
efficiency than that of high molecular weight (MW up to
150 000) (Hofmann, 2000). These compounds seem to act both as
primary and as secondary antioxidants since they have been found to
behave as chain-breakers, reducing compounds and metal chelators
(Homma et al., 1986; Nicoli et al., 1997). The antioxidant capacity of
roasted coffee can greatly vary depending on the roasting degree. In Table
6.3 data of chain-breaking activity and redox potential with reference to
light, medium and dark coffee samples are reported.
While the assessment of the chain-breaking activity allows estimation
of the quenching rate of coffee compounds towards reference radicals, the
redox potential gives indications on the effective oxidation/reduction
efficiency of all the antioxidants present, including the ‘slow’ ones, which
cannot be detected by the traditional chain-breaking kinetic assays. ‘Slow’
antioxidants are expected to play an important role in determining the
shelf life of roasted coffee during storage (Nicoli et al., 2004). While slight
changes in the chain-breaking activity can be observed from light to dark
roasted coffees, indicating the existence of a balance between the antiradical
efficiency of naturally occurring antioxidants, which are degraded
during roasting, and the heat-induced ones, the redox potential
significantly decreases. This indicates that MRPs show higher reducing
properties than natural antioxidants. In broad terms, this means that dark
234 Espresso Coffee
Table 6.3 Chain-breaking activity and redox potential
values of coffee samples having different roasting degree
Sample
Weight loss
(%)
Chain-breaking activity
( Abs 3 min 1mgss
1)
Redox potential
(mV)
Raw 0 0.320 0.13* 109
Light 14.5 0.273 0.03* 45
Medium 16.2 0.283 0.13* 2
Dark 18.9 0.390 0.04 35
*Data not significantly different.
The chain-breaking activity assay was performed using the DPPH test (Nicoli
et al., 2001).
roasted coffee is expected to have a greater antioxidant reservoir
potentially available during storage.
The presence of such reactive species in roasted coffee can explain the
huge variety of redox and radical reactions occurring in coffee during
storage and even, at an increased rate, after beverage extraction (Nicoli
et al., 2001; Anese and Nicoli, 2003). The oxidation of these reactive
compounds has a double relation with roasted coffee shelf life: they are
the main cause of aroma compounds degradation, but they also represent
an efficient barrier against the oxidation of the lipid fraction. In fact,
despite the very low aw value, lipids in coffee show an extraordinary
stability against oxidation thanks to the presence of lipid-soluble
MR antioxidants (Whitfield, 1992). Figure 6.2 shows the influence of
melanoidins on the stability of coffee lipids. It is interesting to note that
samples previously freed from melanoidins showed lower oxidation
stability as compared to that measured for lipids containing melanoidins.
The antioxidant efficiency of melanoidins has been found to be strictly
related to the intensity of the heat treatment used to allow their
formation. The higher the intensity of heating, the higher their intrinsic
antioxidant capacity (Anese et al., 2000). For dark roasted coffee, the
stability of the lipid fraction was observed to range from 4 to 10 months at
25
C storage depending on oxygen availability and packaging conditions
(Nicoli et al., 1993).
Storage and packaging 235
Figure 6.2 Influence of lipid-soluble melanoidins on oxidation stability of coffee
lipids (from Anese et al., 2000). The uncoloured samples were obtained by
removing melanoidins with active carbon. Data are expressed as induction time
(hours) prior to oxidation detected by Rancimat test
The ability of MRPs to act as antioxidants can in part explain their
inhibiting action against the growth of some food-poisoning microorganisms.
However, the extent of MRPs antimicrobial activity appears to be
linked to their concentration and to the bacteria tested, indicating that
different mechanisms other than radical scavenging activity may be
involved, since diluted coffee shows a weaker antibacterial activity than
concentrated coffees. In addition, the extent of the antimicrobial activity
strictly depends on the species tested (Einarson, 1987; Sheikh-Zeinoddin
et al., 2000; Daglia et al., 2000).
An increasing interest towards coffee MRPs is justified by the fact that,
by virtue of their antioxidant activity, they are expected to exert a range
of positive health effects on the human organism (O’Brian and Morissey,
1989; Faist and Erbesdobler, 1999). Thus, the preservation of the intrinsic
antioxidant properties of roasted coffee during storage could be of a
certain importance to obtain coffee products with high functional
properties. Unfortunately, up to now no data are available about the
influence of different environmental and processing conditions on the
evolution of the overall antioxidant properties of roasted coffee during
storage.
6.1.2 Staling kinetics
6.1.2.1 Degassing
Carbon dioxide formed during roasting is trapped in the cellular structure
of the bean and is only released over a period of weeks following roasting,
resulting in a 1.5–1.7% weight loss. The amount of gas released can be
estimated at 6–10 litres per kilogram of beans depending on roasting
degree, the higher figures being valid for dark roasted blends (Clarke,
1987b), recently confirmed by the results of Shimoni and Labuza (2000).
Degassing rate is inversely related to time from roasting. The massive
degassing that takes place in the early hours after roasting slows down
gradually, and it may take months for all the CO2 to be released from the
beans. The process is slow because much of the CO2 is bound to the bean
structure. From a physico-chemical point of view, carbon dioxide in the
coffee bean can be solubilized partly in the lipid phase, partly in the water
(moisture). Some will be adsorbed on the apolar sites, and, finally, a part
will be blocked within the pores, which have collapsed because of the
phase changes in the structure of the bean due to the roasting process.
The amount of CO2 adsorbed in the lipid and aqueous fractions can be
roughly estimated at 15 10 5 mg of CO2 per gram of fresh roasted
coffee in the lipid phase, and 2.13 10 5 mg of CO2 per g of fresh roasted
236 Espresso Coffee
and ground coffee in water, considering coffee in air at normal CO2
pressure (Shimoni and Labuza, 2000; Labuza et al., 2001).
The driving force at the basis of carbon dioxide and volatile release
from roasted coffee is given by a diffusion flow due both to concentration
and pressure gradients. The release begins at a considerable rate just after
the end of roasting; then it gradually slows down as the CO2 and volatile
concentration in the headspace increases, reducing the mass transfer
driving force.
Figure 6.3 shows an example of CO2 and volatiles released from roasted
coffee beans and roasted ground coffee.
Recent studies carried out by Labuza et al. (2001) suggest that CO2
release from ground coffee is controlled by two different mechanisms: at
the beginning the diffusion is regulated by pressure gradients between the
outside and the inside of the coffee particles, then by molecular diffusion.
The time at which the changeover in mechanism occurs in STP is about
120 min. The few data available about volatile partition kinetics from
roasted coffee packed in air indicate that the rates of CO2 and volatiles
release are of the same magnitude. The volatiles present in the headspace
at equilibrium with the product after three days’ storage were 50%, 35%
and 18% at 4, 24 and 40
C respectively. As regards CO2 evolution, the
residual gas percentage in the headspace after the same storage time and
temperature was 63%, 37% and 18% (Nicoli et al., 1993).
As mentioned above, the rate of volatile compounds and gas release
from coffee is strongly affected by temperature. Additional factors are
pressure, particle size distribution, and during the degassing process,
packing, bed depth and entrainment gas composition.
Storage and packaging 237
Figure 6.3 Changes in headspace volatile and CO2 concentration, expressed as
percentage, in (left) roasted coffee beans and (right) roasted ground coffee stored
at 25
C in air (Nicoli et al., 1993)
6.1.2.2 Effect of temperature
The temperature dependence of CO2 and volatile release from roasted
coffee can be well described by the Arrhenius equation. An example of an
Arrhenius plot, referring to volatile release from roasted coffee beans, is
shown in Figure 6.4. The corresponding activation energy was 28.1 kJ/
mol, which gives a temperature sensitivity, expressed in terms of Q10, of
around 1.5. It means that, in the experimental conditions adopted, for any
10
C increase, the rate of volatile release increased 1.5-fold. This result
agrees with data on the temperature dependence of diffusion coefficients
for CO2, recently reported by Labuza et al. (2001). Moreover, the Q10 of
CO2 diffusion coefficients calculated for ground coffee were shown to be
approximately twice those of the beans.
The importance of the temperature variations due to day–night
fluctuations and to transport is difficult to assess. It may be assumed
that, because of temperature changes, competing degradation reactions
occur at different times, so that the resulting deterioration is likely to
occur faster than at constant temperature. Besides temperature, additional
factors affecting staling reaction rates of coffee during storage are aw and
O2 availability.
6.1.2.3 Effect of moisture
Water in roasted coffee may have different origins: despite the strong
dehydration occurring during the first stage of the roasting process, new
238 Espresso Coffee
Figure 6.4 Arrhenius plot of volatile release from roasted coffee beans. Data are
expressed as ln volatile release rate vs. temperature–1 (K). (Elaborated from Nicoli
et al., 1993)
water molecules are formed as a consequence of the Maillard reaction.
Additional water may come by the use of water quenching instead of aircooling.
Furthermore, the eventual sorption of ambient moisture from
roasted coffee during storage represents an additional cause of moisture
and hence of aw increase. Moisture sorption isotherms at 22
C of roasted
coffee beans and ground coffee are shown in Figure 6.5. Due to the higher
exposed surface, which affects concentration and availability of polar
active sites, ground coffee shows a considerable higher capacity to bind
water molecules compared to beans.
In addition, depending on aw, the bean structure can be brittle or
elastic as shown in Figure 6.6.
It is well known that at normal storage temperatures the bean structure
is brittle and easy to grind, as shown by the low fracture force measured for
air-cooled roasted coffee samples with aw values ranging at 0.1–0.2. This is
due to the fact that roasted coffee beans are greatly below the glass
transition temperature, which was recently observed to range from 130
to 170
C (Karmas and Karel, 1994; Eggers and Pietsch, 2001). However,
increasing aw, the fracture force was shown to double up to aw 0.85. This is
due to the plasticizing effect of water responsible for a decrease in the glass
transition temperature (Tg), making the coffee structure rubbery and
more elastic. For aw values higher than 0.85, solubilization of coffee
compounds and a sharp decrease in the fracture force are detected.
Storage and packaging 239
Figure 6.5 Moisture sorption isotherm at 22
C of roasted coffee beans and
ground coffee. (Data from Labuza et al., 2001; Pittia, 2002)
6.1.2.4 Oxygen uptake
As mentioned above, oxidation reactions represent an additional cause of
quality loss of roasted coffee during storage. In general terms the rate of all
the oxidation reactions occurring in coffee, including those of aroma
compounds, increases with increasing oxygen pressure, temperature and
aw. A study of the influence of these three variables on the overall
oxidation rate of ground roasted coffee was recently reported by Cardelli
and Labuza (2001). The oxygen uptake and sensorial evaluations were
used as indicators of product acceptability. By matching the map of rate of
food deterioration reactions as a function of water activity with the water
sorption isotherm of roasted and ground coffee at room temperature it
could be possible to understand what type of chemical reactions are
involved in coffee deterioration. In coffee, moisture content ranges from
2% to 5%, which corresponds to 0.2–0.4 water activity. Using these data,
it is evident that the oxidation of the coffee lipids is the prevalent
reaction.
The influence of aw and temperature on the shelf life of roasted ground
coffee, calculated on the basis of the oxygen uptake index, is illustrated in
Figure 6.7. On the basis of these results it can be stated that temperature
has a smaller effect than moisture on the shelf life of the product.
The increase in the rate of degradation can be quantified, as a function
of oxygen, by introducing a QO2 index, which represents the relative
variation of the rate of degradation caused by an increase in oxygen
content of 1%. According to Cardelli and Labuza (2001), QO2 is 10.5,
240 Espresso Coffee
Figure 6.6 Changes in fracture force of dark roasted coffee beans as a function of
water activity (Pittia, 2002)
when oxygen concentration is low (0.1–1.1%). It is 1.1 when oxygen
increases up to 5%. Further increase of oxygen, up to 21%, does not affect
the index, which remains 1.1. Summarizing, it can be stated that the rate
of oxygen uptake increases 10-fold, from 0.1 to 1.1%; above 1.1% of
oxygen concentration, for each 1% increase of oxygen there is a
consequent increase for the rate of degradation of 10%.
6.1.3 Other physico-chemical changes
O. Savonitti
6.1.3.1 Volatile compounds
The aroma complex formed during roasting consists of several hundred
compounds covering many different chemical classes. Some of them are
stable and undergo no change during storage other than loss due to
evaporation. Apparently, such compounds do not contribute much to the
quality of the perceived aroma. Aroma-impact components are mainly
unstable substances particularly prone to oxidation; some, aldehydes and
ketones, are present in intermediate states of oxidation, whilst many more,
alcohols, phenols, thiols, etc., are in a reduced form. There is then a significant
risk of aroma changes occurring as a result of oxidation, particularly
of compounds such as the thiols, which have very low threshold values.
The mechanisms responsible for the deterioration of aroma constituents
have not yet been fully elucidated; immediately after roasting it may
be simply due to evaporation and physical loss of the most volatile
compounds; conversion to other volatile or non-volatile compounds,
retained in the coffee matrix, also takes place; intermolecular reactions
are also likely to occur – for instance, -dicarbonyls and aldehydes can
Storage and packaging 241
Figure 6.7 Water activity and temperature dependence of the shelf life of ground
and roasted coffee (Labuza et al., 2001)
bind to compounds containing free amino groups. These reactions are
apparently influenced by the presence of atmospheric oxygen, suggesting
an oxidative route to degradation. The change in aroma profile cannot be
simply explained by the loss of the compounds affected, as the reaction
products themselves may make a sensory contribution. In the case of
thiols, for instance, the corresponding condensation products, the
disulphides, have a major sensory impact, different from that of the
fresh product. Afterwards, oxidation reactions become predominant, as
indicated by the correlation between deterioration and oxygen uptake;
and oxidation of the aroma complex will lead to a further dramatic shift
in aroma quality.
Oxidation, responsible for the typical stale flavour of old coffee, is the
sum of two actions: the loss of pleasant aroma components accompanied
by the formation of off-flavours. A study has concentrated on identifying
the compounds responsible for the freshness of coffee aroma, defined as
the fine and pleasant smell arising from freshly roasted beans (Steinhart
and Holscher, 1991). The conclusion was that low-boiling compounds –
such as methanethiol, Strecker aldehydes and -dicarbonyls – were the
most important; in particular, loss of methanethiol was considered as the
most important indicator. All of these compounds are volatile and
extremely reactive species, therefore, easily oxidized. Figure 6.8 shows
that their disappearance, particularly noticeable among those with the
lowest boiling point, occurs mainly over the first three weeks of storage,
and is principally due to evaporation.
Successive research during the 1990s led to a set of 28 compounds in
roasted and ground coffee as the most odour-active components associated
to fresh coffee aroma (Grosch, 1999, 2001; Czerny and Schieberle, 2001).
242 Espresso Coffee
Figure 6.8 Loss of volatile compounds during storage
One of these, namely methanethiol, is frequently used in many
freshness indices for coffee samples stored in different conditions (Sanz
et al., 2001). Many of these compounds can be used, in a definite linear
combination, as indicators of the change of freshness for a roasted and
ground coffee exposed to air at different temperatures (Cappuccio et al.,
2001).
The loss of volatile substances from coffee is not only related to their
volatility, but also to the way they are trapped in the cells. In fact, two
additional mechanisms of aroma retention play an important role. The
cell wall consists of polar polysaccharides and melanoidins, which can
adsorb and retain a wide range of volatile substances, particularly polar
ones. Oil, on the contrary, preferentially retains lipophilic substances,
such as alkylpyrazines, etc. In order to evaporate, volatile particles
‘trapped’ in the cell structure of the beans must first migrate across the cell
walls. In ground coffee this is made easy by the small particle size, so that
aroma loss occurs at a higher rate in ground coffee than in coffee beans
and is inversely proportional to particle size. Hence, finely ground
espresso blends lose their aroma just a few hours after being exposed to air,
and start smelling stale due to oxidation, if not stored in packages.
However, even in this case coffee aroma can undergo modifications
through time, if stored in packages somehow permeable to oxygen. Coffee
staling is associated with a degradation of some particular odorants, like
odour-active aldehydes, -dicarbonyls and thiols (2-furfurylthiol has the
main impact), rather than to the creation of off-odorants. This is due
again to the presence of oxygen, because of the non complete
impermeability of the package (Czerny and Schieberle, 2001).
6.1.3.2 Non-volatile compounds
Production of carbon dioxide during roasting forms an effective barrier,
which excludes most of the atmospheric oxygen from the cellular
structure so delaying the onset of oxidation. As degassing goes on, lipid
oxidation can no longer be stopped by the antioxidant activity of the
Maillard reaction products formed during roasting (Homma, 2001;
Steinhart et al., 2001), and in a few months, at the end of degassing,
the beans are rancid.
Ground coffee is particularly sensitive to oxidation because grinding
removes the CO2 barrier almost completely, while at the same time the
oil–air contact surface dramatically increases, promoting oxygen uptake.
Lipids, an important fraction in roasted coffee, are easily oxidized
during storage (Huynh-ba et al., 2001). The major unsaturated fatty acid
in coffee is linoleic acid (C18:2), which contains two double bonds and is,
Storage and packaging 243
therefore, rather susceptible to oxidation. Figure 6.9 shows the reduction
in linoleic acid content of rancid beans with respect to fresh beans; while
the major saturated fatty acid, stearic acid (C16:0), remains practically
unchanged (Fourny et al., 1982).
An important contribution to oxidation off-flavours is the formation of
volatile aldehydes, such as trans-2-nonenal, the main breakdown product
of lipid oxidation, which has an odour threshold level of 0.08 mg/l in water
(Flament, 2002).
6.1.3.3 Oil migration
Oil migration starts during roasting and goes on during degassing because
carbon dioxide tends to push oil outwards through the cell pores. The
increase in oil viscosity at lower temperatures slows down the process.
Oil migration to the surface of the beans, where the risk of oxidation is
maximal, is particularly important in fine ground dark espresso blends,
since dark roasting leads both to fast degassing and to increased porosity,
from disruption of cell walls. A further problem linked with oil exudation
is the increase in stickiness of the particles, which tend to aggregate into
lumps, making brewing irregular. The aggregation of particles on storage is
worsened by the absorption of moisture, as discussed in 6.1.2.3.
6.1.3.4 Effect of light
Light plays a catalytic role in many chemical reactions; in the case of
espresso (arabica) blends, particularly rich in unsaturated fatty acids, light
244 Espresso Coffee
Figure 6.9 Fatty acid content of (left) fresh and (right) rancid beans
catalyses the prime trigger of their auto-oxidation reaction, i.e. the
formation of H., R. (alkyl) and ROO. (peroxide) free radicals which then
cause the reaction to propagate.
6.2 PACKAGING OF ROASTED COFFEE
O. Savonitti
All the potential causes of deterioration highlight the importance of
packaging. Indeed, it may be stated that, because of its instability, roasted
coffee should either be consumed straight away or packaged in a waterand
oxygen-tight container. Special packaging materials and techniques,
with careful process monitoring, are required not to jeopardize the final
quality by poor manufacturing practices.
6.2.1 Packaging materials
In order to meet total quality requirements (see Chapter 1) the package
should:
n act as a barrier against water and moisture;
n act as a barrier against atmospheric oxygen;
n preserve coffee aroma and keep out foreign odours;
n be grease-proof;
n be light-proof;
n allow the carbon dioxide released during degassing to escape;
n be chemically inert;
n be hygienically safe and suitable for foodstuffs;
n be long-lasting;
n be sturdy and withstand pressure variations;
n be cheap;
n be practical;
n be environmentally friendly;
n be consumer-friendly.
Most of these requirements are aimed at preventing spoilage; some are
exclusively intended to add value to consumers. Packaging materials
should be grease-proof to prevent air oxidation of coffee oil leaking
through the package, which would then spoil the beans. Chemical inertia
of the material is also important, and metals or other substances, which
could act as oxidation catalysts, should be avoided. Table 6.4 lists the
materials meeting most of these packaging requirements.
Storage and packaging 245
Commonly used materials are the inexpensive flexible polymeraluminium
multi-ply, ensuring an efficient barrier and an optimal use of
space all along the life cycle, and tinplate, particularly useful for positive
pressure-resistant packages. The aluminium contained in multi-ply
materials constitutes the only completely effective barrier; the inner
layer is a film of waterproof weldable material; the centre layer is an
246 Espresso Coffee
Table 6.4 Materials commonly used for packaging roast coffee
Material Advantages Disadvantages
Tinplate Total barrier
Resistant to pressurse
Satisfactory ecobalance*
Recyclable
Sturdy
Expensive
Its rigidity prevents an
optimal use of space
Aluminium Total barrier
Resistant to pressure
Recyclable
Expensive
Its rigidity prevents an
optimal use of space
Poor ecobalance
Difficult to sort from other
wastes
Glass Total barrier
Good ecobalance
Recyclable
Resistant to pressure
Fragility
Heavy material
Expensive
Its rigidity prevents an optimal
use of space
Difficult to sort from other
wastes
Combined flexible
multi-ply polymer
aluminium
Inexpensive
Simple manufacturing technology
Large flexibility and optimal use of
space
Satisfactory ecobalance
Non-tight barrier
Resistant only to negative
pressures
Poor strength
Combined
multi-ply
aluminium
cardboard
Inexpensive
Total barrier
Satisfactory ecobalance
Difficult to sort from
other wastes
Non resistant to pressure
Poor strength
Complex manufacturing
technology
Flexible combined
multi-ply polymer
Inexpensive
Simple manufacturing technology
Satisfactory CO2 permeability
No need to sort for other wastes
*Ecobalance is defined as the total pollution resulting from energy consumption, and that
resulting from the product, during manufacture, use and disposal.
airtight aluminium film and the outer layer consists of a material that
makes the structure more rigid and resistant to mechanical stress.
None of the materials listed in Table 6.4 ensures the level of
permeability to carbon dioxide required to keep a stable internal pressure
during degassing; other means must be applied.
Blow-moulded rigid plastic materials (HD-PE or PET) have lately been
introduced for roast and ground coffee packaging: one-way valves on easypeel-
off membranes or applied directly on the packaging allow degassing
and avoid deformations. This type of container is light, resistant to
breakage, can be easily piled up, and recycled. Production technology is
well known and the form can be chosen at will. However, the material
does not provide a perfect barrier to gases.
6.2.2 Packaging techniques
The techniques used for packaging roast coffee are listed in order of
increasing product protection:
n air packaging
n vacuum packaging
n inert gas packaging
n pressurization
n combined use of one of the previous techniques with active
packaging.
6.2.2.1 Air packaging
Air packaging consists in simply filling and hermetically sealing the
package; coffee is protected against humidity, off-flavours and light, but
the presence of air inside the package means high oxygen levels and
consequently shortened shelf life. This technique can only be used with
degassed coffee to prevent swelling and possible explosion of the airtight
package. If the coffee, particularly coffee beans, is not degassed, swelling
can be prevented by fitting a one-way safety valve on the package
allowing carbon dioxide to escape without letting air in. As CO2 is
heavier than air, it tends to stratify at the bottom expelling most of the
oxygen gas; the flushing effect reduces residual oxygen and increases
product shelf life. Air packaging using a one-way safety valve is an
acceptable technique for air-cooled coffee beans since they contain large
quantities of gas; its main disadvantage is that most of the aroma escapes
together with CO2, so dulling cup taste.
Storage and packaging 247
6.2.2.2 Vacuum packaging
Vacuum packaging meets two different objectives: air extraction with
lowering of oxygen level, and use of flexible materials. The technique,
which can also be used with rigid materials such as tinplate, is commonly
applied to flexible materials to make the coffee ‘bricks’ sold in
supermarkets. The box shape is obtained by an intimate contact between
the material and the coffee, which must be completely degassed to
avoid a decrease in compactness of the package provoked by a release of
carbon dioxide making it floppy or swollen. This is why bricks
are generally used only for packaging water-cooled ground coffee.
Disadvantages of vacuum packaging in flexible containers are linked
with water-cooling, which shortens product shelf life and may decrease
yield. On the other hand, in rigid containers the internal vacuum
widens the difference between the partial pressure of volatile aroma and
ambient, so that more aroma volatilizes to saturate the headspace,
dulling the cup. Furthermore, even if rigid containers can be
strengthened by ribbing to prevent collapse, the vacuum level still
remains less than in flexible bags.
6.2.2.3 Inert gas packaging
In inert gas packaging the air inside the container is replaced by inert gas
either through the compensated vacuum technique or by flushing the
inside of the package with inert gas. In the former case, first a vacuum is
created in the package, then enough inert gas to balance the internal and
external pressures is admitted. In the latter case, a drop of liquefied inert
gas is placed on the bottom of the package, which evaporates pushing the
air out. This process generally uses nitrogen or carbon dioxide which,
although not an inert gas, behaves as such in a moisture-free environment
and, moreover, is naturally present in roast coffee. The use of an inert gas
for coffee packaging increases shelf life three-fold (see Table 6.5) with
respect to vacuum packaging. Even with the same type of package – and
therefore with the same permeability to oxygen and water vapour – the
type of conditioning process will impact on the product’s shelf life, and
consequently on the cup (Alves et al., 2001).
The pressure in the package is in equilibrium with the atmosphere at
the moment of sealing. Just like in air packaging, to prevent internal
pressure rising due to degassing, coffee must be either packed after
degassing or the package fitted with a one-way valve.
From a legal standpoint the gas added to the package is considered a
processing aid rather than an additive, since it dissipates on opening.
248 Espresso Coffee
6.2.2.4 Pressurization
Pressurization is the same as inert gas packaging except that the internal
pressure is higher than atmospheric pressure. If coffee is immediately
packaged after air-cooling, the pressure normally rises due to degassing.
The packaging technique is the same as in compensated vacuum
packaging, but in order to withstand pressure containers must be made
of rigid materials, normally tinplate or aluminium; they must also be
equipped with a safety valve opening when pressure increases by 0.5
atmospheres due to the large amount of gas released (see 6.1.2.1 and
Figure 6.1).
Pressurization has an ‘ageing’ effect with quality improvement after
10–15 days. Indeed, an espresso cup prepared from an aged product has an
improved body and aroma. The ageing may be explained by a binding of
aroma constituents to the oil trapped within the cell structure. During
degassing, pressure rises in the container and the reduced pressure
gradient between the packaging environment and the bean structure
produces two combined effects underlying the ageing mechanism: first, a
decrease in degassing rate reduces oil migration so that more of it remains
in the beans (Clarke, 1987b, p. 204); second, the pressure in the package
becomes higher than the partial pressure of most volatile compounds
Storage and packaging 249
Table 6.5 Barrier properties of packaging
materials for roasted coffee
Material Permeability* Barrier
Tinplate, glass aluminium Complete
Three-ply with aluminium <0.5 Very high
Three-ply with metal 0.5–3.0 High
Two-ply with metal 3.0–30 Moderate
Two-ply 30–150 Low
Single film, treated paper >150 Very low
*Values expressed in:
cm3/m2 24 h bar at 23
C and 0% R.H. for gases: gas
volume (cm3 of gas) that passes through 1m2 of surface
in one day (24 hours), with one bar of pressure difference
between the two layers at 23
C and 0% of relative
humidity g/cm2 24 h bar at 38
C and 90% R.H. for water
values gas mass (grams of water vapour) that passes
through 1m2 of surface in one day (24 hours), with one
bar of pressure difference between the two layers at 38
C
and 90% of relative humidity.
present in the cells, allowing for a larger quantity to be dissolved in the
lipid phase or to bind to melanoidins.
Pressurization, by creating high pressure within the bean, also has the
effect of spreading the oil on the cell walls to form an effective barrier
against oxygen; this effect is clearly visible in Figure 6.10, which shows
the cross-sections of, on the left, a non-pressurized roast bean, and, on the
right, a pressurized one; caramelized material, containing lipids (dissolving
volatile aroma), is shown in orange-yellow, while cell walls are reddish.
(PIC IC B&W)
Due to the difference between internal bean pressure (which at the end
of the degassing process reaches equilibrium with the pressure in the
container) and atmospheric pressure, when the package is opened, there is
still a residual amount of CO2 in the beans to be released. Moreover,
when oxygen enters the beans at the end of the degassing phase, oxidation
is reduced because the aroma constituents are dissolved in the compacted
lipid aggregates, so that, even once the package is open, a pressurized
blend retains its fragrance longer than a blend packaged by other
techniques.
6.2.2.5 Active packaging
The need to stress the performance of existing passive packages and
related packaging techniques on one hand, and the need to guarantee an
adequate product freshness on the other has brought the concept of
‘active packaging’ into consideration for coffee as well. Rooney (1995)
defined active packaging as ‘packaging that performs a role other than an
inert barrier to the outside environment’. A more recent definition of
active packaging is ‘packaging that changes the condition of the packaged
food to extend shelf life or improve food safety or sensory properties, while
maintaining the quality of the packaged food’. This second definition has
250 Espresso Coffee
Figure 6.10 Cross-section of cells from (left) non-pressurized and (right)
pressurized beans
been chosen for the European FAIR project Ct.98-4170. (De Kruijf et al.,
2002).
Active packaging systems can be classified into active scavenging
systems (absorbers) and active-releasing systems (emitters). In the specific
case of active coffee packaging, it includes oxygen and carbon dioxide
scavengers. The oxygen scavenger has the precise task to eliminate
residual oxygen present within the headspace of the packaging in the case
of high barrier packages and conditioning in inert gas, or to eliminate the
oxygen present, in the case of air packaging. In both cases, the
elimination of oxygen contributes to extending the stability of the
product during the storing phase. Moreover, independently from the
conditioning used, in the case of permeable packaging, the oxygen
absorber has the task of capturing oxygen entering through the walls of
the package until it becomes saturated.
In general, the existing oxygen scavenger technologies are based on one
or more of the following principles: (1) iron powder oxidation, (2)
ascorbic acid oxidation, (3) photosensitive dye oxidation, (4) enzymatic
oxidation, (5) unsaturated fatty acids and (6) yeast immobilized on solid
material. They may be used as sachets inserted into the package, or be
bonded by a label to the wall of the package. They can be used as linings
in the closure of the package or can be dissolved, dispersed or immobilized
in the polymer of the package material. The most commonly applied
scavengers in coffee packaging are based on iron powder oxidation; their
advantage is that they are in a position to reduce the level of oxygen
below 0.01%, a value much lower than 0.3–3%, the residual values
typically reached when applying a modified atmosphere packaging
technique.
Carbon dioxide scavengers have the task of removing a proportion of
the carbon dioxide released from the coffee in order to prevent the
package from bursting when the coffee has not been previously degassed.
They can represent an alternative use to the one-way valves or to
materials permeable to carbon dioxide (which are in general also very
permeable to oxygen and water vapour) (Vermeiren et al., 1999).
A combined solution, such as dual action oxygen and carbon dioxide
scavenger systems in the form of sachets or labels, is common in canned
and foil-pouched coffees in Japan and the USA. This dual action system
typically contains iron powder as oxygen scavenger and calcium
hydroxide that scavenges carbon dioxide in the presence of high moisture
levels (http://www.atalink.co.uk/pira/html/menu-packaging.htm ‘A fresh
approach’, Brian Day Campden and Chorleywood Food Research
Association). Some oxygen scavengers are subject to competition
between oxygen and carbon dioxide; for this reason the dual action
Storage and packaging 251
system contains alkaline hydroxide specific for carbon dioxide
(Matsushima et al., 1995).
6.2.3 Control parameters
The packaging process starts from the selection of the packaging materials
and techniques, through conditioning, when coffee is filled into the packs
and sealed, to the final phase when bags and cans are stored and taken to
the place of consumption. Each step of this long chain must be carefully
monitored to prevent or at least correct all possible defects so that the
quality (see 1.3) of the ready-to-use coffee blend complies with
regulations and is up to consumers’ expectations. Various parameters
are monitored throughout the different phases of the packaging process.
6.2.3.1 Packaging material
The packaging material must be adapted to the packaging technique and
to the intended shelf life. As mentioned in 6.2.1, the most important
packaging requirements concern the barrier properties of the chosen
material, i.e., its permeability to gas and moisture. Such barrier properties
can be classified as shown above in Table 6.5.
Glass and metals are air- and watertight; the choice of packaging
materials based on their barrier properties, therefore, only applies to
multi-ply materials, which can be prepared from different polymers. Their
permeability is a function of the type of polymer, of possible surface
treatments on the polymer, their thickness and the combination chosen.
In addition to the packaging material, particularly in the case of tight
containers, the packaging shape must also be decided. The main
parameters to be considered in this choice are volume of headspace,
which should be as small as possible to limit the amount of aroma
volatilized to reach saturation equilibrium, and the ratio of volume to
surface area, which should also be as low as possible, particularly when
incomplete barrier materials are used, since permeability depends on
surface area.
6.2.3.2 Choice of the packaging technique
As seen in 6.1.1, oxygen consumption is the main cause of deterioration
of roast coffee, which should therefore be protected from entering into
contact with oxygen as soon as roasted. The lower the level of residual
252 Espresso Coffee
oxygen in the package, the longer the shelf life of the product. But the
lower the amount of residual oxygen the more effective the package
should be as a barrier to keep oxygen at the same level throughout the
shelf life of the product. The best techniques to improve shelf life are, in
decreasing order, pressurization, inert gas and vacuum packaging; the
technique must be chosen as a function of the desired shelf life.
The choice of the most suitable material is also influenced by the
degree to which it can withstand positive internal pressures, when coffee
is pressurized, or negative ones, when it is vacuum packaged. Table 6.6
shows the implications of these two parameters on shelf life and packaging
requirements.
6.2.3.3 Conditioning
When coffee is air-packed no conditioning is needed; packages are just
filled up and sealed, the only parameter to be monitored being air tightness,
as the package needs only to protect coffee from moisture pick-up.
In vacuum packaging, conditioning consists in creating a 300–500 mbar
vacuum inside the pack before sealing it. The vacuum level must then be
monitored, particularly in rigid cans, which might otherwise collapse. If
the vacuum is replaced by inert gas, the applied gas pressure and/or the
internal pressure of the package must be checked. Pressure levels near
atmospheric pressure are normally applied, while valveless packages are
conditioned under a slight vacuum.
In pressurization, conditioning consists in exactly the same process, but
the cans are pressure-resistant and coffee is not degassed. In this case, in
Storage and packaging 253
Table 6.6 Packaging parameters
Technique
Residual
O2(%)
Shelf life
(mth)
Absolute Pint
(Atm) Material
In air:
tight 16–18 1 nRT/V Rigid
with valve 10–12 3 1.01* Indifferent
Under vacuum 4–6 4–6 0.3 Better
flexible
Under inert atmosphere 1–2 6–8 1.01* Indifferent
Under pressure <1 >18 Up to 2.2 Rigid
*Pressure at which the valve opens.
addition to residual oxygen, the pressure in the package should also be
monitored once degassing has made the package swell.
The airtight seal, already important in air packaging, becomes critical
with all the other types of packages because any leak in the packaging
material would allow oxygen in, drastically reducing shelf life.
Pressure is measured by standard pressure gauges; the airtight seal is
checked by measuring residual oxygen and/or, in the case of positive
internal pressures, by plunging the package in water and checking for
leaks. Various sensor methods are available to measure residual oxygen
levels.
6.2.3.4 Logistics
In logistics, pressure, temperature and time are the key parameters.
The pressure level must be monitored to avoid the risk of swelling or
even explosion from a build-up of pressure in valveless packs. This is why
air transportation of coffee should be avoided, unless pressurized
compartments are available.
The spoilage rate of roasted coffee increases seven-fold with every
10
C rise in temperature. Coffee should therefore be stored at low
temperature. Furthermore, low temperatures slow down evaporation of
volatile compounds, so that coffee kept at low temperature retains more
aroma than at normal temperature, as recently confirmed on roasted and
ground coffee in air by Cappuccio et al. (2001). Cooled coffee beans
should, however, be left at room temperature for a few hours before
grinding to trap the volatile aroma present in the cells in the oil and
melanoidins; a large aroma loss at grinding might otherwise dull the
flavour of the cup. In pressurized coffee the binding of aroma by oil is
hindered if the oil becomes too viscous, as is the case at temperatures
approaching 0
C; pressurized coffee should therefore be kept at room
temperature for a few days after packaging. Temperature fluctuations
should be avoided as they accelerate spoilage.
The last parameter to be monitored is time, i.e. the total storage period,
which should always be kept shorter than the actual shelf life of the
product. On average, coffee is consumed within three months from
production which makes time a critical factor only for the least
sophisticated packaging techniques.
6.2.3.5 Testing of finished products
Although lipid oxidation plays a major role in coffee deterioration, the
peroxide test, commonly performed to assess rancidity in foods, is not
254 Espresso Coffee
sensitive enough for coffee. Peroxides reach significant levels only in the
very last stages of spoilage, when coffee has already become undrinkable.
Linoleic acid content is another parameter of little use to analysts, as it
requires reference values measured on the same blend when fresh.
Electron spin resonance (ESR) measurements have also been carried
out to detect the presence of free radicals; the intensity of the ESR signal
has been found to be inversely related to the level of perceived acidity,
which rises with deterioration; however, even this technique is not
sensitive enough.
The concentration of specific volatile compounds in the headspace of
the package or of the cup, measured by gas chromatography, has been used
to determine the degree of spoilage. For instance, the ratio of
2-methylfuran to butanone, the ‘aroma index’ M/B, drops from 3.2 in
fresh coffee to 2.3 in spoilt coffee. However, such indices depend on blend
composition and on roasting technique, normally unknown to the analyst,
so that these parameters can only be relied on to a very limited extent.
None of the test methods available can yet replace sensory evaluation,
and tasting remains the most reliable of all test methods.
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258 Espresso Coffee
CHAPTER7
Percolation
M. Petracco
For all types of coffee beverages, brewing is intrinsic to the relevant
definition (Petracco, 2001). This holds especially in the case of espresso,
where, among all subsequent technological processes through which
coffee must go before finally getting into the cup, the last one –
percolation – is the stage crucially contributing to the beverage’s qualities.
On the one hand, it is possible to sort, roast, grind and package coffee
following all the established rules for an espresso coffee, as described in
the preceding chapters, and finally prepare a cup by infusion or drip
filtering. The beverage so obtained certainly cannot be called an espresso.
On the other hand, the inverse operation could be reasonably well
performed: it could be possible to prepare an espresso cup (even if not
the best one) by using a low quality raw material, which has been
inadequately roasted and improperly ground, provided that it is
transferred into the cup by an extraction process following the espresso
percolation process, as will be discussed in 7.4 below. With the
importance of extraction as an integral part of the definition of espresso
ascertained, we have to face the necessity of giving a scientific definition
of percolation, and establishing the conditions for a correct espresso
percolation.
7.1 CONCEPTUAL DEFINITIONS
7.1.1 Definition of percolation
Percolation takes place each time a fluid flows through a porous medium.
In hydrodynamics, this concept materializes in many aspects of our
everyday life: rainwater percolates through sand in the soil; the engine
lubricant percolates through the oil filter. While such phenomena are
cited in common language as filtration, this term is used improperly. In
fact, filtration strictly refers to the process of separation undergone by a
suspension (solid particles dispersed in a liquid) flowing through a
filtering baffle: the baffle may consist of a bed of packed particles, a fibrous
tangle or simply a perforated plate. Unlike other forms of liquid/solid
separation (sedimentation, centrifugation, electro-deposition, etc.), filtration
cannot take place without percolation; conversely, a wide range of
percolation phenomena, where there is removal of mass from the solid bed
instead of accumulation, bear no relationship to filtration but rather to
extraction.
The term percolation is, in a more general meaning, used also in the
language of topology, the branch of geometry that studies the characteristics
of forms. Under this meaning, an abstract property of space can
spread from one area to another according to fixed mathematical laws
(Stauffer, 1985). A classical example is Bernoulli’s percolation through a
lattice randomly composed of conducting or insulating segments. If at
least one uninterrupted series of conducting segments stretches from one
end of the lattice to the other, electric current may flow through the
lattice, which is thus called ‘percolating’ (Mandelbrot, 1977). This
reference is pertinent to our case, as the application of topological, or
more generally, mathematical methods may contribute to the understanding
and modelling of hydrodynamic phenomena and, in particular,
of espresso coffee brewing.
7.1.2 Definition of coffee cake
Back to hydrodynamics, it is useful to explain what is meant by ‘porous
medium’. It is an aliquot of space, possibly bounded by walls impermeable
to fluids, and partially filled with a solid material leaving empty spaces,
filled in their turn with a pre-existing fluid that can be displaced by the
percolating fluid (Bear and Verrujt, 1987). A simple example: a bucket
full of gravel has interstices between pebbles, which are filled up by air. By
pouring water into the bucket, the liquid takes up the space previously
held by air and fills up the interstitial spaces. The measure of porosity is
called ‘fraction void’, “, which is defined (Kay, 1963) as
” . empty spaces volume
total volume of the porous body
A porous medium may be composed of a connected structure of
interstices, where the spaces are intercommunicating. It can therefore
260 Espresso Coffee
be filled up by a fluid, a phenomenon called invasion. Conversely, the
interstices may form a non-connected structure (e.g. insulating foam,
made up of many gas cells or bubbles entrained in a solid polymer matrix).
The first case concerns us most, even if ground coffee does not have the
same porosity structure of other typical percolating media with connected
pores, such as tangles of fibres (the most common example is felt) or
sponges (where solid fibres have grown together to form a single body).
Actually, ground coffee behaves as a bed of particles, namely an aggregate
of solid granules in mutual contact under the force of their own weight,
which do not penetrate each other owing to their solid state. Such a bed
may be unstable and prone to rearrangement by vibration, or remain
stable under gentle vibrations but get reshuffled under stronger forces (e.g.
overturning). Particles may also stick together, forming a mass having its
own shape and capable of being handled as one whole solid (e.g. sintered
metals, made up of metal pellets fused together by mutual friction).
Apart from an ideal case, all particles are formed by materials that are
to a certain extent elastic or plastic; therefore, particles are subject to the
force of their own weight and to other external forces causing reversible or
irreversible deformations. Particles form beds that can be compacted by
compression, with reduction of the total apparent volume of the bed.
Owing to this compaction and to a certain degree of adhesion of the
external particle layer, beds can take up a shape stable enough to
withstand handling (with due care in the case of coffee).
This last description provides an adequate definition of a ground coffee
bed, also known as a ‘cake’, ready to be extracted in the espresso machine.
By contrast, ground coffee used in other brewing methods, as for instance
drip filter coffee, lets percolation occur through particle beds that are
much less tightly aggregated than espresso cakes.
7.2 PHYSICAL AND CHEMICAL
CHARACTERIZATION OF THE PERCOLATION
PROCESS
7.2.1 Physical description
From a physical point of view, preparing a cup of espresso coffee means
percolating a definite quantity of liquid through a compact bed of ground
coffee particles, the cake. The latter is usually contained in an impermeable
metal holder, generally made of stainless steel for its inalterability
and easy cleaning. The holder is popularly called a ‘filter’, in spite of the
Percolation 261
inappropriateness of this term as pointed out above: it is approximately
cylindrical in shape, with a perforated bottom. Percolation takes place
when liquid penetrates the interstices between the particles at a certain
spatial velocity, displacing the pre-existing air. The concept of spatial
velocity, borrowed from chemical reactor theory, refers to the flow of
liquid per unit section of the bed. When dealing with espresso, the liquid
involved in this process is water, heated to a temperature just above 90
C
by means of direct or indirect heating.
The driving force that generates the flow is produced by the pressure
drop undergone by the hot water that enters the cake with positive
hydrostatic pressure. Pressure, a well-known notion of general physics,
defines a force exerted perpendicularly over a unit of area. In the domain
of fluid dynamics, pressure refers to an intensive-type property, defined
instant by instant in any point of space taken up by a fluid; it can be
represented by means of a scalar field. The existence of a pressure field
within a fluid yields a potential energy (Bernoulli’s piezometric energy)
that can be easily transformed into kinetic energy, thus giving speed to
elementary masses of fluid.
Pressure energy is partly dissipated as heat by the friction generated
when water flows through the particles, and partly gradually transformed
into kinetic energy of the beverage which flows into the cup, according to
the general equation (Bullo and Illy, 1963):
Pe . Pr . Pc . Pa
where:
Pe . driving pressure (water pressure at the filter inlet)
Pr . pressure drop from bed resistance
Pc . kinetic pressure drop
Pa . atmospheric pressure
The average water velocity must remain constant through the cake in
order to provide a continuous jet, while pressure decreases along the
percolation axis until it reaches atmospheric level. At this point, the
‘liquor’ obtained by percolation flows into the cup underneath by
gravitation.
It is beyond dispute that every percolation method requires some
pressure (at least several millimetres of water column height) to overcome
the head loss required for obtaining a flow through ground coffee beds;
nevertheless, a higher relative pressure (one order of magnitude, or two) is
needed when dealing with finer grinds. Even higher pressure, up to 10 atm
(corresponding to 103 327mmH2O: four orders of magnitude higher),
262 Espresso Coffee
must be used when trying to pass water through a compacted cake of
finely ground coffee, as happens in espresso brewing. The energy
expended during this kind of operation produces interesting effects, like
driving of micron-size solid particles and oil droplets into the cup. This
may influence the beverage’s properties dramatically.
Hydrostatic pressure is generally generated by a device constituted of a
rotary or reciprocating pump: some set-ups nevertheless utilize compressed
air or steam pressure to pressurize a water tank, or exploit the centrifugal
force of a rotary system to push water through the cake. Even a head
formed by a water column (tens of metres high, anyway!) might serve the
task. A description of technological devices that have been developed in
nearly a century, since the birth of espresso coffee, to produce this
characteristic beverage will be discussed in 7.4 below. However, it must be
stressed that an accurate design and a precise mechanical manufacture of
espresso machines are most essential because these machines, even the
more economical models for home usage, are far from being simple (see
later, Figure 7.6).
7.2.2 Physico-chemical description
The intimate contact of water with roasted coffee is the cardinal
requirement for producing a coffee beverage. From a chemical point of
view, percolation induces two processes: the extraction of the watersoluble
substances and the emulsification of the insoluble coffee oils. Both
phenomena together are responsible for the considerable differences
between pure water and the espresso brew (Table 7.1).
Solid-liquid extraction produces the removal of a soluble fraction – in
the form of a solution – from an insoluble permeable solid phase where it
belongs. Two steps are always involved in extraction: contact of solid and
solvent to effect mass transfer of solubles to the solvent, and separation of
Percolation 263
Table 7.1 Espresso brew contrasted with pure water
Colour Brown-black, due to partially defined pigments (melanoidins)
Refractive index Increased by the presence of solutes
Surface tension Reduced by surface-active substances present
pH Lowered by organic acids and by phosphoric acid
Electric conductivity Higher, owing to the ions presents
Viscosity Higher, mainly due to the oils in emulsion
Density Higher, due to the high solute content
the resulting solution from the residual solid, this latter step is usually
done by filtration. The mechanism of the first step is favoured by
increased surface per unit volume of solids to be extracted, and decreased
radial distances that must be traversed within the solids, both of which are
favoured by decreased particle size as it occurs in coffee ground for
espresso.
The main dependent variable that can be objectively measured (along
with the obviously all-important sensory ones, which are largely based on
subjective evaluation) is brewing yield, namely the ratio between the
mass of the coffee material that passes into the cup and the total coffee
material used (the balance to be disposed as spent grounds). Brewing
temperature exerts a preponderant role on yield variability, which may
range between 20% and 30%, and darker roasting procures higher yields.
It is worth mentioning that yield is a different concept from beverage
concentration – or strength, measured in grams of extracted matter per
litre of beverage – and can be set independently, if a different brewing
formula (coffee/water ratio) is chosen.
The general principle ruling extraction is Fick’s law of diffusion, which
gives the quantity s of solute diffusing from a solid particle surrounded by
a liquid. It can be written in the following form (Barbanti and Nicoli,
1996):
s . k T/ A/x (C c)
where:
k . constant, depending on molecular factors
T . absolute temperature
. viscosity of the liquid, f (T)
A . layer cross section around the particle
x . layer depth
C . solute concentration in the solid
c . solute concentration in the liquid
. contact time
In espresso percolation, the short extraction time does not allow
equilibrium to be reached. This calls for consideration of reaction kinetics
too, especially the branch dealing with the diffusion of water entering,
and of molecules of various solutes leaving, a cellular structure. Optical
and electronic microscopic examinations of the cell structure of coffee
show the presence of soluble substances both outside and inside the cell
wall already in the green bean (Dentan, 1977), and no evidence of pores
or channels running through the cell wall has yet been produced.
264 Espresso Coffee
Therefore, it may be assumed that percolation provokes an extraction
mainly by washing out the outer surface of the particle, rather than by
diffusing from the interior of the particle to the solution.
A naive way of thinking could suggest that the more one extracts from
one’s purchased material, the better: nothing is less true, quality-wise.
Since the many chemical species identified in roasted coffee (more than
1800 so far) exhibit different extraction rates (Lee et al., 1992), it is
logical that extraction yield relates to sensory quality too, leading to the
two celebrated converse brewing errors: under- and over-extraction. The
former is due to lower yields (deriving from whatsoever reason, like low
water temperature, short contact time or too coarse grind), when the most
soluble substances – the acidic and sweet ones – are predominantly driven
into the cup, producing the so-called under-extracted beverage. The latter
happens when higher yields are obtained, by forcing more substances –
the bitter and astringent ones – into the cup. It is therefore evident that
brewing yield should be adjusted according to personal taste, in agreement
with the nature of the beans used (origin, blend, roasting).
Only few authors have tackled so far the study of coffee extraction
kinetics (Voilley and Clo, 1984; Nicoli et al., 1987; Cammenga et al.,
1997). The only substance that has been studied to a considerable extent
is caffeine, the constituent producing the most remarkable physiological
effect (Spiro and Selwood, 1984; Spiro and Page, 1984; Spiro and Hunter,
1985; Zanoni et al., 1991; Spiro, 1993). Caffeine exhibits high solubility
in hot water (Macrae, 1985; Cammenga and Eligehausen, 1993);
nevertheless, its quantitative extraction may be obtained only by coffee
preparation methods that permit a long contact period between water and
ground coffee (Peters, 1991). In espresso percolation, caffeine extraction
yield is usually within the range 75–85% (Petracco, 1989); this relatively
low figure may be explained by the fact that caffeine is extracted from
within the cell, by diffusion, and not by simple washout kinetics.
The other important chemical phenomenon occurring during coffee
percolation is the emulsification in water of non-soluble matter, mostly
composed of lipids, present in coffee in quantities ranging from 8 to 16%
dry basis (see 4.3.2.3), that are mainly contained inside the cells of roasted
coffee. By means of the energy transferred to the bed by the water pressure
drop, the lipid fraction in broken cells may easily form an emulsion.
Optical microscopy of emulsified lipids in an espresso brew (see Figure
8.2) (Heathcock, 1988) shows that they are present as small droplets
varying from a minimum of 0.5 mm to a maximum of 10 mm in diameter.
As will be seen in 8.1.2, the stable emulsion formed during espresso
percolation does not coalesce spontaneously, even after weeks of rest.
None of the artifices commonly used to break up emulsions (filtration,
Percolation 265
dilution, addition of electrolytes, etc.) overcomes the reluctance of
espresso coffee to be separated into two distinct liquid phases. The small
diameter of droplets is a factor that helps in precluding coalescence;
moreover, the density of the disperse phase is not very different from that
of the solution – this prevents droplets from buoying up and concentrating
on the free surface. The key factor of emulsion’s stability, whose
importance in the sensory context will be discussed in 9.5, is constituted
by a layer of surface-active substances surrounding each droplet and thus
generating the chemical stability (Navarini et al., 2004).
7.3 MODELLING OF THE PERCOLATION
PROCESS
The first attempt to set up a mathematical model for the espresso brewing
process dates back to the early 1960s (Bullo and Illy, 1963), when the key
role of the pressure drop due to the hydraulic resistance of the ground
coffee cake was first recognized. As a starting approximation, it can be
assumed that the percolation process can be hydraulically defined by an
equation associating five variables, sufficient to characterize the process in
its macroscopic physical aspect:
V .
Zt
0
p.t.
R.t; T.t. dt
where:
V . volume of beverage in the cup
T . temperature inside the coffee cake
t . percolation time
p . pressure difference above and under the cake
R . hydraulic resistance of the cake.
This choice of variables is not the only possible one: for instance, the
volume of beverage in the cup, V, might be replaced by its derivative in
relation to time, the instantaneous flow Q. Moreover, the hydraulic
resistance R depends on sub-variables as the dose and particle size of the
ground coffee, as well as the compacting of the bed. Anyway, five
variables are sufficient to characterize the process in its macroscopic
physical aspect, in agreement with the traditional rule of the barista (the
espresso bartender):
266 Espresso Coffee
To prepare an espresso cup correctly, it is necessary to set the right
temperature and pressure, then to adjust the hydraulic resistance (by
grinding and compacting), until the right volume of beverage is obtained
at the right time.
To model the phenomenon theoretically, that is to forecast the value of
a dependent variable once the values of the independent ones have been
set, a semi-empirical approach uses flow, pressure and temperature
continual sensors to obtain flow curves as a function of time (Petracco
and Suggi Liverani, 1993). Classical hydraulics would apply Darcy’s law –
‘The loss of pressure in a pipe is proportional to the flow rate of the liquid
through it’ – or:
p . RQ
where:
Q . flow
R . hydraulic resistance of the system
But two observations that lie in clear contradiction with this equation
emerge from the experimental curves (Figure 7.1):
Percolation 267
Figure 7.1 Pressure dependence of flow during percolation
n flow is not constant for a constant pressure drop, but after an initial
transient peak it decreases in time until it reaches an apparently
asymptotic value (dependent on temperature);
n the mean flow is not proportional to the pressure applied, but increases
with it up to a certain value, then remains constant or even decreases.
The anomalous behaviour of the pressure is not linked to the extraction
reactions, started by high water temperatures not far from 100
C. It is
actually feasible to percolate ground coffee with as cold water as possible
(in practice not lower than 4
C to prevent freezing) so as to minimize any
influence of solubilization on the physical phenomenon. Data show that
the asymptotic non-monotonic behaviour persists also under such
conditions, even if higher flows are needed (Figure 7.2).
However, the flow’s quasi-exponential decay suggests that the bed of
ground coffee particles should present a time-dependent geometry
(Baldini, 1992). An indication that some form of modification does
occur inside the bed derives from electron microscopy of the bed structure
(Figure 7.3), which is made up of coarse particles containing fragments of
cell walls, called fines. The latter are assumed to migrate to and eventually
to concentrate at the bottom of the bed. Indirect confirmation comes from
268 Espresso Coffee
Figure 7.2 Temperature dependence of flow during percolation
tests where the direction of percolation was reversed, using an extraction
chamber that could be turned upside-down.
The direct flow behaves normally, remaining unchanged even when
the pump is stopped for a brief moment. Surprisingly, flow increases when
the percolation chamber is inverted (Figure 7.4). This effect can be
explained by assuming that the finest particles, which concentrate in a
lower section of the cake causing an increase in the hydraulic resistance,
now counter-migrate, with an increase in hydraulic conductivity in the
opposite direction, until the system reaches a new steady state.
Attempts have been made (Baldini and Petracco, 1993) to formulate a
law providing an explanation for this experimental behaviour, but finding
Percolation 269
Figure 7.3 Microstructure of ground coffee particles
Figure 7.4 Direct/inverse discharge curve
an exact analytical solution to the consequent system of differential
equations still remains a difficult, if not impossible, task. Computer-aided
calculation of the numerical solution to these equations becomes feasible
only by fixing the boundary conditions; the coefficients, however, must
always be determined experimentally. The theory has yet to be expanded
to include not only the physical, but also the chemical aspect of
percolation and possibly to allow also forecasting sensory features like
foam, body and flavour.
Another method, alternative to differential equations, is offered by
‘molecular ontology’, a branch of ‘naive physics’ that studies physical
phenomena starting from experimental and qualitative observation via a
heuristic approach (Bandini and Cattaneo, 1988). A model is constructed
by breaking down a physical phenomenon into its constituent elements
(in the present case, coffee particles and elemental volumes of water), and
by describing their local interactions, whose synergism builds up the
system’s overall behaviour.
Simulations of espresso percolation obtained by powerful computers
(Figure 7.5) demonstrate that particles with a given possibility of
movement tend to migrate and to accumulate in a specific critical
section simulating a metallic filter (Bandini et al., 1997). This technique
shows micro-vortexes and allows the trajectories of the liquid and the
particles to be followed (Cappuccio and Suggi Liverani, 1999).
7.4 THE ESPRESSO MACHINE
As can be seen from what has been said so far, preparing an espresso coffee
requires equipment able to:
n keep a portion of ground coffee in the form of a compact cake inside a
container that allows the brew to drip out while holding back the
particles (spent grounds);
270 Espresso Coffee
Figure 7.5 Stages of cake percolation simulation
n impart to the water a temperature close to 90
C and a pressure
customarily fixed at 9 relative atmospheres.
Such equipment is called an ‘espresso coffee machine’, often shortened
into ‘espresso machine’ or, in Italy, ‘coffee machine’. Coffee machines are
available on the market in many models, ranging from compact light ones
for occasional home usage to the professional computerized units. The
main parts of the machine are the pump, the heat exchanger and the
extraction chamber, as sketched in Figure 7.6.
The origin of espresso coffee machines dates back to the beginning of
the 1900s, when the pioneer Bezzera patented a machine in which
pressure was applied to water by simple boiling in an autoclave. The
drawback of that design is that the attainment of the desired pressure
levels (around 1.5 atm) required a very high temperature, well above
100
C. These conditions cause the extraction of substances normally
insoluble, imparting an unpalatable bitter flavour to the brew. With the
knowledge of the espresso technique available now, a brew obtained with
such a machine could no longer be called an espresso, due to the much
lower pressure applied; this principle is still widely employed today in
Italian home coffee brewing by the commonly used inexpensive machine
called a ‘Moka’ (Petracco, 2001).
This drawback can be avoided by separating the generation of pressure
from that of temperature. After the compressed-air machine, whose
technology was probably ahead of its time (Illy, 1935), a lever-system was
developed in 1945 by Achille Gaggia. It consisted in a spring which,
wound up by the muscular force of the barista, released energy – via a
Percolation 271
Figure 7.6 Schematic flow diagram of an espresso machine
piston – to the water contained in a pressurization chamber. This system is
still sometimes favoured in traditionalist coffee houses; its performance
depends on the operator’s manual ability in properly dosing and applying
force and time, rendering espresso preparation an ‘art’ with uncertain
results. The system has the advantage of allowing the manufacture of
machines that do not require electric energy, as heating can be
accomplished by a gas burner, and no pump is needed to pressurize the
water.
7.4.1 The pump
The spreading of electricity allowed the automation of espresso machines,
mainly through the introduction of a pump as the source of pressure.
Professional pumping devices consist in an impeller with fins, driven by
an electric motor, and in an adjustable bypass valve. The purpose of the
latter is to withdraw part of the water delivered by the rotor at the
discharge outlet of the pump, and to recycle it to the suction inlet: this
power dissipation is legitimate because it enables the pump head (back
pressure) to remain constant, independent from the variability of the
hydraulic resistance (coffee cake) downstream. Also, the repeatability of
percolation has considerably benefited from the use of such pumps,
because an electric motor is far less prone to imprecise behaviour than
muscular force.
Machines for home usage, where neither the encumbrance nor the cost
of a centrifugal pump is justified, utilize preferably a vibration pump. This
is a volumetric pump where a small piston is set in a vibrating
reciprocating motion by an electromagnet coupled with a return spring.
These pumps exhibit a very high head (up to 30 atm), heavily depending
on the hydraulic resistance downstream. As a consequence, the quality of
the beverage in the cup is strongly influenced by the amount of ground
coffee used, and by the degree of grinding and of compacting. A less
common alternative, used to generate water pressure in commercial
domestic machines, is centrifugal force.
7.4.2 The heat exchanger
The invention of waterproof electric resistors, which permitted the
manufacture of direct heat exchangers, was another major innovation. In
modern espresso machines (see Figure 7.6), the heating of water is based
on a low-pressure boiler of a capacity of 3–10 litres, heated by a
waterproof electric resistor immersed in it (seldom by a gas burner). This
272 Espresso Coffee
reservoir supplies hot water for tea and other infusions, and saturated
steam for cappuccino steamed milk. In addition, it works as heat storage to
warm up the espresso percolation water, which runs through a completely
different circuit. This water comes directly from the tap and is driven by
the pump through a coil immersed inside the boiler, where it reaches
dynamically the desired temperature (many degrees lower than the
temperature in the boiler itself). This system, particularly cumbersome
and expensive, is applied in all professional machines and only in some of
the classiest home-use machines, where it permits an ample production of
steam for cappuccino preparation.
In smaller machines, the boiler is replaced by a direct heat exchanger,
consisting of a metal block with an enclosed electrical resistance fed by a
bimetallic thermostat or, in more modern models, by an electronic power
supply driven by a thermocouple sensor. Cold water is forced by the pump
to pass through a sinuous circuit inside the block, and then passes directly
through the coffee cake beneath. In professional machines, conversely,
temperature is regulated by varying the boiler pressure (see also 7.5.6).
7.4.3 The extraction chamber
The extraction chamber consists in an upper block, on which is inserted
(like a bayonet) a filter-holder cup. The ground coffee portion is poured
and compacted into this filter. Since the block protrudes from the body of
the machine, it is recommended to heat it by means of a small calibrated
flow of boiler water passing through a built-in tube. When the pump is
switched off, water is no longer able to flow through the cake and remains
under pressure within the chamber, due to the increased bed resistance
caused by hydration of the spent coffee grounds. If not overcome, this
residual pressure would cause troublesome splashes when removing the
filter-holder from its block. Therefore, the hydraulic circuit of all espresso
machines is equipped with some kind of device that allows depressurization
of the extraction chamber at the end of percolation, and this is
usually achieved by calibrated check valves, or servo-controlled valves
synchronized with the pump power switch.
7.4.4 Water pre-treatment
Another all-important piece of equipment is the pre-treatment unit for
the water, as the latter usually comes directly from the mains water supply
system with substantial calcium and magnesium content. Any system of
heating hard water must forestall the formation of alkaline-earth
Percolation 273
carbonate deposits on heated surfaces, and the consequent harmful
decline in heat exchange efficiency (Cammenga and Zielasko, 1997). An
operation most commonly implemented to prevent malfunctioning in
professional machines is water softening. It consists in passing the water
through a bed of ion-exchange resins, which capture calcium ions present
in the water and replace them with sodium ions, whose salts are
completely soluble. Water softeners need regular regeneration by washing
with sodium chloride: this is a vital operation for ensuring efficient water
conditioning and a long service life of the espresso machine. Finally, the
effect of water acidity and hardness on espresso cake may affect extraction
(Fond, 1995). More details about water influence on espresso will be
found in 7.5.5 below.
7.5 PARAMETERS INFLUENCING PERCOLATION
The three main features that lead to the definition of espresso are:
n ‘on the spur of the moment’: the brew must be prepared just before
serving it;
n ‘in a short time’: brewing must be fast;
n ‘under pressure’: mechanical energy must be spent within the coffee
cake.
Whereas the feature ‘on the spur of the moment’ (or, more colourfully,
‘extemporaneously’) refers not so much to the coffee brewing method as
to the ‘espresso lifestyle’, the other two features are strictly related to
percolation.
These general features are necessary for the identification of an
espresso. Nevertheless, they are insufficient to establish whether that cup
has been prepared correctly, that is, whether it falls within the interval of
the n-dimensional space of the fundamental preparation variables that
allows the consumer to expect a sensory experience that is up to the wellestablished
fame of espresso. In this interval, the ideal espresso is
represented by a point, and probably not always by the same point nor by
one single point. Reaching this point is an art, and is so related to the
subjective sphere of the individual that it cannot be completely clarified
in a technical book. The present work can, however, strive to fix the
boundaries of the n-dimensional interval. They could be imagined as the
limits of an ‘excellent espresso’, with each one being a necessary, but not a
sufficient condition (Swartz, 1997).
274 Espresso Coffee
The first difficulty encountered is establishing the number of dimensions
needed to describe an ‘excellent espresso’. Certainly, they are more
than those five variables mentioned in 7.3, because each of the latter is
the aggregation of several sub-variables. Moreover, some variables have
not as yet been mentioned, inasmuch as, if we wished to tackle every
aspect of coffee preparation, a myriad of variables should be considered
(Illy, 2002). For practical reasons, we shall limit ourselves to discussing
the following ten:
n ground coffee portion
n particle size distribution
n cake porosity
n cake shape
n water quality
n temperature
n pressure
n percolation time
n cake moistening
n machine cleanliness
The first five variables refer to the raw materials: four of these are related
to the product to be extracted (the ground coffee cake) and can be seen as
factors influencing its hydraulic resistance; an additional one is dedicated
to the second main raw material of espresso coffee, water, which is often
neglected because of its inexpensiveness. The last five variables refer to
the machine. Three of them, temperature, pressure and percolation time,
are, together with hydraulic resistance, the primary independent variables
of the percolation equation (see 7.3). The last two, cake moistening
methods and machine cleanliness, are also important, yet neglected,
factors.
7.5.1 Ground coffee portion
A ‘portion’ is the weight of roast and ground coffee required for preparing
one cup. The acceptable extreme range of values for a portion may be
from a minimum of 5 g to a maximum of 8 g. Minimum values are
sometimes used in coffee houses that wish to ‘save’ on coffee: these values
are only barely tolerable when dark roasted coffee is used, for it has a
higher content of soluble substances. Maximum values are often
encountered in home-brewed coffee, as the host inclines to overdose
the serving in order to offer the guest the best possible coffee cup. This
Percolation 275
practice is risky, however, because, as will be seen in 7.5.4, an excessive
amount of ground coffee does not permit sufficient expansion during cake
wetting. This causes over-compacting (deriving from the imbibition
forces), which disturbs percolation and may cause the deposit of solids
into the cup.
In professional practice, a double portion is generally used because
coffee-bar espresso machines are designed to meet consumer demand in
peak hours. This need is fulfilled in two ways: the simultaneous use of
more than one percolation head (up to five, or most often three per
machine) or by preparing two cups per head at the same time. The social
role of an espresso, particularly in Italy, is such that a person wishing to
enjoy a cup of espresso hardly ever enters a coffee house alone, therefore
espresso machines are usually optimized for a double portion, equal to 13 g
of ground coffee. Accordingly, the optimal portion used for one cup
(namely the single serving asked for by an individual customer) is of 6.5 g
in the case of arabica blends, something less for robusta blends because of
higher extraction yield. Also roasting degree influences yield, up to a
point: therefore darker roasts allow for slightly reduced dosages.
Unlike in quality-control laboratories, where the portion is determined
by weighing on analytical balances (with a precision of 0.1 g), in espresso
bars it is measured volumetrically using the so-called ‘grinder-doser’. It is
an additional appliance fitted on the professional grinding machine (with
a capacity of at least 2 kg of ground coffee per hour), consisting in a device
with rotary paddles into which ground coffee powder drops from the
grinding tools. The height of the pallets can be adjusted, so that the
portion released at each ‘stroke’ corresponds to a standard weight. At
home, the easiest way to determine a portion is by using a measuring
spoon, which allows the right volume of coffee powder to be drawn from
the package, or the right volume of beans to be ground each time. The
volume-to-weight equivalence is not absolute, as it depends on the
density of the powder, which in turn depends on the degree of compacting
in the package; variability is even larger for whole beans. In practical
terms, however, the error is negligible.
7.5.2 Particle size distribution
Undoubtedly, coffee granulometry is a crucial factor for espresso, but it is
also one of the least investigated, and probably least understood, variables
of percolation. Particle size refers to the measurement of the geometric
characteristics of a population of particles constituting a powder, and to
its representation. The most studied characteristic is size distribution,
276 Espresso Coffee
while particle shape is also a matter of great interest. Common ways of
measuring it have been described in 5.3.
Size distribution of a powder sample may be represented by a histogram
or, when many measure points are used, by a chart showing a grading
curve. The curve is plotted on a diagram (see Figure 5.4) in which the
axis of the abscissa, usually on a logarithmic scale, carries the ‘equivalent
diameter’ variable: this variable can be defined differently according to
the technique adopted, but it is generally linked with particle size. The
percentages of particles belonging to each size group are plotted on the
axis of the ordinates in differential (frequency chart) or integral form
(cumulative curve). The percentage may refer to the number of particles,
to their volume or to their surface, or even to their mass or weight. By
calculation, it is possible to pass from one form of chart to the other,
once the relationships between particle shape and density factors have
been determined. The comparison of ground coffee batches obtained
from different grinding equipment shows differences that characterize
each ground coffee by its own grading curve, as fingerprints identify a
person.
A fundamental parameter to obtain a correct percolation, the so-called
‘fineness’, refers to the average particle diameter and depends on the
adjustment of the grinder. The most suitable particle size distribution for
espresso percolation is not, however, a single-size grading; consequently,
neither the average diameter nor the mode of the curve are sufficient to
describe the correct size distribution.
An empirical compromise reached by grinder manufacturers resorts to
an intermediate distribution, which can be modelled either by power-law
or log-normal distribution (Vicsek, 1991), occasionally with a typical
bimodality or even trimodality. Such a complex characteristic of particle
size is believed to produce a double effect: on the one hand, it forms a
coarse fixed structure, which allows the correct flow through the cake; on
the other hand, it forms a large quantity of fines of high specific surface,
which permit the extraction of a large amount of soluble and emulsifiable
material. Furthermore, the coffee cake appears to behave as a selffiltering
structure, retaining the fines that have been displaced by the
flow.
7.5.3 Cake porosity
Theoretically, the most suitable particle size distribution is the one that
exposes the maximum surface area to the action of water. However, such
a distribution would not permit any porosity of the cake capable of
Percolation 277
maintaining the hydraulic resistance within limits that allow a correct
flow. While in aggregations of particles having the same shape the
fraction void (see 7.1.2) does not vary with different powder fineness, it
can be profoundly modified when size distribution is changed (for
example, by transforming a monomodal into a bimodal distribution: as
is the case when sand is added to gravel in order to render it less
permeable). The same statement of bimodality holds true for the shape of
particles, because irregular particles (such as fines made up of fragments of
fractured cells) may easily sneak through regular-shaped particles and
increase specific surface.
Another factor affecting cake porosity is compacting. The bed of loose
ground coffee poured into the filter by dropping must be compacted,
usually by hand. The compacting force may vary from a few kilos, for a
vertical upward thrust (a tamping plate is usually built-in with the
grinder), to approximately 20 kgf for downward compacting by a hand
tool, the tamper: in the latter case, the force of the thrust may be
increased by part of the operator’s own weight.
Compacting influences percolation even when exerted weakly, that is,
the difference in hydraulic resistance between a loose bed and a weakly
compacted one is large, but there is only a minor variation between
weakly or more forcefully compacted beds. A plausible explanation for
this phenomenon could be the formation of small localized channels in
the upper layer of a loose bed which, when struck by a jet of boiling
water, does not resist to impacting water as firmly as a compacted cake
does.
Compacting forces much larger than those just mentioned (40 kgf or
more) are employed in the industrial production of a patented system of
single portions of ground coffee, sealed between two layers of filter paper
(Illy, 1982). These portions (also called pods or servings) are percolated in
especially designed semi-automatic coffee machines aimed at the
preparation of an espresso at home, where they free the operator from
manual dosing of ground coffee and from most possible errors in
preparation.
7.5.4 Cake shape
The shape of the coffee cake, determined by the filter’s shape, is
important when considering peculiar forms that deviate from the
traditional ones, universally accepted by current coffee machines. The
classical shape of the double dose is cylindrical, around 12mm in height
by 60mm in width, the exact dimensions depending on the machine. If
278 Espresso Coffee
this height-to-diameter ratio of about 0.2 is lowered, very fine grounds are
required for coffee to percolate within a standard time; this choice would
degrade the reproducibility of percolation, because of the creation of
localized channels by water. A larger height-to-diameter ratio would give
the cake the shape of a column; in this case, optimal extraction would
demand an excessively high pressure (as in liquid-phase chromatographic
columns). Conversely, for normal pressure values the grind would be so
coarse as not to offer a sufficient number of fractured cells, resulting in a
low extraction. Consequently, only minor variations from the optimal
form of the established filter can be recommended.
An important phenomenon to be taken into account when considering
the shape of the cake is the expansion of the bed due to the swelling of
coffee particles when wetted. This is a phenomenon of colloidal
imbibition, where a chemical hydration of the organic materials (such
as the polysaccharides) takes place. The water involved in wetting the
coffee grounds is bound, therefore, in an aggregation state differing not
only from the ordinary liquid state, but also from that of interstitial water,
which is subject to the laws of capillary phenomena. This is shown by the
fact that imbibition water cannot be eliminated by oven heating at
100
C for a protracted length of time, but only by raising the oven
temperature by several degrees. During expansion, wet coffee grounds
exert a pressure comparable to that of the wooden wedge used in the past
to cleave marble blocks, whose force of expansion, once it is driven into
the block and wetted, prevails over the cohesive forces of the stone and
splits the block into slabs. On account of this behaviour, an empty space is
left over the ground coffee cake inside the extraction chamber. The actual
expansion of the cake varies with blend, roasting degree and dose, and
determines the exact headspace (around 5–6 mm) needed to prevent
over-compacting.
A final shape characteristic worth mentioning is the filter perforation.
In 7.2.1, the term filter was mentioned as inappropriate for indicating the
perforated plate that constitutes the base of the cup containing the
ground coffee bed. Actually, this plate does not act as a filter but as a
supporting structure for the compacted coffee cake. The diameter of the
holes (usually 0.25 mm) is designed to prevent the coarse particles that
break away from the lower layer of the cake from passing into the cup. In
order to prevent solidified caramelized residue from obstructing the holes,
they are triangular in vertical cross-section: the coffee powder comes into
contact with the smaller sized opening of the hole, beneath which there is
an open truncated-conical cavity. The filter contribution to the overall
hydraulic resistance is small, and becomes negligible when the holes get
worn.
Percolation 279
7.5.5 Water quality
The second most important ingredient of espresso – after the coffee itself
– is water, constituting more than 95% of any coffee beverage. However
self-evident this statement may seem, the role of water in coffee
preparation must be taken into due consideration. It would be otherwise
senseless to devote utmost care and attention to only one of the two
ingredients, while neglecting the other. Yet, this regularly occurs, partly
because the choice of the proper water is left to the end user and partly
because that person (espresso bartender, home user) has an extremely
limited influence over the choice of the water. In fact, very few people
brew coffee using mineral water, whose characteristics are stated on the
label and are supposed to be constant. Most consumers trust tap water
quality, which they rarely purify in any way.
It must be stressed that the quality of treated water coming from public
waterworks is perfectly suitable for human consumption (WHO, 1993)
and further purification or disinfecting treatments are unnecessary.
However, treatment of drinking water may turn out to be beneficial in
two critical cases; one is linked to sensory perception, and the other
concerns the use of the espresso machine.
In the first case, the beverage must be free from any unpleasant foreign
flavours left over in the water by generalized disinfecting treatments, such
as chlorination. Salts that release free chlorine (hypochlorites) lend the
beverage their peculiar taste, which is perceptible when the initial
concentration of Cl2 exceeds 0.5 mg/l. A system most commonly used for
removing chlorine taste involves passing the liquid through a bed of
granules of activated carbon, which has the ability to adsorb many odourbearing
substances. A drawback in the use of activated carbon is that
saturation cannot be easily detected, so that it is necessary to regenerate
or replace the bed regularly.
The second critical case is related to water hardness, namely to the
water’s calcium and magnesium content. On heating, these cations
produce insoluble salts (mainly carbonates, but also sulphates and
silicates). These salts tend to precipitate in the form of compact plaques
(scaling), particularly on the heated surfaces. Such scales form a coating
on the surfaces, which detrimentally affect the heat exchange coefficient.
This may lead to serious technical troubles with the espresso machine, in
particular causing a reduction in heat transfer effectiveness with a possible
consequent widening of the fluctiations in temperature (too hot/too cold)
and risk of failure in electrical resistance due to overheating.
Potable water may be nearly always defined as hard, that is, having a
total hardness that exceeds 15 French degrees, corresponding to a
280 Espresso Coffee
concentration of 150 mg/l of CaCO3. In some waterworks in Italy, for
instance, water hardness may reach a peak of 37 French degrees. This is
not a reason for health concern, because the hardness for drinking water
recommended by Italian law (DPR 236, 24/5/88) ranges from 15 to 50
French degrees. It is also worth remembering that mineral waters may
exceed 100 French degrees.
Direct use of hard mains supply water in both professional and home
espresso machines produces unacceptable deposits in 1–3 months.
Manufacturers of home-use machines advise removing these deposits by
periodic washing (see 7.5.10) with weak acid solutions: citric or formic
acid, even vinegar, will do. In professional machines, scaling is prevented
by fitting the machine upstream of a cleanup system specifically active on
calcium and magnesium ions, but nearly as active on other metal ions.
Two types of treatments are commonly available: softeners or demineralizers.
A softener consists of a bed of ion exchange resins that traps calcium
ions and replaces them with sodium ions (which do not affect hardness,
and do not form deposits) to a hardness depending on the saturation of
the resin. Optimal water hardness to avoid scales and have good
percolation is around 9 French degrees, as recommended by manufacturers.
The resins must be periodically regenerated by washing with NaCl
so as to restore their effectiveness. In the case of treatment of very hard
water, the amount of sodium released (in combination with other ionic
species present) may become perceptible to taste and increase water pH
on heating. Two troubles may be encountered with softeners:
n A functional problem, inasmuch as localized channels form in the
resin bed after some time, diminishing the effect of ion exchange.
This can be avoided by stirring the resins from time to time.
n A hygiene risk, namely the danger of a possible proliferation of
microorganisms in the bed. This can be averted by using beds
containing a bacteriostatic additive.
Demineralizers eliminate Ca.. ions without introducing any foreign
ions. Ion exchange of the demineralizer employs either separate beds of
resins, fitted in series, or one single bed in which resins have been mixed
but that can be separated by floating. One resin is of the so-called cationic
type and captures Ca.. and Mg.. by replacing them with H.; the other
resin is of anionic type and captures the negative ions (Cl–, HCO3
–,
SO4
– – and others) by replacing them with hydroxyls, which neutralize
hydrogen ions forming H2O. These types of resins must be regenerated
separately with HCl and with NaOH, respectively. A more modern type
Percolation 281
282 Espresso Coffee
of demineralizer is based on the principle of reverse osmosis. Pressure
energy is employed to filter water through a semi-permeable membrane
capable of retaining ions, which can be discarded as concentrated
solution, while letting the smaller H2O molecules through. With the use
of a suitable membrane or of a graded bypass, water can be obtained at the
desired degree of hardness.
The main influence of hardness on beverage quality is an increase in
percolation time, compared to soft water below 2 French degrees.
However, only minor differences occur when water hardness is raised
above 8 French degrees to typical mains water values (Rivetti et al.,
2001). Hence, it is not recommended to soften or demineralize water
below this value, otherwise a coarser grind should be used to compensate
for the increase in percolation time. Water pH also affects percolation
time even if in a less predictable way, probably in association with
hardness. The influence of hardness and pH on percolation might be
explained by the change, due to Ca and H ions, in foam-producing and
emulsifying properties of the natural surfactants present in roasted coffee
(see 8.1.2.4). It is unclear, though, whether the increase in percolation
time is to be ascribed to increased foam and emulsion viscosity, with
respect to pure water, or to the interstitial formation and precipitation of
insoluble calcareous soaps or other pH-dependent material.
7.5.6 Temperature
Quantitatively, this relationship between the rate a reaction proceeds and
its temperature is determined by the Arrhenius equation:
k . A e–E/RT
where:
k . reaction rate
A . constant
E . activation energy
R . gas constant (0.082 litre atm/mole
K)
T . temperature
Extraction of coffee, like any dissolution reaction of a chemical species in
water, depends on the constant of equilibrium and on the rate. Higher
temperatures are matched by increased percentage of extracted material.
It is disputable whether the additional fraction of substances extracted
at higher temperature improves the taste of the brew, or enhances any
desirable property. The appeal of an espresso prepared under optimal
Percolation 283
hydraulic conditions is known to increase with temperature, up to a
maximum at 92
C. The extraction of bitter and astringent substances at
higher temperatures reduces the overall sensory appreciation.
Unfortunately, in practice, temperature cannot be easily singled out
from the other percolation variables, because hydraulic resistance of the
cake depends on extraction temperature. For instance, reference has been
made in 7.3 to the fact that cold water flows are larger than hot water
ones, when obtained through the same cakes and under the same
conditions. This difference becomes noticeable when trimming the
canonical temperature of 92
C down to 70
C; the effect is slighter, yet
significant, at temperatures around the optimal value (Andueza et al.,
2001). This makes temperature regulation one of the most important
operations in designing an espresso machine (see 7.4.2).
The control of espresso water temperature in professional machines is
established by varying steam pressure in the boiler, where water is heated
passing through an immersed coil acting as a heat exchange surface. The
temperature of steady water in equilibrium with its own steam is univocally
dependent on pressure. Espresso water heated this way has a reasonably
constant temperature, thanks to over-dimensioned exchange surfaces.
Home-use machines, when not equipped with a boiler, are fitted with a
thermostat controlling the temperature of the metallic block, which acts
as a heat exchanger. This thermostat is often built in on the unit, and
fixed; in models where it is adjustable its fine-tuning can usually be done
using a small screwdriver until the temperature of the water dripping from
the empty extraction chamber reaches 90 2
C.
The temperature of the water is certainly the main thermal factor in
percolation, but not the only one. Also important is the cake temperature,
induced by both the mobile and the fixed equipment constituting the
extraction chamber. In order to homogenize temperature in professional
machines, a small quantity of circulating boiler water constantly heats up
the fixed equipment (the block). When using home espresso machines, it
is recommended to pre-heat the extraction group by letting a small
quantity of hot water drip through before inserting the cake. The same
operation may also be used to pre-heat the empty cup, filling it up with
hot water before percolating the coffee; an excellent espresso is always
served in a warm cup. To this purpose, professional machines are equipped
with a cup-warming grid.
7.5.7 Pressure
The role of water pressure on percolation is widely acknowledged, but the
influence of the method of exerting pressure on the dry cake has not been
adequately studied and deserves more attention. Pressure does not remain
constant during the process of percolation (as often assumed), but varies
as a function of the characteristics of the hydraulic circuit above the cake.
This can be verified by continual pressure sensors: the recordings yield a
function p . p(t), which exhibits an initial transient rise, increasing more
or less slowly. The dry coffee bed still lacks adequate cohesion and is
susceptible to resettlement; therefore the initial phase appears to be
decisive in attaining the definitive microstructure of the cake. After
wetting, ground coffee particles swell up, interpenetrating each other
firmly, and percolation can follow its stationary course. This effect may
explain why, as already mentioned in 7.3, the flow is not constant during
percolation, rather showing a non-monotonic dependence on the average
pressure and an asymptotic decay.
The pressure value universally applied in professional espresso
preparations, where a centrifugal pump is employed, is 9 relative
atmospheres or, technically, 9 kg/cm2 (equivalent to 10 ata, i.e. absolute
technical atmospheres). It is easily adjusted by regulating the bypass valve
located downstream of the pump, which, as already mentioned in 7.4.1,
can supply a flow exceeding percolation needs.
This value results from a series of ‘trial and error’ attempts in the early
years of espresso machine technology, whose results were measured by
consumer satisfaction. It is interesting to point out how an alert espresso
operator is in a good position to test any slight variation in coffee
preparation procedures, judging immediately from the consumer’s face (a
genuine sensor) and drawing useful indications for further trials. In the
course of time, such a method has established the optimal pressure level
that minimizes failure possibilities in obtaining a perfect espresso.
Hydrodynamic experiments (Baldini and Petracco, 1993) demonstrate
that the average flow ceases to depend linearly on pressure in the
proximity of 9 atm, and that an increase in pressure actually causes a
decrease in the average flow despite the already discussed law of Darcy.
This suggests that the combined machine/cake system is capable of selfadjusting
to overcome pressure variations. Such a phenomenon may lead
to cups of coffee that are apparently (hydraulically) similar, though
originating from completely different extraction conditions, and thus
potentially having different brew qualities.
In home espresso machines, which are driven by a small vibration pump
without bypass valve, pressure cannot be adjusted and is settled by the
hydraulic resistance of the cake. This makes the grind even more
important: it should be adapted to each machine model, because the
wrong particle size does not only influence percolation time, but
extraction pressure as well.
284 Espresso Coffee
7.5.8 Percolation time
Percolation time is the variable that most depends on the person who
prepares the espresso. Nevertheless, in consumers’ perception, extraction
time is not an independent variable, but rather the result of the other
preset parameters (pressure and temperature, grind and tamping/compacting).
The real variable allowed to consumer’s choice is actually the
volume of brew in the cup, as it is the easiest thing to check: once the cup
is full, percolation is stopped without minding the prescribed percolation
time.
The volume in the cup is therefore left to the personal taste of the
consumer, who chooses it according to the local food tradition and
personal eating habits. Optimal results are attained with approximately
25–30 ml volume reached in 25–30 seconds, common in Southern Europe
(Italy, Spain, Greece).
Very short extraction times (less than 10 s) would produce a brew that
is under-extracted, tasting dull and diluted.
Local tastes may ask for a volume up to 70, 90 or even 120 ml or more,
particularly in northern Europe (Germany, Holland, Scandinavia) and in
the USA. However respectable this criterion may be, it appears quite
insidious because it does not set any rule for the duration of percolation,
which can become too long (over 60 s). The resulting brew is then overextracted,
tasting bitter and astringent.
It is therefore recommended that the volume in the cup be considered
as a consequence of percolation, and that the extraction time be
controlled as the main variable. Long expertise produces a consensus on
a percolation time in the optimal range of 25–35 seconds, obtained by
commanding other variables, particularly grinding. If a larger brew
volume is desired, a better cup is prepared by increasing the particle
size of the grounds instead of prolonging extraction time. However, its
body and flavour cannot be compared to those of a full-fledged Italian
espresso, prepared following the full set of rules (see 8.3).
7.5.9 Cake moistening
The moistening of the ground coffee cake is a term that indicates a
specific operation, also called pre-infusion, for which every espresso
bartender has their own personal method. It consists of pouring a certain
quantity of hot water into the extraction chamber so as to wet the ground
coffee, starting the actual percolation a few seconds later. The resulting
beverage is claimed to have a richer body and better flavour, which could
Percolation 285
derive from an improved extraction caused by the swelling of the bed
during the pre-infusion period.
Dynamics similar to pre-infusion can also be observed in the common
straightforward preparation. Water never enters the extraction chamber
instantaneously, not even when the hydraulic circuit consists simply of a
tube linking the pump to the chamber; this is because the start-up
transient of the pump produces a wave of pressure that reaches the ground
coffee surface in a finite time. Moreover, a number of espresso machines
are equipped with an empirical device – a choke nozzle (also known by its
French term ‘gicleur’ or jet), which somewhat renders the pre-infusion
operation automatic by slowing down the water entering into the
chamber. The opening of the device, inserted in series on the hydraulic
circuit just above the chamber, is usually about 0.1mm wide.
7.5.10 Machine cleanliness
Let us conclude the list of variables influencing percolation by dealing
with cleanliness – truly irksome and yet also a delight for baristas and
home users alike. Following the same hygiene rules recommended for
other kitchenware, percolation equipment must be kept clean to prevent
foreign matter from being dragged into the cup and to eliminate any
possibility of proliferation of microorganisms. Nevertheless, it must be
stressed that the organoleptic qualities of the brew may also be negatively
influenced by excessive cleanliness. An established tradition dictates that
coffee pots (and espresso machines too) must be washed with warm
running water, avoiding soap or common household detergents to avert
the risk of off-flavours from residues.
This rule stems from two considerations, the first of which is easy to
explain: the presence of metal filters densely arranged along the water and
coffee path may hold back some cleansing agent in spite of the most careful
rinsing. The residual detergent is then gradually released, spoiling the
flavour of the cups brewed soon after washing. The second reason to forbid
using common detergents is subtler and concerns the surface state of the
metallic parts in contact with coffee. After the first few percolations, a thin
layer of coffee oil and extremely fine solid particles deposits on the metallic
parts, acting as a passivator of the metal surface. Excessively clean coffee
machines produce a beverage having the typical metallic flavour of new
material, probably produced by the catalytic activity of the metal (often
copper or its alloys, but also aluminium, steel, chrome) or, possibly, by
metal ions released into the water. This activity is apparently inhibited by
the formation of a protective layer of coffee matter, which can be washed
286 Espresso Coffee
away if strong or abrasive detergents are used. Mild detergents, devoid of
scents or foaming additives, have been developed specifically for coffee
machines; their regular use is recommended to prevent the accumulation
of sediments in critical sections of the equipment.
A particular aspect of cleanliness regards the removal of calcium
deposits formed by hard water, as detailed in 7.5.5. This is a common
occurrence in home-use espresso machines, where periodical decalcification
is recommended in order to prevent serious damage to the valves and
heating resistor. It is performed by re-circulating for a few minutes a water
solution of a weak acid (acetic, formic, citric), or any decalcification
liquid available on the market, through the looped hydraulic circuit of the
machine. Thorough rinsing must always follow washing.
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Percolation 289
CHAPTER8
The cup
M. Petracco
At the beginning of this book, a first portrait of an espresso coffee serving
was presented as ‘a small heavy china cup with a capacity just over 50 ml,
half full with a dark brew topped by a thick layer of a reddish-brown foam
of tiny bubbles’. A chemist would not like such a description and prefer a
less poetic but more explicit wording. Nevertheless, this definition
characterizes the espresso beverage quite well, at least from a macroscopic
point of view, pinpointing its main features:
n a small volume, indicating that the brew is more concentrated than
usual;
n a composite physical nature, where the foam is as important as the
liquid.
These features are, of course, the result of a very special percolation
method used to prepare the cup, but they could be recognized and
characterized even if nothing were known about the history of the cup
presented to the analyst’s examination. In the following sections espresso
will be described in detail and characterized both from a physico-chemical
and an organoleptic point of view.
8.1 PHYSICAL AND CHEMICAL
CHARACTERIZATION OF THE ESPRESSO
BEVERAGE
Espresso is unstable: all the parameters that are prone to fast modification
after brewing must be determined as soon as possible. The most rapidly
varying property is gas content. Gas release occurs owing to the sudden
pressure drop undergone by the beverage when issuing out from the
espresso machine (see 7.5.7).
Dissolved gases – mainly CO2 – quickly effervesce in the cup, and
bubble up to build a layer of froth. This makes espresso a composite
beverage where two distinct constituents are present: supernatant foam
and underlying liquid.
8.1.1 Foam
Espresso foam – also known by the Italian term of crema – is by itself a
biphasic system composed of gas globules framed within liquid films (called
lamellae) constituted by a water solution of surfactant. These films tend to
set in a configuration of two layers of surface-active molecules facing the
gas, with water molecules between them. The high molecular force in the
film allows its peculiar geometry: a bubble if isolated, or a honeycomb-like
structure of many bubbles growing close together (Figure 8.1).
An important characteristic of the crema is persistence, and it must
survive at least a couple of minutes before breaking up and starting to
show an uncovered spot on the dark surface of the beverage. The
disappearance can be imputed to the phenomenon of drainage, which
causes entrapped water to flow out of the films, leaving only surfactant
molecules to bear the stress of the structure. The films eventually fail due
to their limited elasticity, thus releasing the entrapped gas. Persistence
can be easily measured by watching the surface of a standard cup at
constant temperature and recording the first sign of weakening of the
foam’s layer, revealed by formation of a hole.
As espresso foam is fairly short-lived, its quantity must be measured
soon after percolation. This is done by transferring the whole beverage
The cup 291
Figure 8.1 Pattern of foam in espresso: (left) at beginning of brewing; (right) after
resting overnight, proteins have agglomerated
from the cup to a graduated cylinder, allowing it to stabilize and recording
the volumes of both foam and liquid. Their ratio constitutes a ‘foam
index’ that in well-prepared cups should be at least 10%.
Solidity is a noteworthy rheological property of foam, denoting its
capability to bear for some time the weight of a spoonful of sugar. It can be
quantified in the laboratory by loading the foam with a calibrated wire
mesh ring of a standard weight and measuring the time it takes to
submerge.
8.1.2 Liquid
The liquid part of the espresso beverage, where the foam stays afloat, is an
even more complex system. Along with the properties of the liquid as a
bulk, the presence of several dispersed phases must be taken into account
to explain the peculiar characteristics of espresso. Physical and chemical
parameters of the beverage are detailed in Table 8.1, which shows typical
analyses of correctly prepared espresso cups (30 ml/30 s).
As already pointed out in Chapters 1 and 7, the polyphasic nature of
the liquid is what makes espresso stand out in comparison with all other
brews, both from a physico-chemical and from a sensory standpoint. The
peculiarity of espresso beverage is the simultaneous presence of three
292 Espresso Coffee
Table 8.1 Typical analysis of an espresso brew
Parameter Pure arabica Pure robusta
Density at 20
C (g/ml) 1.0198 1.0219
Viscosity at 45
C (mPa s) 1.70 1.74
Refractive index at 20
C 1.341 1.339
Refractive index of filtrate at 20
C 1.339 1.339
Surface tension at 20
C (mN/m) 46 48
Total solids (mg/ml) 52.5 58.2
Total solids of filtrate (mg/ml) 47.3 55.6
Total lipids (mg/ml) 2.5 1.0
Unsaponifiable lipids (mg/ml) 0.4 0.2
pH 5.2 5.2
Chlorogenic acids (mg/ml) 4.3 5.0
Soluble carbohydrates (mg/ml) 8.0 10.0
Total elemental nitrogen (mg/ml) 1.8 2.3
Caffeine (mg/ml) 2.6 3.8
Ash (mg/ml) 7.2 7.0
Elemental potassium (mg/ml) 3.2 2.9
dispersed phases coexisting within a matrix, namely a concentrated
solution of salts, acids, sugars, caffeine and many other hydrophilic
substances. These phases are:
1 an emulsion of oil droplets;
2 a suspension of solid particles;
3 an effervescence of gas bubbles, which evolves into a foam.
While the presence of dispersed phases is evident, for the beverage is
opaque, some microscopic investigation is needed to enlighten this
feature. Light microscopy, scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) have been employed to observe
the discontinuities of the matrix. To optimize light microscopy, some
form of highlighting of the particles is needed: interference techniques are
quite useful to reveal bubbles, while staining media specific for lipids (e.g.
osmium tetroxide or ‘Oil Red O’) have proved useful to examine oil
droplets (Heathcock, 1989).
The study of the dispersed phases requires special care, because they
differ in their stability. The effervescence, which can be observed for no
longer than one or two minutes, must be tackled as soon as possible by
some form of ‘freezing’, while the suspension remains stable for days, and
the emulsion even longer. Furthermore, the last is difficult to break in
order to collect the oil phase for analysis.
The relative amounts of stable particles, namely lipid droplets and solid
particles, are strongly skewed in favour of droplets. An optical microscope
picture of a perforated filtering sheet, retaining particles larger than
0.2 mm, shows a few cell wall fragments along with dozens of droplets of
various sizes, up to 10 mm (Figure 8.2).
Their size distribution (Figure 8.3) was evaluated by digital image processing
of light micrographies (Heathcock, 1989) and shows dependence
from the percolation variables used to produce the beverage (see Figure
8.3) (Heathcock, 1988).
Distribution is then one factor affecting the organoleptic appreciation
of the cup. Particle size can be measured by laser scattering analysis, and
average values of 2 to 5 mm have been reported (Petracco, 1989).
8.1.2.1 Density
Density is little influenced by coffee solids. Differences with respect to
pure water are limited to the second decimal digit; this could be explained
by the presence of coffee lipids dispersed as an emulsion, which are
less dense than water and decrease the overall density. Addition of sugar
The cup 293
294 Espresso Coffee
Figure 8.2 SEM of coffee lipid droplets and solid particles filtered out from their
liquid matrix
Figure 8.3 Droplet size distribution
can raise density to around 1.08 g/ml, while synthetic sweeteners like
saccharin or aspartame add sweetness with little or no density increase.
This changes the perception of the sensory variable ‘body’ between
naturally and artificially sweetened espresso.
8.1.2.2 Viscosity
Viscosity is considerably higher than that of pure water, with reported
values approximately double at usual consumption temperatures (Table
8.2). This property is influenced by the presence of dispersed phases,
related to the amount of lipid droplets in emulsion. Increased viscosity has
been associated with improved body, as shown by the comparison of a
correctly prepared cup with a poorly prepared one (Figure 8.3), where
viscosity differences as high as 33% have been recorded (Table 8.2)
(Petracco, 1989). This may be explained by the fact that emulsions
usually display higher viscosity than pure solutions.
8.1.2.3 Refractive index
The refractive index is usually determined to evaluate solute concentrations
in transparent liquids. Espresso’s polyphasic nature makes this
method difficult to apply, and scarcely convenient to predict the body
character. In comparison with the index of pure water (1.333 at 20
C),
espresso beverages display values ranging from 1.339 to 1.341. Refractive
index is influenced mainly by the solute concentration, and the dispersed
phases are less important as indicated by the minor differences found after
filtering aliquots of espresso. Commercial refractometers specially
graduated for coffee are available and correlate with total solids content.
8.1.2.4 Surface tension
Surface tension is a property of interfaces, due to the tendency of
molecules to diffuse from one material to another: pure water–air
The cup 295
Table 8.2 Viscosity of high- and
low-body espresso, compared with
water (mPa s)
C Pure water Low body High body
45 0.61 1.32 1.64
30 0.81 1.51 2.01
interfaces have a tension of 73 10 3N/m at 20
C. The lower values found
in coffee beverages, down to 46 10 3N/m, are due to the presence of
surface-active agents, the surfactants, which accumulate at the interface
between two phases building up monomolecular layers. These molecules,
composed of an apolar chain with a polar head, help to form and stabilize
foam and emulsion, and may also influence the fluid’s behaviour during
percolation through the narrow interstices of the packed coffee cake,
where capillarity plays an important role. The chemical nature of natural
coffee surfactants has not yet been fully clarified: several classes of
complex molecules, like glycolipids or glycoproteins, might exhibit such
behaviour (Nunes et al., 1997). Apparently, in espresso beverages there
are two classes of compounds responsible for two different properties:
foamability seems to be influenced by a proteic fraction, while foam
persistency depends more on a polysaccharidic fraction (Petracco et al.,
1999). An additional interesting effect of coffee surfactants, besides foam
and emulsion promotion, is that they help the beverage to come into close
contact with taste buds in the tongue, thus enhancing the sensory
perception.
8.1.2.5 Total solids
Total solids content – the beverage’s overall concentration – is the most
important characteristic in the chemical composition of espresso, often
perceived by lay consumers as ‘strength’. This property is determined by
drying the brew in an oven to constant weight without filtering.
Accordingly, suspended solid material, emulsified lipids and solutes are
included. Total solids concentration in the beverage is dictated by the
brewing formula, namely the coffee/water ratio, but varies greatly also
depending on both roasting degree and percolation temperature: a darker
roasting produces more solubles, and a higher temperature extracts them
more efficiently (Nicoli et al., 1991). Values as low as 20 mg/ml up to
60 mg/ml may be encountered. The true soluble fraction, typically 90% of
the total solids, can be determined by oven drying after filtering the
liquid.
8.1.2.6 Lipids
The emulsified lipid fraction can be determined by liquid–liquid solvent
extraction. Different amounts have been reported in both espresso and
non-espresso coffee beverages (Petracco, 1989; Peters, 1991; Ratnayake et
al., 1993; Sehat et al., 1993). For an espresso made from a 100% arabica
blend, a figure of the order of 5% of the total solids may be considered as
296 Espresso Coffee
typical (Table 8.1). This is a little less than one-tenth of the total lipids
present in the roast and ground coffee portion, and represents only a
minor contribution to the dietary energy intake: less than 1 kcal per cup.
Within the unsaponifiable fraction of coffee lipids, two compounds
belonging to the diterpene family – cafestol and kahweol – are of
relevance to blood cholesterol level (see 10.3.3); present in roasted
arabica beans at levels around 0.6%, they are transferred to the espresso
brew in small amounts (Urgert, 1997).
8.1.2.7 Acids
Acidity of coffee beverages is an important organoleptic parameter
(Woodman, 1985). The presence of acetic, formic, malic, citric and lactic
acids has been detected in the brew, along with quinic and chlorogenic
acids (Peters, 1991; Severini et al., 1993). The content of the last ones is
of course smaller when using dark roasted coffee, in which they have
largely disappeared (Clifford, 1985). A mineral acid – phosphoric – has
also been found in brews (Maier, 1987), deriving from thermal
degradation of phytic acid, the phosphoric ester of inositol. As already
mentioned in 4.3.2.6, espresso acidity cannot be described just by pH.
Measurements of this variable obtained using electrode-type instruments
report values ranging from 5.2 to 5.8, showing an increase with roasting
degree (Dalla Rosa et al., 1986a; 1986b) and depending on extraction
time (Nicoli et al., 1987).
8.1.2.8 Carbohydrates
This broad class of compounds is present in coffee seeds, as in all plant
materials, both as simple sugars and as polysaccharides (Trugo, 1985).
Upon roasting, heavy reactions and transformations happen, changing the
balance between soluble and insoluble carbohydrates. While only the
former are relevant to the beverage, the brewing method may influence
the hydrolytic degradation of insoluble carbohydrate polymers, adding to
the solubles content. Typically, only small amounts of monosaccharides
can be found in the brew, but no sucrose is present, because it has already
been transformed on roasting into bitter tasting Maillard compounds
(Trugo and Macrae, 1985). Much research has been done on coffee
carbohydrates (Leloup et al., 1997), but few data on their content in
coffee beverages are available: in espresso a typical figure for soluble
carbohydrates is 8 g/l, corresponding to some 15% of total solids
(Petracco, 1989).
The cup 297
8.1.2.9 Nitrogen compounds
Nitrogen-containing compounds are present in coffee beverages as
transformed proteic material – grouped under the broad name of
melanoidins – and caffeine, while trigonelline has largely disappeared
during roasting to yield volatile aroma compounds (Macrae, 1985). A way
to analyse this complex class of compounds is by determining total
elemental nitrogen, subtracting the caffeine nitrogen and multiplying for
the standard factor 6.25 to supply a conventional protein content value.
Typical figures are in the neighbourhood of 1.4 mg/ml total N,
corresponding to approximately 4 mg/ml of proteic material.
Caffeine is the most studied coffee component, due to its physiological
activity (see 10.2). Its solubility in water displays a marked dependence
on temperature, with a more than 30-fold increase from standard
conditions to atmospheric boiling temperature, where almost quantitative
extraction takes place at brewing provided enough time is allowed to cope
with the kinetics (Spiro, 1993). Also water hardness seems to influence
caffeine extraction to a certain extent (Cammenga and Eligehausen,
1993).
Caffeine presence in espresso beverages has not been much studied.
Data suggest that caffeine extraction from coffee grounds is incomplete
in espresso percolation and, as already indicated in 7.2.2, the yield is
usually within the range 75–85%. This is due to the short time available
with espresso brewing to extract caffeine from the cellular structure, as
shown by fractionated extraction curves (Figure 8.4) (Petracco, 1989).
Therefore, caffeine concentration in the brew ranges between 1.2 mg/ml
up to 4 mg/ml, depending on cup size and blend composition.
Corresponding cup contents range between 60 mg for pure arabica
blends, and 130 mg for pure robusta blends.
8.1.2.10 Minerals
The presence of minerals in the espresso brew is low, as can be seen from
ash content scarcely exceeding 7 mg/ml. Nevertheless, they constitute
about 15% of total solids and, since potassium forms a substantial part of
them, they might be considered as a beneficial contribution to the diet.
8.1.3 Volatile aroma
The meaning of the substantive ‘aroma’ is traditionally considered to be ‘a
strong pleasant smell, usually from food or drink’ (Procter, 1995). In the
298 Espresso Coffee
domain of food science, however, it is better addressed as ‘the quality or
principle of plants or other substances which constitutes their fragrance;
agreeable odor; as, the aroma of coffee’ (Webster, 1913). The following,
more general, definition applies even better to our chemical context: ‘any
property detected by the olfactory system’ (WordNet, 2003), with the
additional specification that by ‘aroma’ we shall designate any set of
‘odorants’, i.e. chemical substances capable of reaching our
olfactory organs.
There is little doubt that the aroma of any coffee beverage is dissimilar
from that of the roast and ground coffee used to brew it, inasmuch as the
composition of its odorants is different (Grosch, 2001). This is the result
of extraction dynamics, which itemizes fast and slow extractors among the
several hundreds of volatile compounds produced on roasting (Mayer et
al., 2000).
Further work is required to identify over-the-cup aroma compounds
specific to espresso, as compared with those above a cup of filter coffee
(see Figure 4.13). Along with a stronger overall concentration, espresso
features a dispersed lipidic phase (see 8.1.2), which is likely to carry
and release some oil-soluble volatiles to the headspace. Moreover, the
The cup 299
Figure 8.4 Fractionated extraction of caffeine during the brewing of a ristretto
coffee cup
presence of a thick layer of foam would certainly change the composition
of the latter.
Actually, the pleasant sensation of sniffing freshly ground coffee beans
is much more stimulating than breathing in the rather weak aroma
exhaling from a cup of coffee. The strong sensory advantage of the
beverage appears only when taking a sip in the mouth, where more
complex release and hydrolysis reactions are likely to occur due to the
biological environment. Thanks to new methodologies (Bu¨ttner and
Schieberle, 2000), the field of the measurement of volatiles evolving in
vivo is open to future research.
8.2 ORGANOLEPTIC CHARACTERISTICS OF
ESPRESSO (PRACTICAL ASPECTS)
The word of Greek origin ‘organoleptic’ translates literally as ‘brought by
organs’, and figuratively as ‘learnt through perceptions’, that is, by direct
information coming from our organs of sense: eyes, ears, skin, nose, or
mouth. According to ISO vocabulary, it denotes the attributes of a
product that are perceptible by sense organs (ISO, 1992). Its implementation,
called sensory analysis, is used to examine and measure objective
characteristics such as taste, aroma, colour and other analogous factors of
foods and beverages via an evaluation of the subjective sensations they
elicit by the gustatory, olfactory, haptic (tactile), visual and auditory
spheres (see also Chapter 9).
Few everyday experiences can compete with a good cup of coffee, as
long as sheer sensory pleasure is considered. It is clear that most of the
quality of such a beverage is determined by its overall sensory impact. In
this context, espresso is the brewing method that offers the consumer the
most potent experience, even if a high quality cup it is not easy to obtain:
espresso’s very strength – the ability to concentrate aromas – is also its
weak point, because, while enhancing qualities, it shows up at the same
time all the possible defects of the raw material.
A cup of well-prepared espresso is a powerful stimulator of our senses:
the rich colour, the intense aroma, the intense and long-lasting taste, all
provide pleasure to the nose, the mouth and the eyes. Only the sense of
hearing seems to be extraneous to this multi-sensory experience (if we
disregard the call of a steaming espresso machine). Sipping from this
small cup creates a link between the amazing complexity of the
chemical composition of coffee and the as complex sensory perception
system.
300 Espresso Coffee
8.2.1 Visual characteristics
As far as sight is concerned, the main attribute of the cup is foam. The
beautiful aspect of the foam is conferred by the tiny gas bubbles framed by
viscous liquid lamellae, where minute cell wall fragments float producing a
‘tiger skin’ effect (Figures 8.5 and 8.6b).
Consumers who ascribe great importance to the colour and the texture
of foam are right, because perfect foam is the signature of a perfect
preparation. Any error in grinding or in percolation, in temperature or
extraction level, is immediately denounced by the colour, the texture and
the persistence of the foam. For example, if the foam is light-coloured,
inconsistent, thin and evanescent, it means that espresso has been underextracted,
probably because the grind was too coarse or the water
temperature too low (Figure 8.6a).
An over-extracted espresso, on the contrary, may display either a white
foam with big bubbles, if water temperature was too high, or just a white
spot in the middle of the cup (Figure 8.6c).
Other extraction mistakes can derive from too low a cake porosity or
from too high a coffee dose, both indicated by very dark foam with a hole
in the middle.
The foam layer also acts as an aroma-sealing lid, trapping the volatile
compounds responsible for the odour of espresso brew. The foam preserves
them for our pleasure if the blend used to brew the cup was good, or for
our torment if it was defective: it works as an amplifier, enhancing both
virtues and sins.
8.2.2 Degustation characteristics
For the sake of a systematic description of organoleptic attributes, it is
important to clearly show the boundaries between the multi-faceted
The cup 301
Figure 8.5 A perfect espresso showing (left) the depth of foam and (right) the
‘tiger skin’ effect on the surface of the foam
concepts of taste and after-taste, odour and flavour, body and astringency,
since they all are determinants in the appreciation of a beverage such as
espresso, when mouth and nose intervene. While Chapter 9 will deal in
depth with these concepts from the perspective of the science of
perception, in the following sections they will be addressed mostly from
the hedonic point of view.
8.2.2.1 Gustatory senses: taste
Taste is perceived by the homonymous sense whose primary elements,
called buds and which number several hundreds, are located on the
surface of the tongue (see 9.2.1 for more on this). Specialized buds used to
be associated with the perception of mainly one of the four pure taste
sensations: sweet-sensitive buds were deemed to cluster on the tip of the
tongue, whereas bitter-sensitive buds were to be found in its back part,
with acidic and salty sensations coming mostly from the tongue’s sides.
However, this theory has recently been questioned (Bartoshuk, 1993) (see
9.2.4).
Sweetness, a character always positively correlated with coffee quality
and price, is particularly appreciated in the espresso cup, while the
contrary holds for bitterness. Nevertheless, the overall description of a
fine espresso sounds like ‘bitter-sweet with a initial slightly acidic note’.
Acidity is not a desideratum in espresso because it tends to give an
unbalanced feeling amplified by the high concentration of the brew.
Consumers in different countries show marked differences in the
appreciation of the ratio of bitterness to acidity. In southern European
countries, consumers prefer an espresso with a bitter dominance (and
302 Espresso Coffee
Figure 8.6 Different espressi: (a) under-extracted; (b) correctly extracted; (c) overextracted
consider body very important), while in the northern ones a more
balanced taste is preferred and excess bitterness is considered a defect.
This stems also from the concentration in the espresso cup, very small and
concentrated in the south, where it is drunk without milk; more diluted
and frequently with milk or dairy cream in the north. Unfortunately, the
preparation of an espresso of more than 50 ml without over-extracting is
very difficult: a large water volume also extracts more of the less soluble,
and less pleasant, components. For this reason, in the north, consumers
asking for their favourite rather diluted espresso shot often receive a
bitter-tasting, woody and sometimes astringent one. Espresso percolation
resembles a fractional extraction, with an increase in less soluble
substances as extraction proceeds. Alas, not all that is soluble is agreeable:
one must know when to close off.
Both blend formula and roasting level can be used to change the
bitterness–acidity ratio. Fine washed coffees, frequently having a strong
acidic tone, are rewarding in the preparation of filter coffee since they
will withstand hydrolysis, and keep their quality during the time
elapsing between preparation and consumption (Feria-Morales, 1989).
In an espresso blend they will help with the aroma, but a blend of pure
washed coffees will taste excessively acidic and lacking body. Sun-dried
coffees will, on the contrary, enhance both body and balance. Also, the
roasting intensity can be used to change the bitterness–acidity ratio, as
seen in 4.3.2.6. Caution must be used when roasting washed coffees: if
roasted too dark they will develop a pungent, burnt character usually
found unpleasant. Another objectionable acidic, astringent and metallic
taste is the consequence of very fast roasting, as already discussed in
4.2.2.
8.2.2.2 Gustatory senses: after-taste
The rheology of espresso is rather special compared to other coffee brews.
Its strong concentration is responsible for high density and viscosity,
whereas the presence of natural surfactants lowers surface tension. These
apparently contrasting characters could explain the intense taste and the
long-lasting after-taste (and after-flavour) sensations, namely those felt
for a longer period after emptying the mouth. When drunk, espresso first
soaks the surface of the tongue, colouring it; the brew is then trapped by
the taste buds, and oil droplets fix themselves to the mucous membrane.
There they slowly release the volatile substances dissolved, so that they
are perceived for a while (up to 15 minutes) after the beverage has been
swallowed.
The cup 303
Coating of the tongue may possibly explain a reduction in the
perception of bitterness. This statement can be indirectly supported by
experiments in which an expert cup-testers’ panel is presented with a
quinine solution, whose bitterness is rated 100. Then a colloidal
polysaccharide is added to the bitter solution at a dilution of 1% and,
surprisingly, bitterness is rated at 40. The conclusion is that colloids are
able to reduce the perception of bitterness by receptor blockade. Droplets
of the emulsified oil in espresso probably behave like colloids, and they
act on perceived bitterness owing to their dimensions, which are smaller
than 10 mm.
This theory may also explain the phenomenon of increased perceived
bitterness when diluting an espresso. Dilution normally reduces the
intensity of any taste, but in this case the molecules responsible for the
bitter taste are present in the liquid in several billions, and the oil droplets
in several orders of magnitude less. The probability of a bitter molecule
catching an unblocked receptor is increased by the dilution of the
beverage, so that diluted espresso tastes more bitter.
8.2.2.3 Olfactory senses: odour
Odour here is meant as the olfactory sensations caused by inhaling.
Whereas the aroma of freshly ground coffee is a powerful stimulus, coffee
beverages do not release a large amount of volatile compounds into the
environment that surrounds the cup, and this is even more so for espresso,
as already explained in speaking about the foam’s role in trapping aroma.
On the contrary, many volatile molecules of aroma are released within
the mouth after drinking and reach the olfactory receptors in the nose by
retro-diffusion, namely the movement of gaseous molecules from the
mouth through the pharynx up to the nose. A simple experiment that can
prove the existence of this phenomenon is to munch and swallow a bite of
apple while pinching one’s nostrils. The missing sensation is exactly what
will be hereinafter called flavour (but which – just to add to the confusion
of terms – is referred to as ‘arome’ in French, at least according to ISO
5492 vocabulary:
3.33 aroma (noun): NOTE – The sense of the terms ‘aroma’ in
English and ‘arome’ in French is not exactly equivalent. (1)
French sense: Organoleptic attribute perceptible by the olfactory
organ via the back of the nose when tasting. (2) English sense and
French informal language: An odour with a pleasant connotation.
(ISO 1992)
304 Espresso Coffee
8.2.2.4 Olfactory senses: flavour
To make the distinction between aroma and flavour even clearer, it is
useful to explain that the human sense of olfaction perceives the presence
of odorous volatile molecules – the ensemble of the ones given off by
coffee is, in this context, called aroma – by means of thousands of
receptors located in the inner mucosae of the nose. Olfaction is in charge
of both odour, by direct inhaling of the molecules arising from the roast
and ground coffee or from the cup, and flavour, the nasal perception of
the volatile substances evolving in the mouth and reaching the nose
cavity by the pharyngeal pathway (Petracco, 2001). While evaluation of
pure taste is quite easy by closing up the nostrils and excluding in this way
the olfactory side, the opposite is unfortunately not feasible (at least
without nerve surgery . . .): no retro-pharyngeal flow can occur without
actually passing the sample through the mouth. Probably for this reason,
most authors, even in normative papers (ISO, 1992; Civille and Lyon,
1996), define flavour as a complex combination, in which olfactory
sensations merge with gustatory and somatic (haptic and trigeminal) ones.
This concept is shared by other contributors to this book (see 9.4.3).
Contrarily, the present author’s opinion calls for reserving the term
‘flavour’ to the pure olfactory component of this overall sensation, while
the latter is felt sadly to lack a specific term: ‘palatability’ could be a good
candidate, if freed from its current hedonic meaning (taken to be positive
by default).
Among the organoleptic characters of coffee, flavour is one of the most
debated (Wrigley, 1988). Both intensity and quality of the olfactory
component of taste are important, especially when espresso is concerned.
More than 800 volatile species have been identified so far in coffee
headspace, and many of them are liposoluble. Since espresso brewing
extracts a substantial quantity of coffee oil as a dispersion of small
droplets, it also transports into the cup the liposoluble volatile substances
dissolved in the lipid phase of roast coffee during roasting and storage.
After brewing, these gaseous molecules tend to escape from the liquid and
finally reach the nasal receptors of the consumer. This complex
mechanism explains the difference between the flavour of espresso and
filter coffee: not only a matter of concentration, but a distinct aromatic
spectrum as well.
Terms to describe flavour are usually borrowed from everyday life, and
recall the world of flowers, fruits and fresh or baked foods. Unfortunately,
there is sometimes a need to describe negative flavours or taints, where
terms like ‘stinker’, describing a rotten vegetable tang, or ‘mouldy’,
reminding of fruits infected by mildew, or ‘peasy’, reminiscent of pea pods,
The cup 305
are used (see 3.7.2.10). A consumer-oriented list has been developed by
the International Coffee Organization (ICO, 1991), but each expert panel
usually has its own terminology.
8.2.2.5 Haptic senses: mouthfeel
Mouthfeel is a tactile sensation perceived by buccal mucous membranes,
along with the thermal response due to the temperature of the beverage.
Most of its nature is related to small movements of the tongue against
palate and gums, which apply a shear stress to the liquid performing a sort
of rheological measurement of viscosity and texture. Body, an important
attribute of espresso coffee, is felt in this way and can be considered either
as a synonym of mouthfeel or, with astringency, as part of its definition.
It is sometimes claimed that body is a peculiarity of robusta espresso
brews, and, as a matter of fact, those beverages ‘fill the mouth’. It is
difficult to believe that oil droplets of colloidal size are present in higher
amounts in robusta than in arabica espresso, given that the lipids content
of roast and ground robusta is much lower than that of arabica (some 9%
as opposed to 13%). Cup-testing experiments have been reported,
indicating that freshly percolated robusta espresso has actually a stronger
body than arabica, but only as a first sensation: if the sip is held in the
mouth, after some seconds arabica’s body is still present while robusta’s
fades away. Moreover, an inversion of assessment takes place if the
beverage is allowed to rest for 2 minutes or so, with arabica again
displaying stronger body. It has been postulated that this behaviour could
derive from a transient viscosity rise induced by gas bubbles of colloidal
size, possibly more evident in robusta (Petracco, 1989).
8.2.2.6 Haptic senses: astringency
Another tactile sensation, astringency, reminds one of medicine and is
always considered as negative. It is related to a chemical phenomenon,
namely the precipitation of the proteins of saliva due to specific phenolic
compounds present in some beverages and unripe fruits (Rider, 1992). In
coffee, it has been ascribed to the presence of immature beans containing
dicaffeoylquinic (dichlorogenic) acids (Ohiokpehai et al., 1982), which
are deemed to be astringent to mucous membranes. Deplorably, immature
beans are nowadays found more and more frequently, because green coffee
producers seem to pay less attention to the quality of their crops due to
the low price of recent periods.
306 Espresso Coffee
8.2.3 Forecasting sensory quality
As seen so far, the traditional cup-testing approach still remains the
ultimate assessment tool for coffee quality. After all, the reason why
coffee has become so popular, achieving the position of the second most
largely consumed beverage after water, is its flavour or, even better, its
overall impact on our senses. Sensory evaluation, which used to be
considered the magic because ‘taste is a matter of taste’, is nowadays
earning the status of a highly respected analytical tool, able to produce
key information with good reliability (Meilgaard et al., 1999).
8.2.3.1 Espresso cupping practice
In industrial coffee routine, some form of objective evaluation is needed
to ascertain product overall quality, along with the constancy of that
quality in time and in varying process conditions. For a product that is
consumed for pleasure, there is little doubt that the final word must be left
to the sensorial sphere. While the general aspects of organoleptic
laboratory activity will be dealt in 9.4.3, we present here some practical
guidelines to approach the assessment of espresso.
The ‘tool’ commonly used is a panel of assessors, who may be either
coffee experts (professional cup-testers) or naive consumers after a basic
training. The reason for employing more than one person is obvious: by
averaging responses, the risk of incorrect judgement due to a possible
indisposition of one person is minimized. Another panel potential is the
synergy that can be gained by debating coffee characteristics among the
assessors during open sessions: this procedure may extract more information,
since individual sensitivities and perception thresholds may be
different.
Sensory tests used in espresso evaluation may be grouped in three basic
types, listed by increasing difficulty for the panel members:
n Trio tests, used simply to determine whether any perceivable
difference exists between two samples. In this configuration the
two beverages are split in two cups each, but only three of the four
samples are presented to the panel: the assessors are requested to tell
which is the ‘foreign’ single cup, as opposed to the pair of ‘sister’ cups.
n Duo tests, where two or more single cups are presented to the panel,
who are asked to rank them in relationship to one sensory variable.
When more than one variable is to be determined, a pre-filled card
proves to be useful to summarize the evaluations.
The cup 307
n Absolute tests, in which some complex variable, like aromaticity or
overall merit, is to be determined by comparison with a mental
paradigm present in each assessor’s memory based on previous
experience. Coffee aroma profiling (ICO, 1991) can also be included
in this type, since it is based on assessors’ recall of variegated flavour
knowledge.
In regular day-to-day espresso cup-testing sessions in support of the
purchasing activity, the panel should be presented with a maximum of a
dozen samples, each served in three different cups according to three
fundamental preparation techniques:
n Infusion: it is a brewing method that is widely used in northern
Europe and in the USA, in which boiling water is poured on coarsely
ground coffee powder and allowed to rest for a given period before
filtering away the spent grounds from the remaining clear beverage.
The concentration of the beverage is low (below 20 /l) and only the
soluble substances may pass into the cup, giving an aromatic pattern
typical of filtered coffee beverage.
n Espresso: the classic espresso cup is prepared under standardized and
thoroughly controlled conditions, allowing the panel to evaluate its
foam and body along with the taste and flavour characteristics.
n Diluted with hot water up to reach the concentration of the infusion
method: in this way the high concentration of regular espresso does
not hinder the evaluation of a weaker aroma’s nuances, and the
difference between the aromatic pattern of the solution and the
emulsion can be determined.
Espresso cup-testing sessions cannot be too long nor too frequent during
the day because some fatigue develops after the first dozen or so espresso
cups, due to the lingering after-taste deriving from the sticking of coffee
oil droplets on the tongue and mouth membranes (Petracco, 1989).
Rinsing the mouth with water, albeit necessary between each sampling, is
not effective to remove the taste completely. On the contrary, cool whole
milk seems to act better to this purpose, perhaps because, being itself an
oil-in-water emulsion, it can displace coffee oil droplets from the tongue
by dilution.
8.2.3.2 Instrumental testing
Unfortunately, it is fairly evident that implementing the cup-testing
practice as explained in the previous section is neither simple nor
308 Espresso Coffee
inexpensive. As a consequence, industry strives to take advantage of
sensory data collections, using them as raw experimental data to calibrate
instrumental screening methodologies.
A good instance of such a rapid, non-destructive fingerprinting technique
is near-infrared spectroscopy. It is based on absorption measurements
of scanned monochromatic near-infrared light (wavelength between
1100 nm and 2500 nm), whose energy is dissipated in rotational and vibrational
movements of the molecular bonds of the material under examination,
and ultimately transformed into heat (Murray and Williams, 1987).
Energy absorption patterns contain a lot of implicit information about
molecular response to specific wavelengths and to their combinations.
Spectrophotometers may operate by filters or by scanning monochromators:
the latter present the advantage that a higher amount of
information can be collected at the same time, and are therefore most
useful for research and new applications. Spectra are obtained by
recording either the transmitted part of the incident light (NIT, used
mainly for transparent materials like solutions) or the reflected one (NIR,
applied to opaque materials like grains or powders), and by plotting its
reciprocal logarithm against the wavelength. The procedure, generally
employed to analyse solid materials opaque to visible light, is called
diffuse reflectance. Since infrared radiation is able to penetrate some
millimetres under the surface of solid materials, the term near infrared
transflectance (NIRT) may better describe the technique as applied for
the analysis of coffee beans.
All spectra of vegetal materials do not differ that much at a first glance:
this is the reason for the need for sophisticated statistical elaboration of
the raw data in order to correlate spectra with the reference analysis data
of interest. The branch of chemistry studying the best fit of data from such
secondary analytical methods with the primary ones is called chemometrics.
It uses computerized regression methods like principal components
analysis or partial least squares to calibrate, that is, to build an
equation able to convert spectra into chemical previsions. Calibration sets
of some dozens of analysed samples are needed, and the robustness of the
result is linked to their careful choice.
NIRT has been shown suitable for supplying simultaneous forecasts of
many chemical characteristics of the sample examined, provided that a
good calibration has been previously obtained by statistical correlation
with traditional, time-consuming analytical methods. This secondary
method has widely been used with agricultural products (Shenk, 1992)
and sometimes in the specific domain of coffee too.
Applications to coffee have been described for both green beans and
roast and ground products: they include prediction of chemical characters
The cup 309
like moisture and caffeine content (Guyot et al., 1993), trade features like
pureness (Downey and Boussion, 1996) or arabica/robusta content
(Davrieux et al., 2001), and also for modelling sensory data (Feria-
Morales, 1991).
Several instrument brands, among are which NIRSystems, PerTen and
Buhler, sell equipment with extensive autodiagnostics and powerful
software: some claim to have developed specific calibrations for coffee
too. It would be wise nevertheless for users to check the equations on the
specific products of interest, and be prepared to develop their own
calibrations.
8.3 ESPRESSO DEFINITION AGAIN
At the end of Chapter 1, a tentative definition of Italian espresso was
proposed to the reader. Let us discuss it once again:
Italian espresso is a small cup of concentrated brew prepared on request by
extraction of ground roasted coffee beans, with hot water under pressure
for a defined short time.
At this point, after having analysed and characterized espresso as carefully
as possible, we are able to complete our definition. The above features,
related to ‘espresso style’ and to ‘espresso method’, can be merged with
those characteristics of the beverage which assure the fulfilment of
consumer’s expectations:
Italian espresso is a polyphasic beverage, prepared from roast and ground
coffee and water alone. It is constituted by a foam layer of small bubbles
with a peculiar tiger-tail pattern, on top of an emulsion of microscopic
oil droplets in an aqueous solution of sugars, acids, protein-like material
and caffeine, with dispersed gas bubbles and colloidal solids.
The analysis of the beverage is:
Viscosity at 45
C >1.5 mPa s
Total solids 20–60 g/l
Total lipids 2 mg/ml
Droplet size count (90%) <10m
Caffeine <100 mg/cup
310 Espresso Coffee
The distinguishing sensory ensemble of characteristics of Italian espresso
includes a rich body, a full and fine aroma, a balanced bitter-sweet taste
with an acidic note and a pleasant lingering after-taste. It must be
exempt from unpleasant flavour defects, such as stinking, mouldy,
grass-like or other.
Owing to its instability, Italian espresso must be prepared on request from
roasted and ground coffee beans, by means of a specific brewing method
defined as:
Italian espresso is a brew obtained by percolation of hot water under
pressure through a compacted cake of roasted ground coffee, where the
energy of the water pressure is spent within the cake.
The variables ranges are:
Ground coffee portion 6.5 1.5 g
Water temperature 90 5
C
Inlet water pressure 9 2 bar
Percolation time 30 5 s
the volume in the cup is to be left to the personal taste of the consumer
inside the range 15 to 50 ml, with an optimal outcome at 25 to 30 ml.
8.4 ESPRESSO–MILK MIXES
A chapter about espresso coffee beverages cannot ignore the enormous
number of coffee-with-milk cups consumed every day. Caloric effects of
espresso, from a nutritionist’s point of view, are insubstantial (see 8.1.2.5
and 8.1.2.6). Nevertheless, coffee does play a significant role in the diet
when drunk together with other foods such as milk or sugar, inasmuch as
its cup behaves as a carrier of nutrients not inherent to coffee.
The nutritional importance of milk in everybody’s diet is not to be
understated: a complete feed for newborns on its own, it helps adults to
keep in good health mainly via its calcium content. Therefore, blending
coffee drinks with milk may be seen as a great way to increase milk
consumption and foster calcium intake, to the benefit of bone strength.
Conversely, it must be remembered that the caloric content of milk is
rather high (60–80 kcal/100 ml for whole milk, around 40 for skim milk),
and may contribute seriously to the total energy intake, exceeding by far
the effect of coffee when they are mixed together.
The cup 311
A plethora of recipes are present in various cultures, ranging from a
drop of coffee in a glass of milk, just to add some flavour, to a drop of milk
to discolour a coffee cup. Milk adds to appearance, to texture and to aftertaste
persistence thanks to its fat content, which contributes mouthfeel
and flavour too, and helps to distribute fat-soluble flavour components of
coffee, principally when used in espresso drinks. It may be added cold or
hot, and some are now even questioning the priority: is milk to be added
to coffee, or coffee to milk?
Perhaps the most attractive marriage between the two products happens
when milk is added in the form of foam, originating the family of
cappuccino-like drinks. The chemistry of milk foam formation is at least as
complex as that of coffee, ensuing from casein–lipid interactions mediated
by phospholipids (Goff and Hill, 1993). Both the protein and fat contents
in milk are critical to foam development: skimmed milk contains the
greatest percentage of protein and foams better than low-fat or whole milk,
while fat content helps to maintain foam stability. The freshest milk –
coming directly from the cow – is seldom used for hygienic reasons.
Pasteurized milk is favoured because it can be kept up to 14 days in
refrigerated storage with no noticeable change in foaming properties. It
can be produced either by conventional process (heated to 64
C for 30
min) or by HTST process (72
C for 15 seconds) (Hinrichs and Kessler,
1995). Aseptically processed milk (the so-called UHT, heated up to
144
C for 4 seconds) can be kept sealed at room temperature for several
months. From a practical standpoint, it has many advantages: less
refrigeration is needed in a business that is usually carried out in a limited
space, and less time and temperature is needed for heating a product
already at room temperature. However, it is not widely used by baristas.
The rule-of-thumb for making a good cappuccino is ‘thirds’:
n first, make a standard espresso shot in a larger cup, where espresso
should take about one third of its volume;
n then add a third of hot liquid milk; and
n a third of steamed, frothed milk.
Any variation on the above prescription is permitted, producing
beverages that have been baptized with fancy, often exotic-sounding
names like cafe´ au lait, cafe´ cre`me, macchiato, latte, frappuccino and
mochaccino.
Several pitfalls may spoil cappuccino: off-flavour water can taint the
steamed milk; damaged or unclean steaming tools can add a burnt or
scorched flavour; lengthy steaming can produce milk temperatures above
70
C, imparting a cooked flavour. Typically, more than 75% fresh
312 Espresso Coffee
(unsteamed) milk should be present when producing the foam. Failure to
drain steam pipes before frothing can dilute milk thus reducing mouthfeel
and creaminess, as well as decreasing foaming capacity.
A final crucial warning: unlike roast and ground coffee, milk is a
‘perfect’ food not just for people but for microorganisms too, so when
handled incorrectly it is prone to spoilage. Keeping it refrigerated as long
as possible and maintaining all relevant tubing and vessels clean is
mandatory.
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The cup 315
CHAPTER9
Physiology of
perception
R. Cappuccio
9.1 INTRODUCTION
When we take a sip of espresso, we can have a hedonic reaction of
pleasure, caused by the rich and smooth aromatic notes, the balanced
acidity and bitterness and the intense body of the beverage, or a rejection
reaction, caused by a low quality blend or an incorrect extraction process.
These hedonic reactions are consequences of a chemical reception
process, which occurs in the mouth and retronasal cavity, and a
transduction process, which transforms the chemical signals into
electrical ones. The information is then transmitted to the central
nervous system (CNS), where an integration process occurs, giving that
overall complex mixture of sensory inputs – composed of taste (gustation),
smell (olfaction) and the tactile/trigeminal sensations (temperature and
mouthfeel) – which we call flavour (Heath, 1988). This integrated
sensation is evaluated, and a response in terms of acceptance and liking is
given. Comprehension of how sensory information is processed and
integrated is important in understanding the evaluation process, even if
we do not fully understand all the steps (Stone and Pangborn, 1968).
Three semi-independent molecular sensors to check the external
environment have been identified in mammals:
n The vomeronasal organ (VNO) in the nasal septum is specialized in
the detection of particular chemical signals, called pheromones,
which are correlated to mating and aggressive behaviours. Recently,
evidence of this organ has been found also in humans – to the great
joy of perfumers (Meredith, 2001; Thorne et al., 2002).
n The main olfactory epithelium (MOE) in the back of the nasal cavity
detects volatiles. Its main features are an extraordinary sensitivity; the
human threshold of detection can be as low as 10 11M for
trichloroanisole – rio corky flavour (see 3.7.2.6). Humans are thought
to be able to discriminate between several thousands of different
odours, even if in theory the number is billions, but bloodhound dogs
have been shown to detect some odours at a concentration of
10 17M (Dulac, 2000a), and to possess an extremely large discriminatory
power.
n The taste sensory epithelium of the mouth responds principally to
water-soluble chemicals present in the oral cavity. Chemicals from
food termed tastants dissolve in saliva and contact the taste cells
through the taste pores, to give a fast response, in terms of acceptance
or rejection, to food to be ingested.
These chemical senses are semi-independent because, even if they are
located in different zones and their reception/transduction mechanisms are
different, they often work together to give the organism a unique answer.
The perception of coffee quality involves both olfactory and gustative
sensory inputs: after drinking and swallowing, volatiles released in the
throat reach the nasal cavity and stimulate the olfactory epithelium.
9.2 GUSTATION
Gustation has been studied for many years, but only recently has progress
in genetics and molecular biology enabled greater understanding of its
fundamental characteristics, namely reception and transduction mechanisms.
Lately many reviews have appeared, among which should be
mentioned (in alphabetical order) Dulac (2000a), Kinnamon (2000),
Lindemann (2001) and Smith and Margolskee (2001).
Even though people often use the word ‘taste’ to mean ‘flavour’, in the
strict (and correct) sense, the word is applicable only to the sensations
coming from specialized cells in the oral cavity. Gustation or taste is the
sensory system whose scope is essentially an evaluation of the food to be
ingested, and in fact all organisms, be they bacteria or mammals, perform
this check by chemoreceptive evaluation. Taste evolved in order to help
organisms to detect hazardous substances, such as acids or alkaloids, and
to recognize nutritionally important compounds like sugars, salts and
aminoacids (Lindemann, 2001). Actually, even a few days’ old baby is
able to discern between sweet and bitter and react with a smile when
tasting a sweet substance, or with a cry when tasting a bitter one
(Ganchrow et al., 1983).
While the olfactory system can recognize and discriminate a huge
number of different odours in humans, the taste system discriminates only
Physiology of perception 317
a few basic taste qualities: sweet, salty, sour, bitter and umami – the
sensation elicited by monosodium glutamate.
Taste receptor cells (TRCs) supply information for the recognition and
response to the nutrients we need. The sweet taste of sugars, for example, is
a stimulus for eating carbohydrate-rich foods. Salty and acid tastes are
markers for ions in food: evidence is reported that humans and other
animals with sodium and other dietary deficiencies will seek foods rich in
sodium and minerals, as well as vitamins (Smith and Margolskee, 2001).
Umami can be considered a signal coming from protein-rich food, whereas
the perception of bitterness is fundamental for its protective role, since
many harmful substances, like strychnine and other alkaloids, often
present a strong bitter taste (Dulac, 2000a). The sour taste of spoiled foods
also contributes to their avoidance. All animals, including humans,
generally reject acids and bitter-tasting substances at all but the weakest
concentrations. And in fact thresholds of perception are very different,
ranging from about 10 10M for denantonium benzoate (benzyldiethyl
[(2,6-xylylcarbamoyl)methyl] ammonium benzoate), the most bitter
substance known, to 8 10 4Mfor citric acid and 9 10 3Mfor sucrose.
Taste signals also evoke physiological responses, such as insulin release,
that aid in preparing the body to use the nutrients effectively. Individual
taste stimuli, even within one quality, often differ greatly in molecular
size, lipophilicity and pH (Kinnamon, 2000). For example, as far as sweet
taste is regarded, the molecular weights can be as low as 92 for glycerol, up
to 180 for glucose, 205 for saccharin, 342 for sucrose, but also as high as
11 233 for monellin or 22 204 for thaumatin.
It is easy to understand that there may be multiple mechanisms for
transducing even a single taste quality such as sweetness. Moreover, there
is a wide range of variation in people’s sensitivities to compounds
belonging to the same ‘taste-category’ (Delwiche et al., 2001), and in fact
some researchers provocatively assert that there may not be any
‘fundamental’ tastes (Ishii and O’Mahony, 1987; Delwiche, 1996). The
division of responses to taste stimuli into five types might be nothing but a
useful categorization that humans do, according to the particular function
that the molecules associated to those chemical stimuli have.
9.2.1 Reception taste buds
A field of taste research is devoted to the comprehension of how
molecules corresponding to the taste qualities become electrical signals
that carry information to the brain about what substance is in the mouth.
This process is called transduction and starts in the oral cavity.
318 Espresso Coffee
Human TRCs can be found as single cells or packed in onion-shaped
structures called taste buds (Figure 9.1), which consist of up to one
hundred cells. Taste buds are located predominantly on the tongue, but
can be found also in the soft palate, in the epiglottis. The taste marker
molecule gustducin can be found even in nasal mucosa and in the
stomach (Lindemann, 2001).
Gustducin is a taste-specific G-protein: the name ‘G-protein’ derives
from the fact that the activity of such proteins is regulated by a chemical
called guanosine triphosphate (GTP). G-proteins are made up of three
subunits: an subunit that can be considered the active portion, and
and subunits that regulate. When a tastant binds to G-protein coupled
receptors (GPCRs) on a taste cell’s surface, the subunit dissociates from
the portion, and activates a nearby enzyme. The enzyme starts a
transduction cascade, ending with a firing of an electrical impulse, which
is sent to the brain. The same transduction mechanism can be found in
olfaction.
In humans the tongue contains around 5000 taste buds, but the number
of buds can vary considerably from one person to another. Most of taste
buds on the tongue are located within specialized structures called
papillae, which give the tongue its velvety appearance (Smith and
Margolskee, 2001).
Physiology of perception 319
Figure 9.1 Tastebud with two receptor cells (Lindemann, 2001). On the top
surface of the bud, microvilli are noticeable; on the bottom part the synapses are
shown. Tastants enter the bud through the pore, are processed in the cells, and a
signal is sent to the brain by the nerve fibres. Reproduced with permission.
Taste buds in the anterior two-thirds of the tongue reside in fungiform
papillae, each containing one or at most a few taste buds. The posterior
region of the tongue contains the papillae circumvallatae and the papillae
foliatae, each of which contains hundreds of taste buds (Dodd and
Castellucci, 1991). The most numerous papillae on the tongue, called
‘filiform’, lack taste buds and are involved in tactile sensation.
9.2.2 Taste coding in the brain
Many research papers on animals and humans demonstrate that there is
not always a strict correlation between taste quality and chemical class,
particularly for bitter and sweet tastants. Bitter compounds can be salts
like magnesium sulphate, aminoacids like L-phenylalanine, alkaloids such
as caffeine, or glycosides of phenolic compounds (naringin in grapefruit).
Many carbohydrates are sweet but some are not. Furthermore, very
disparate types of chemicals can evoke the same sensation: the chemical
structures of artificial sweeteners aspartame and saccharin have nothing
in common with sugar. On the contrary, the compounds that elicit salty
or sour tastes are less diverse and are typically ions.
Coding in the periphery of the taste system could occur at two levels:
taste receptor cells and afferent fibres. In principle, a taste receptor cell
could be tuned to a single modality (e.g., sweet, sour, bitter, or salty), or to
more than one modality. Likewise, subsets of cells having similar response
profiles could be innervated by a common fibre, or single fibres may carry
information from different types of cells. Whether and how individual
taste cells respond to multiple chemical stimuli is still a matter for debate.
9.2.3 Transduction mechanisms
Given a so widely varying class of compounds eliciting a particular taste,
much effort has been devoted to understanding the molecular components
of the taste transduction mechanism, leading to the conclusion that
multiple reception–transduction systems occur for different tastes and
even for different tastants of the ‘same’ taste.
Ionic stimuli, such as salts and acids, can depolarize taste cells by direct
interaction with particular ion channels. More complex stimuli, such as
aminoacids, sugars and most bitter-tasting compounds, bind to surface
receptors that trigger a series of signals within the cell, the result of which
is the opening and closing of ion channels.
320 Espresso Coffee
9.2.3.1 Salty and acid tastes
Two tastes can detect ions in the oral cavity: salty and sour.
Salty taste can regulate the assumption of NaCl and other minerals,
and it is therefore very important for homeostasis processes; it is elicited
by many ionic species, but the component that is probably most
important, and surely most studied, is the one due to the presence of
sodium ions. Sodium ions penetrate into TRCs through Na-channels,
which are located on the top or lateral surface. The accumulation of
sodium ions leads to an electrochemical change, called depolarization,
which eventually is transformed into an electrical impulse that is sent to
the brain through nerve cells. TRCs then repolarize, that is, reset.
Na
.
-channels play a role in NaCl taste transduction, as well as in the
transduction process of other salts, like KCl or NH4Cl. Thus salty taste
detection and transduction can both depend on the use of certain
combinations of common and specific transcellular pathways (Lundy et
al., 1997).
Acidity can be considered a quality in many foods and beverages when
of low intensity, and it is in fact a marker for good quality coffee, but it
becomes unpleasant as intensity grows (Ganchrow et al., 1983). Its role for
the organism could be to give an alarm for an unripe fruit or spoiled food.
Acids taste sour because they dissociate into hydrogen ions (H.) in
solution. Those ions act on a taste cell in three ways:
n By directly entering the cell as ions or undissociated weak acid.
n By blocking potassium ion (K.) channels on the microvilli: K.–
channels are responsible for maintaining the cell membrane potential
at a hyperpolarized level. Block of these channels causes a
depolarization, Ca2. entry, transmitter release and increased firing
of electrical signals.
n By binding to and consequently opening channels and thus allowing
other positive ions to enter the cell. These positive charges
eventually depolarize the cell and an electrical signal is then sent
to the brain.
A fundamental issue surrounding sour taste reception is the identification
of the sour stimulus. It must be pointed out that no strong correlation has
been found between the pH of a solution (e.g., coffee) and the perceived
sourness, even if to some extent the intracellular pH of taste cells is
correlated with extracellular changes in pH (Figure 9.2). Recently a
hypothesis has been successfully tested, that acids induce sour taste
perception by penetrating plasma membranes as H. ions or as
Physiology of perception 321
undissociated molecules and decreasing the intracellular pH of TRCs.
The results evidence that taste nerve responses to weak acids (like acetic
acid) are independent of stimulus pH but correlate strongly with the
intracellular acidification of polarized TRCs (Lyall et al., 2001).
With regard to coffee, there is so far no clear consensus (Woodman,
1985; Heath, 1988; Balzer, 2001) as to which acids are the most important
in determining acidity by pH measurement and perceived acidity, and
whether pH or titrable acidity (Maier, 1987) is a good chemical estimator
for perceived acidity. Extending the above-mentioned results to humans,
titrable acidity at pH 6.5, which is the average pH of human saliva, can be
considered the optimal chemical measure of perceived acidity.
9.2.3.2 Bitter and sweet tastes
The sensations of bitter and sweet tastes derive from the interaction of
tastants with GPCRs in the apical membranes of TRCs. The top surface
of TRCs, which makes contact with the oral cavity, is rich in microvilli
containing GPCRs, ion channels and other transduction elements. The
lateral region of TRCs contains ion channels and synapses with afferent
taste nerves (Margolskee, 2002).
Like acidity, bitterness is also acceptable when of low intensity, but it
becomes absolutely disagreeable when intense. It is usually associated with
the idea of something harmful, and in fact many toxic substances (as
nicotine and strychnine) taste bitter. Bitterness is elicited by a number
322 Espresso Coffee
Figure 9.2 Recorded nerve signals from responses to different acid stimuli with (a)
same pH and (b) same concentration (Reproduced with permission from Lyall et al.,
2001)
of chemically heterogeneous substances, but they present the same
transduction mechanism, through GPCRs and second messengers.
By scanning mouse genomic databases, a group of new GPCRs, the
T2R family, was discovered in 2000 by different research groups (Adler et
al., 2000; Chandrashekar et al., 2000; Matsunami et al., 2000). This family
of receptors is very large, consisting of 40–80 genes, thus explaining the
large number of different bitter-tasting compounds. In humans, the T2R
family comprises more than 20 genes coding for GPCRs.
Sweet taste is mainly caused by soluble carbohydrates, and it may
represent a stimulus for the intake of calorie-rich food. However, a large
number of non-carbohydrate molecules are also sweet. Sweet taste elicits
a strong hedonic effect (Ganchrow et al., 1983). Like bitter stimuli, sweet
tastants also bind to GPCRs on a taste cell’s surface and trigger the
downstream process of secondary messengers, which eventually send the
electrical impulse to the brain. The candidate receptor for sweet taste is
T1R3, a GPCR, which was found simultaneously by four laboratories in
2001, by means of the latest tools and techniques of genetics and
molecular biology (Nelson et al., 2001; Montmayeur et al., 2001).
Ultimately, the study of sweet taste perception should help us explore
the hedonic aspects of taste and perhaps understand why ‘a spoonful of
sugar helps the medicine [and the cup of coffee] go down’ (Sherman and
Sherman, 1964).
9.2.3.3 Umami taste
Umami, a term derived from the Japanese umai (delicious), corresponds to
a taste sensation that is qualitatively different from sweet, salty, sour and
bitter. Umami is a dominant taste of food containing L-glutamate, like
chicken-broth, meat extracts and ageing cheese. The biological significance
of this basic taste, discovered about 100 years ago, is comparable to
that of sweet taste, being elicited by protein-rich food.
A taste receptor for L-glutamate could be linked to one of the receptors
for glutamate already known from neuronal synapses. Starting from this
hypothesis, Roper and colleagues (Chaudhari et al., 2000) found a subset
of gustative cells which presents a structure adapting the receptor to the
concentrations of glutamate present in food: thus a receptor has been
identified which responds to aminoacids.
Other laboratories (Matsunami et al., 2000; Nelson et al., 2002), have
identified and characterized a mammalian aminoacid taste receptor,
which respond to L-aminoacids and monosodium glutamate, belonging to
another family of genes, sharing many analogies with T2Rs, recognized as
receptors for bitter compounds.
Physiology of perception 323
Umami stimuli are known to bind to G-protein-coupled receptors and
to activate second messengers. But the intermediate steps between the
second messengers and the release of neurotransmitters are unknown.
9.2.4 Challenging the tongue map myth
One of the greatest myths about taste – which is unfortunately still found
in textbooks – is the so-called ‘tongue map’, showing a regional
specialization of the tongue for a particular taste quality, indicating that
sweetness is detected by taste buds on the tip of the tongue, bitterness at
the back and saltiness and acidity on the sides.
The misconception arose early in the twentieth century as a result of
misinterpretation of a German work (Ha¨nig, 1901), and still resists in the
literature. In the actual research it had been, on the contrary, determined
that each chemoreceptive area of the human tongue responds to each of
the qualities of sweet, sour, salty and bitter taste. Only minor differences
in subjective sensitivities, computed as the inverse of the detection
threshold, were noted across areas (Lindemann, 2001). The misinterpretation
and the lack of check of the original reference led to an interesting
graphical ‘evolution’ of those images, providing the impression of
specialized areas for taste reception. In reality, taste buds responsive to
each category of taste stimuli are found in all regions of the oral cavity,
even if with minor differences; moreover, each individual taste bud
contains taste cells with sensitivity to the different types of tastants.
Taking bitter compounds as an example, some of them are perceived more
intensely on the tip of the tongue, others on the back of the tongue and
others in the throat after swallowing. The lack of specificity of individual
taste buds across the tongue is directly confirmed by the large distribution
of the T2R genes in taste buds of all types of papillae (Dulac, 2000a).
9.2.5 PROP sensitivity as a marker for food choice
There are both cultural and genetic reasons why we perceive foods
differently, and have different habits as far as eating is regarded. For
example, thresholds of perception and sensitivity to a bitter compound
called 6-N-propylthiouracil (PROP) vary enormously across individuals.
The differences are so sharp that scientists tend to categorize people into
‘non-tasters’, ‘tasters’ and ‘super-tasters’, according to their sensitivity to
PROP (Bartoshuk, 2000). The general dislike of bitterness led early
investigators to suspect that ‘tasters’ and ‘super-tasters’ would tend to
dislike bitter foods, coffee included.
324 Espresso Coffee
However, other scientists are more sceptical (Mattes, 2002). While
significant differences in bitter taste perception have been observed under
laboratory conditions, their impact on actual food choices and eating
habits may be limited. The research has not reached a definitive answer.
As far as coffee is concerned, out of respondents who consumed drip
coffee, tasters were more likely to drink coffee with milk/cream and
sweetener than non-tasters were. Conversely, a greater percentage of nontasters
preferred their coffee black as opposed to tasters (Ly and
Drewnowski, 2001).
9.3 OLFACTION
Despite the aesthetic and emotional role to which humans have
confined olfaction, for most animals it represents a fundamental sense,
on which they rely in order to recognize predators or preys, food, kin and
mates. Today’s big challenge of olfaction research is to understand how
scents are recognized and how recognition of odours in the nose is
transferred to the brain to provide interpretation. Coffee, for example,
emits a specific combination of some 1000 different odour molecules
(Flament, 2002): therefore the central olfactory system has to integrate
signals from a huge number of odorant receptors (OR) in order to
provide recognition and interpretation, which in the end is translated
into a smile of appreciation for a lovely sensory experience or a grimace
for unsatisfied expectations.
Olfaction is the chemical sense whose scope is to inform the organism
about the chemical composition of the environment. The breakthrough
for understanding the mechanisms of this sense came in 1991 with Buck
and Axel’s discovery of the genes encoding ORs in mammals (Buck and
Axel, 1991), and since then new findings appear regularly in scientific
journals, adding to our understanding of this sense. Among the latest
reviews on the subject are (in alphabetical order): Axel (1995), Doty
(2001), Firestein (2001), Leffingwell (2002), Mombaerts (2001a).
The human nose is an extremely sophisticated device of molecular
recognition, still far superior to the electronic noses found on the market
(Mombaerts, 2001a). A common misconception is that humans can smell
some 10 000 odours; this view arises from confusion between detection,
discrimination and identification of odours. Theoretically there is no
upper limit to the number of possible detected smells. It is true, however,
that humans are very good at detecting odours, but lack the ability to
identify them. In other words, our lives are not ruled by smells, whereas
odours and chemical signals in general play a fundamental role in the life
Physiology of perception 325
of most animals, from basic tasks like finding foods to complex functions,
like social behaviour.
When we smell an odour out of its normal context (e.g. in a laboratory
vial), it is very common to experience the ‘tip-of-the-nose’ phenomenon:
we are familiar with it, but just cannot remember its name (Pines, 2001).
That is, it is easier to recognize the scent of coffee in a coffee shop than in
a theatre, because the context helps to narrow down the list of potential
odours. Moreover, the emotional role of smell is high.
Memories associated with odours are linked to emotions; conversely
they appear to play a special role for memories: it is the ‘Proust effect’. InA `
la recherche du temps perdu, French novelist Marcel Proust described his
feelings after consuming a spoonful of tea in which he had soaked a
madeleine:
The taste [meaning, of course flavour] was that of a little piece of
madeleine which on Sunday mornings in Combray . . . my Aunt Leonie
used to give me, dipping it first in her own cup of tisane . . . Immediately
the old grey house on the street, where her room was, rose up like a stage
set . . . and the entire town, with its people and houses, gardens, church,
and surroundings, taking shape and solidity, sprang into being from my
cup of tea.
What if he had been drinking an espresso?!?
9.3.1 What are odours?
As far as odorants are concerned, olfaction scientists have to face a
complex problem: there is no ‘smell scale’, and odorous molecules vary
widely in chemical composition and three-dimensional shape (Pines,
2001): there is no comparison with sight, where wavelengths and
corresponding colours can be placed on a linear scale. Represented in
the olfactory repertoire are aliphatic and aromatic molecules with varied
carbon backbones and diverse functional groups, including (in alphabetical
order) alcohols, aldehydes, alkenes, amines, carboxylic acids, esters,
ethers, halides, imines, ketones, nitriles, sulphides and thiols (Firestein,
2001). The requirements a molecule must fulfil in order to be an odorant
are that it should be volatile and hydrophobic. These requirements can be
due to the fact that molecules have to reach the nose and have to pass
through membranes. Moreover, their molecular weight should be less
than approximately 300 daltons (one dalton is one-twelfth the weight of
an atom of 12C). Actually, the largest known odorous molecule is labdane,
326 Espresso Coffee
which has a molecular weight of 296. There is no definitive explanation
of this requirement, although there may be some clue in that vapour
pressure (and therefore volatility) decreases rapidly with molecular size.
Conversely, some of the strongest odorants present high molecular
weights. Moreover, ‘the cut-off point is very sharp: for example,
substitution of the slightly larger silicon atom for a carbon in a benzenoid
musk causes it to become odourless’ (Turin and Yoshii, 2003). Another
hint of the correlation between dimension and chemoreception is that
occurrences of specific anosmias are correlated with the molecular weight.
Anosmia (from the Greek; a [no] osmia [smell]), or, more correctly
hyposmia, is the decreased sensitivity to some or all odorants. Everybody
is anosmic to some compounds, and there are molecules to which up to
one-third of the human population is hypo-osmic, as, for example, benzyl
salicylate.
9.3.2 Physiology: olfactory sensory systems,
olfactory sensory neurons, olfactory
epithelium, glomeruli
Receptors of three different neural systems can be found in most land
mammals. They are located in the nasal cavity, and their role is to check
the external environment. The main olfactory system is the sensor of the
environment, responsible in large part for the flavour of foods and
beverages. Additionally, this system serves as an early warning system for
spoiled food and noxious or dangerous environmental chemicals.
A second olfactory system (the vomeronasal system) has developed for
the specific task of finding a receptive mate (Dulac, 2000b). The nature of
vomeronasal sensations, if any, is unknown to humans (Doty, 2001), even
if chemical communication appears to occur (Meredith, 2001).
The trigeminal system provides a response to both chemical and nonchemical
stimuli, inducing somatosensory sensations (e.g. cold, hot, spicy
and tickling), and provoking reactions, secretion of mucus and halting of
inhalation, in order to minimize possible damage to the organism.
Odorant stimuli are detected by olfactory sensory neurons (OSNs)
located in the olfactory epithelium (OE), which is situated in the dorsal
part of the nasal cavity. The OE is only a few square centimetres wide and
it is composed of some millions of OSNs, which are generated in situ from
supporting stem cells. Like in other epithelia, cell renewal persists
throughout adult life to replace OSNs, which have a lifespan of weeks to
months (Mombaerts, 2001a). The OSNs are bipolar neurons with a single
Physiology of perception 327
dendrite that reaches up to the surface of the tissue and ends in a knoblike
swelling with some 20–30 very fine projecting cilia. These cilia,
which actually lie in the thin layer of mucus covering the tissue, are now
known to be the site of the sensory transduction apparatus. A thin axon
projects from the cell directly to higher brain regions (Dodd and
Castellucci, 1991) (Figure 9.3a).
The OE is a complex melange of different OSN populations randomly
distributed, with the aim of maximizing the probability of interaction of
an odour molecule with its receptor. All axons of OSNs which express a
particular receptor, no matter what their position is in the OE, converge
to a single ‘target’ in the olfactory bulb (Mombaerts, 1999), called
glomerulus. Glomeruli are spherical specialized synaptic arrangements, in
which thousands of axons from OSNs converge (Firestein, 2001).
Olfactory glomeruli have been viewed as functional units since they
were first described in the nineteenth century, but their role was unknown
until the discovery of odorant receptor genes. A century after its
anatomical description, molecular biology has proved that a glomerulus
receives the axonal input from OSNs that express a particular OR gene,
thus providing for integration of information and for an increase in the
signal-to-noise ratio (Mombaerts, 2001a). The concept of glomerular
convergence is consistent with a widely accepted view of olfactory coding,
in which the particular combination of activated glomeruli informs the
brain on what the nose is smelling (Buck, 1996).
Sensory information is processed within the bulb and relayed to the
olfactory cortex and several other centres in the brain, where it gives rise
to the olfactory perception of, for instance, coffee. The olfactory pathway
is unique among the senses because in olfaction the cell detecting the
stimulus, the OSN, is a neuron that projects its axon directly to the brain
(Mombaerts, 2001b).
9.3.3 Olfactory receptor genes
Scientists have long been convinced that receptor proteins capable of
recognizing and binding odorants are on the cilia in the OE, but the
discovery happened only in 1991, when Buck and Axel found the family
of transmembrane proteins that were believed to be the ORs and some of
the genes that encode them, thus giving a partial answer to how the OS
responds to the thousands of molecules of different shapes and sizes, i.e.
the odorants, and how the brain makes use of these responses to
discriminate between odours (Buck and Axel, 1991). They cloned and
characterized 18 different members of an extremely large multigene family
328 Espresso Coffee
Physiology of perception 329
Figure 9.3 Schematic synaptic organization circuit, from the olfactory sensory
neurons (OSNs) to higher regions in the brain (Mori et al., 1999). The olfactory
epithelium is mapped onto the olfactory bulb in the brain with a convergent
topography. Axons from OSNs expressing the same odorant receptor converge to
defined glomeruli. (Reprinted with permission from Mori et al., 1999. Copyright
2005 American Association of the Advancement of Science)
that encodes the seven transmembrane proteins whose expression is
restricted to the OE. All the proteins they found contained sequence
similarity to other members of the ‘G-protein’ coupled receptor family
(Leffingwell, 2002).
A second giant step in understanding the olfactory reception process
happened in 1998, when Firestein and colleagues (Zhao et al., 1998)
proved by experiment, measuring the electrical activity in the neurons,
what Axel and Buck had found were actually ORs. In 2001, Lancet and
co-workers reported a first draft of the human OR repertoire through a
data-mining process (Glusman et al., 2001). This dataset includes 906
human OR genes, of which more than two-thirds seem to be pseudogenes
(i.e., unexpressed genes) (Mombaerts, 2001b). This extremely high
number of genes for ORs in the mammalian genome (it is always
approximately 1000) makes it by far the largest family of GPCRs, and
probably the largest gene family in the entire genome (Firestein, 2001).
This family represents 1% of the genome, which means that, at least in
animals like dogs or rats, where almost all genes are expressed, 1 out of
every 100 genes is used for the detection of odours. This astounding
number of genes reflects the crucial importance of smell to animals
(Pines, 2001).
9.3.4 Transduction and processing in the brain
Once the receptor has bound an odour molecule, a cascade of events is
initiated that transforms the chemical energy of binding into a neural
signal, which is sent to the brain.
The ligand-bound receptor activates a G-protein, causing a cascade of
events that ends with the firing of an electrical signal to the brain. The
central olfactory system receives the odour molecule information through
axons of sensory neurons. The information is processed and integrated as
the olfactory quality of objects. The human perception of an odour is
characteristic in that it is usually associated with pleasant or unpleasant
emotions (Mori et al., 1999). In order to understand the rules for
interpreting the olfactory stimuli, it is useful to use a ‘molecular receptive
range’ for OSNs (Figure 9.3b). Individual OSNs express only one type of
OR gene out of a repertoire of up to 1000 genes. It appears that to match
the ability of organisms to detect far more than 1000 odours, the odours
must participate in some kind of combinatorial processing: that is, one
receptor must be able to interact with several discrete odorants. Olfactory
sensory neurons should respond to many odorants with varying affinities.
Conversely, an odour molecule should interact with more receptors. In
330 Espresso Coffee
other words, an individual odour will activate multiple glomeruli in the
olfactory bulb, and different odours activate overlapping but nonidentical
patterns of glomeruli. One particularly striking case involves
enantiomers (molecules that are optical isomers, or mirror images, of one
another).
It seems that receptors that recognize similar odours tend to stimulate
the same area of the olfactory bulb (Figure 9.3b) (Firestein, 2001). Recent
studies have proved that the olfactory axon projection follows a specific
convergence pattern towards glomeruli (Mori et al., 1999). There are
specific zones of activity in the olfactory bulb, caused by odour-specific
patterns. These zones seem to correspond to individual glomeruli, since
there is a correspondence between the size distribution of the discrete
responses and the size distribution of glomeruli (Bozza and Mombaerts,
2001).
Despite these findings, the basic understanding of the functional
organization of the axonal projection to the olfactory cortex and of the
neuronal circuits in the olfactory cortex is still unknown. The next
fundamental step will be therefore the comprehension of the neuronal
mechanisms by which the olfactory cortex combines or compares signals
from all different glomeruli (Mori et al., 1999).
9.3.5 How many odours can we detect?
Different figures are given in the literature on the number of odours
humans can detect, ranging from approximately 2000 to more than
100 000. Theoretically there could be billions, based on the possible
combinations of 1000 receptors. In fact, the question is probably
irrelevant, just as it makes little sense to ask how many colours or hues
we can see (Firestein, 2001). Perfumers, chefs, sommeliers, coffee cuptasters
or highly trained animals can discriminate more odours than the
average, but this is due to experience, not to a difference in the
‘measurement instrument’. Physical chemistry may be the primary
limiting factor, as odour chemicals must possess a certain volatility,
solubility and stability to act on the nasal sensory tissue.
The variable sensitivity of individual glomeruli produces distinct maps
for different odorant concentrations. It seems that odorants and their
concentrations can be encoded by distinct spatial patterns of glomerular
activation (Rubin and Katz, 1999). For example, methanethiol, which
smells like rotten cabbage at high concentrations, has a sweet coffee-like
aroma at low concentrations (Flament, 2002). However, most odours
remain constant in their quality over orders of magnitude of concentra-
Physiology of perception 331
tion. Amylacetate, a pleasant fruity smelling substance also found in
coffee aroma, can be easily identified at concentrations from 0.1 mM to
10mM (Firestein, 2001).
9.3.6 Cross-cultural differences
Social and cultural cues provide another interpretation of non-sensory
influence on perception, and are potent modulators of responses to odours
and foods. Cross-cultural studies show how cultural experience can affect
expectations, thereby altering sensory perception (Cristovam et al., 2000;
Hu¨bener et al., 2001; Prescott et al., 2002).
Much work has been carried out in the past decade, with increasingly
accurate and standardized psychophysical tests leading to the following
broad results, reported by Doty (2001):
n Women, on the average, have a better sense of smell than men –
superiority that goes beyond cultures and is noticeable already at the
age of four; they also retain the ability to smell longer than men.
n There is a substantial genetic influence on the ability of humans to
identify odours.
n Starting about in the fourth decade of life, many individuals begin to
notice a gradual decline in their ability to smell, affecting both
sensitivity and ability to identify odours. Because olfactory stimulation
makes up a major component of food and beverage flavours,
people with olfactory decrements often report a diminished ability to
taste. A major loss of olfactory function occurs after age 65, with over
half of those between 65 and 80, and over three-quarters of those 80
years of age and older, having such loss.
n The decrement in olfactory function associated with smoking is
present in former smokers and recovery to pre-smoking levels, while
possible, can take years, depending on the duration and amount of
past smoking.
n The olfactory function is compromised in urban residents and workers
in various industries.
9.4 HUMAN CHEMOSENSORY
PSYCHOPHYSICS
So far, reception and transduction mechanisms for chemical senses have
been described. When we drink a cup of espresso, the chemical stimuli
332 Espresso Coffee
coming from taste and smell are transduced, together with tactile
sensations, into electrical signals, which are sent to the brain. Taste,
smell and trigeminal systems are separate systems with separate functions,
as discussed above. As in vision, where movement, position and
identification of objects are perceived separately, without, however,
perceiving them as independent properties, the experience of flavour
integrates information from smell, taste and the trigeminal system. The
orbitofrontal cortex may be the brain structure that performs this
integration (Abdi, 2002), with an analytical and hedonic reaction and
a final response in terms of recognition and liking of the characteristics of
the coffee (or whatever food or beverage).
How can this response be correlated to the physico-chemical characteristics
of the coffee and the chemoreception process? Psychophysics
is the branch of research that interprets perception, that is, the
psychological process that provides the link between the physical stimulus
and our everyday experience of odour, taste, tactile sensations and sight.
Psychophysics uses specific behavioural methods to determine the
relationship between the physical stimuli and the perceptions they elicit,
that is, people’s subjective experience. It is a method for measuring some
characteristics of an object, specifically those features eliciting a reaction
from our senses, which goes beyond the ‘I like it, I do not like it!’
judgement.
In an analytical measurement method, like, for example, headspace
gas chromatography, a sample is prepared and injected into the column,
and the detector gives, as an output, the aroma profile of the sample, in
terms of quality and quantity of volatile compounds. When dealing with
sensory analysis, the detectors are our senses of gustation and olfaction.
The output is a profile in terms of quality and quantity of sensory
attributes.
Among the literature on best sensory analysis practice and methods
is the pioneer work of Amerine and Pangborn (Amerine et al., 1965),
and more recent works by Stone and Sidel (1985), Moskowitz
(1988), Piggot (1988), Lawless and Klein (1991) and Meilgaard et al.
(1999).
9.4.1 Psychophysical scales
An important issue, which has to be faced in sensory analysis, is the use of
scales. Scales are psychophysical metrics used to evaluate the intensity of
a stimulus. Usually five types of scales are used:
Physiology of perception 333
n Category scale (e.g., barely detectable, weak, moderate, strong, very
strong), which is easy to use but has a fixed number of categories and
is sensitive to bias.
n Discrete numerical scale, the numerical counterpart of the category
scale. The most commonly used is the 9-point scale (Peryam and
Girardot, 1952).
n Visual analogue scale, that is a straight line (with or without semantic
or numeric anchors) on which the intensity of the sensation has to be
rated. This scale presents an infinite number of categories and it is
easy to use, but it does not allow between-subject comparison,
because different assessors have their own ‘mental’ scale.
n Ratio scale, in which the assessors evaluate the ratio between two
stimuli. It provides information about the relative intensity of stimuli,
but it requires considerable training and does not allow betweensubject
comparisons.
n The category-ratio scale, like the Labelled Magnitude Scale (Green et
al., 1993; 1996), combines features of ratio and category scales,
allowing comparisons between subjects and providing absolute
intensity estimates. However, when evaluation of a complex stimulus
is involved, like in coffee, it becomes quite cumbersome.
No scale is correct or incorrect, in an absolute sense: the use of one or the
other depends on the task being performed, on the differences looked for,
and on the panel being used. When comparing very different items, such
as a robusta with an arabica, the category scale, or numeric discrete scale,
would be sufficient; but in the comparison between, for example, two lines
of the same cultivar, then an analogous scale or a ratio scale should be
used, to look for fine differences. Non-linear anchored scales can be used
in some situations, like log scales or power scales. The problem that the
use of anchored scales brings along is the tendency of assessors to stick to
the anchors.
In a preference test with consumers, a 5-point or 9-point scale for the
degree of liking (1 . not at all . . . 5 or 9 . very much) would be
sufficient: a more detailed scale could lead to confusion for the consumer.
9.4.2 Stimuli-reaction categorization: type of
sensory tests
The reactions to the presentation of a sensory stimulus can be divided
into three types:
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n qualitative perception of intrinsic characteristics – analytical;
n quantitative perception of the intensity of the stimulus – analytical;
n hedonic reaction, i.e. the degree of pleasure caused by the stimulus –
hedonic.
A sensory stimulus (e.g., an espresso coffee) is described first in terms of its
characteristics (e.g., acid or bitter taste, smooth mouthfeel, chocolate and
toasted bread aroma notes), then these characteristics can be quantified
(on a given scale), and finally the degree of pleasure caused by the
stimulus is noted. This last, being a hedonic judgement, is not an attribute
of the product itself. It can vary with culture, gender, age, and it
represents the basis of consumer segmentation science. The two types of
assessments (analytical or hedonic) should not be performed together by
the same panel, because trained assessors are not representative of
consumers’ preferences, and, moreover, they are particularly critical of the
product. On the other hand, consumers are unable to describe correctly
the sensations they experience, for lack of training (Majou, 2001).
However, statistical techniques, like preference mapping, allow the
drawing of correlations between sensory characteristics and consumers’
preferences.
9.4.2.1 Analytical tests
Analytical tests can be essentially of two types: discriminative or
descriptive.
Discriminative tests are used to check whether two products can be
differentiated. For example, the roasting process is changed, but the
sensory characteristics of the coffee are not to be affected, or, conversely,
one ingredient of the blend is changed, in a way to be perceived by
consumers. For these tasks discriminative tests are be used, with a trained
panel, not necessarily composed of experts, who are asked to identify
which are the identical products. Commonly used tests are the paired
comparison test, the triangle test and the ‘two-out-of-five’ test. What
changes is the probability of a correct guess, and therefore the number of
judges and correct answers required in order to reach a result with a
certain degree of confidence.
Descriptive tests are used to characterize the sensory profile of a
product, by describing its perceived sensory features. The characterization
of coffees from different origins, as well as the profiles of different brewing
methods, is achieved through such tests.
Descriptive analysis techniques involve a panel to specify the
intensities of specific attributes. In order to obtain reliable results, a
Physiology of perception 335
trained panel of expert judges is required, who should be trained with
reference standards.
The test methods are classified according to the result they provide.
There can be qualitative tests, like the flavour profile, and quantitative
tests, like the Quantitative Descriptive Analysis (QDA). The panel must
rate the perceived intensity of each attribute based on a psychophysical
model for subjective intensity, which accepts the proposition that
individual ratings vary in subjective intensity as a function of stimulus
concentration. The result of these tests is usually visualized in ‘spider’
plots.
It must be noticed that it is usually assumed that each descriptor is
unrelated to the others (i.e., they are orthogonal in mathematical terms)
and it is therefore necessary for an exhaustive profile of the product.
Moreover, it is assumed that they are perceptually separable features that
we can perceive distinctly, which is not the case for complex stimuli
(Lawless, 1999) like coffee, which consists of taste qualities and a mixture
of hundreds of different, distinct odours.
The intensity-based descriptive approach can bring with it several
hidden problems, and therefore it can be dangerous to assume a perfect
correspondence between the descriptive data that are obtained and the
sensations actually experienced by the panellists.
These considerations do not imply that the profile data cannot be used
to differentiate and characterize products, but it is scientifically more
honest to state that ‘ratings for coffee A were higher or lower than for
coffee B’, rather than ‘Coffee A was perceived as more intense in its bitter
and smoky notes and less flowery’. Such an inference about perception
would be unjustified or even incorrect (Lawless, 1999).
9.4.2.2 Hedonic tests
Hedonic tests evaluate the liking or acceptance of a product. They should
be carried out with an untrained panel of consumers, which must be
representative of the population; quite large numbers are therefore
required. In the sensory testing sequence, acceptance testing usually
follows analytical tests, which have reduced the number of possible
alternatives. If one ingredient of the blend is changed, to the extent that
consumers will likely perceive it, and the sensory profile of the new blend
is established, a check on the acceptance of the product must be
performed before launching it into the market.
Hedonic tests can be preference tests, where two or more products (e.g.,
the old blend and the new blend) are compared, or acceptance tests,
where the degree of liking (for example on a 9-point scale) is evaluated.
336 Espresso Coffee
9.4.3 Espresso cupping technique (fundamental
aspects)
Espresso cupping has been dealt with in 8.2.2 from a practical point of
view. Here more theoretical aspects will be considered.
Cupping methods followed in the industry are quite heterogeneous. In
the following some general rules will be given for a correct cupping
procedure. These are valid for all brewing methods, espresso included.
Cupping should be performed alone, without external stimuli, in order to
minimize noise and bias introduced by interaction with other assessors.
A sip of some 10–15 ml of coffee should be taken. Salivary-induced
changes in pH of sour taste stimuli affect taste perception, and therefore
smaller volumes of espresso would be perceived as less sour than larger
volumes. Saliva induces an increase of solution pH, and a small quantity
(4 ml) can be totally buffered by saliva (Christensen et al., 1987), thus
distorting the perception of acidity.
In order to avoid the consequences of caffeine intake, when evaluating
many samples a sip-and-spit procedure is recommended. Flavour perception
during eating or drinking depends on the movement of odorants
released in the mouth via the retronasal cavity to the olfactory
epithelium. An accurate analysis of the swallowing process, by means of
real-time magnetic resonance imaging (Bu¨ttner et al., 2002) has proved
(what already was well-known in medicine) that during mastication and
swallowing there is no connection to the dorsal oropharynx, the
nasopharynx and the airways, preventing leaking and aspiration of food.
When the bolus has left the pharynx, the upper oesophageal sphincter is
closed and the larynx, epiglottis and velum palatinum return to their
original position, followed by a short pulse of respiration, the so-called
‘swallow breath’, by which volatiles reach the retronasal cavity (Figure
9.4). A simple proof of this phenomenon can be performed by drinking an
espresso with a nose-clip: only 5% of the population would recognize it as
coffee.
The ‘slurping’ technique (Heath, 1988), that is the deliberate opening
of the velum palatinum border when coffee is in the oral cavity, is used to
avoid swallowing and yet perceive the aroma. In this way the amount of
volatiles reaching the olfactory epithelium ranges from 30% to 130% of
the values observed with swallowing, depending on how effective each
panellist is in the control of exhaling (Bu¨ttner and Schieberle, 2000).
Frequent rinsing has been prescribed to minimize adaptation of salivary
influence to taste perception, as well as to eliminate or minimize saturation
or carryover effects, and to enhance discrimination among samples. Palate
cleansers are commonly employed in sensory evaluation to rinse the mouth
Physiology of perception 337
before and in between samples. Despite the widespread use, little literature
regarding the effectiveness of these materials or methods is available, and
some studies report that no significant differences between palate cleansers
were found on any measure (Johnson and Vickers, 2002), unless in very
particular test conditions.
When tasting coffee, several main aspects have to be evaluated and
their definitions ought to be stated unequivocally. Unfortunately,
between areas of investigation there is a wide range of definitions for
the same terms used for describing a sensory experience. In the following,
a brief survey of most commonly used definitions will be given with a
stress on our own interpretation, which tends to align with the consensus
in the field of sensory science.
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Figure 9.4 View of the mouth and retronasal cavities (adapted from Bu¨ ttner et al.,
2000). When the bolus has left the pharynx, volatiles can reach the retronasal
cavity, thanks to the so-called ‘swallow breath’
9.4.3.1 Appearance
Appearance, mainly that of the crema, attracts consumer attention. It has
been said that espresso is consumed first by sight. This is true also for
other beverages, like wine, where wine tasters create perceptual illusions,
describing the odours of a wine according to its colour (Morrot et al.,
2001). As discussed in 8.2.1, the crema should be persistent (some
minutes), and made up of very small, hazelnut-coloured bubbles.
9.4.3.2 Gustatory sensations
Gustatory sensations, produced by the beverage’s taste, encompass acidity,
bitterness and sweetness (no salt or umami sensations are reported in
coffee literature), whose perception-transduction mechanisms have
already been discussed (see 9.2).
The stimulus eliciting acidity/sourness sensations acts on the same
receptors – and sensory scientists use them as synonyms. Nevertheless, the
reaction to it can have both positive and negative connotations, due to
the fact that this stimulus is often coupled with other sensations.
Therefore, two distinct terms are commonly used in the coffee field,
according to the hedonic reaction that the stimulus causes:
n Acidity, a basic taste produced by the solution of an organic acid. The
International Coffee Organization defines it as ‘A desirable sharp
(producing a strong physical sensation as if of cutting) and pleasing
taste is particularly strong with certain origins’ (ICO, 1991), which
applies, for instance, to Central American wet-processed coffees.
n Sourness, a different basic taste descriptor that ‘refers to an
excessively sharp, biting and unpleasant flavour’ (ICO, 1991). It is
used when associated to an astringent or metallic sensation, typical of
fast roasted coffees where the short time does not allow the extensive
elimination of chlorogenic acids (Ohiokpehai et al., 1982).
As in the case of acidity, the definition of bitterness has hedonic
connotations. Bitterness is ‘considered desirable up to a certain level’
(ICO, 1991). When it brings about an unpleasant reaction, the attribute
‘harsh’ (rough or sharp in a way that is unpleasant, especially to the
senses) is associated with the product.
In coffee cupping, sweetness is a marker of good quality coffee, and it is
often associated with aroma descriptors such as fruity, flowery, ‘chocolatey’
and caramel-like (ICO, 1991).
Physiology of perception 339
9.4.3.3 Olfactory sensations
Olfactory sensations, produced by the beverage’s aroma sensu lato, are
commonly described by the terms odour and/or aroma.
Aroma can be defined as ‘the property of certain substances, in very
small concentrations, to stimulate chemical sense receptors that sample
the air or water surrounding an animal’ (Encyclopaedia Britannica, 2003).
According to the ASTM, it is the ‘perception resulting from stimulating
the olfactory receptors; in a broader sense, the term is sometimes used to
refer to the combination of sensations resulting from stimulation of the
nasal cavity’ (ASTM E253-03). ISO does not give a unique definition,
describing it as ‘an odour with a pleasant connotation’, according to
English sense and French informal language, or, according to French
sense, as an ‘organoleptic attribute perceptible by the olfactory organ via
the back of the nose when tasting’ (ISO, 1992a). Other sources
characterize aroma as ‘a sensory perception based on one’s olfactory
senses; i.e., sense of smell’ (IFT, 2003), or prefer to define it from a
chemical point of view, as illustrated in section 8.1.3, leaving the term
odour to describe olfactory perception. One author in the coffee field
states that it is due to the volatiles, perceived retronasally after
swallowing, or by the ‘slurping’ technique (see above) (Heath, 1988). In
other fields of research the definition is applied to pleasant sensations
perceived by direct inhalation in non-specialized language (i.e. orthonasally),
whereas, technically speaking, aroma is referred to as ‘pleasant
sensations perceived indirectly (i.e. retronasally) by the olfactory organ
when tasting food or beverages’ (Consejo Oleicola International, 1987).
We accept the last definition, which brings about a difference between
odour and aroma, as volatiles perceived orthonasally or retronasally. The
latter has been called ‘flavour’ elsewhere in this book (see 8.2.2.4).
Odour is the sensation arising from the ‘sniffing’ of the cup (i.e., the
volatiles smelled orthonasally). Sensory scientists tend to use the term
odour to indicate an ‘organoleptic attribute perceptible by the olfactory
organ on sniffing certain volatile substances’ (ISO, 1992a).
9.4.3.4 Flavour
Flavour is by far the most debated term and it varies according to the field
of research. An early sensoric definition was ‘the sensation realized when a
food or a beverage is placed in the oral cavity. It is primarily dependent
upon reactions to taste and olfactory receptors to the chemical stimulus.
However, some flavours also involve tactile, temperature and pain
receptors’ (Beidler, 1958). Several standards have accepted it: for example,
340 Espresso Coffee
‘complex combination of the olfactory, gustatory and trigeminal sensations
perceived during tasting. The flavour may be influenced by tactile,
thermal, painful and/or kinaesthesic effects’ (ISO, 1992a). The definition
by ASTM is similar: ‘Flavour is the complex effect of the basic tastes,
olfactory sensations – perceived retronasally – and chemical feeling factors
all stimulated by foods and/or other materials in the mouth’ (Civille and
Lyon, 1996). Or, ‘flavour is the combined perception of mouthfeel,
texture, taste and aroma’ (British Standards Institute, 1975). A recent
definition (Encyclopaedia Britannica, 2003) is: ‘in sensory perception,
attribute of a substance (apart from its texture and temperature) that is
produced by the senses of smell, taste and touch and is perceived within
the mouth. These sensations help to identify substances and are sources of
enjoyment when eating and drinking’. One independent author proposes
to reserve the term flavour for a pure olfactory sensation (Petracco, 2001):
as elaborated in 8.2.2.4, this would ask for a new term to cover the overall
feeling produced by aliments in the mouth. Summarizing, we could take as
our definition the conclusion from a recent survey (Delwiche, 2003),
defining flavour as a complex reaction to chemical stimuli, generated in
the orbitofrontal cortex. Taste and smell are considered to be essential in
the perception of flavour.
9.4.3.5 After-flavour
After-flavour is the sensation produced by the lingering of taste and
aroma for a while (up to 15 minutes in the case of espresso) after having
swallowed a beverage. It might be ascribed to the mouth-coating effect of
the espresso, which has been related to the wetting properties of the
beverage on the oral cavity, deriving from its surface and interfacial
behaviour (Navarini et al., 2002).
9.4.3.6 Astringency
Astringency, a tactile sensation belonging to ‘mouthfeel’ with a negative
connotation for espresso coffee, is described as ‘the shrinking of the
epithelium probably related to precipitation of proteins of the saliva, and
somehow related to pH viscosity and anion species’ (Smith and Noble,
1997; Sowalsky and Noble, 1998). It is characteristic of an after-taste
sensation consistent with a dry feeling in the mouth (ICO, 1991). A
typical case is tannic acid, which precipitates gelatine to give an insoluble
compound used to perform the task of tanning leather.
Physiology of perception 341
9.4.3.7 Texture
Texture is primarily the response of the tactile senses to physical stimuli
that result from contact between the oral cavity and the food. It is a
multi-parameter attribute, which derives from the structure of the food
and is detected by several senses (Szczesniak, 2002). This is another
difficult term to define since it means different things to different people
(Bourne, 2002). Texture refers mostly to solid foods. When liquids are
studied, other terms are used, like body and mouthfeel.
Body (see also 3.8.8) is defined by ASTM as ‘the quality of a food or
beverage relating either to its consistency, compactness of texture,
fullness, flavour, or to a combination thereof’ (ASTM E253-03). In the
coffee field this attribute descriptor is used to characterize the physical
properties of coffee beverages (ICO, 1991). We would tend to eliminate
olfactory sensation from its definition, and describe the body of an
espresso coffee as the integration of tactile sensations due to the
interaction between the beverage and the oral cavity, the tongue and
the palate. This definition cannot easily be applied to other brewing
methods, where differences in tactile properties are minimal and not
supported by scientific literature. However, with the term ‘body’ having so
wide a range of definitions, and espresso being a typically Italian brewing
method, we could propose the Italian term corpo to describe it. Adapting
Szczesniak’s definition of texture (Szczesniak, 2002), corpo is a sensory
property and, thus, only a human being can describe it. It is a multiparameter
attribute not just oiliness or creaminess but a set of
characteristics; it derives from the structure of the beverage (molecular,
microscopic or macroscopic) and is detected by the senses of touch,
pressure and sight (Navarini, 2003).
Mouthfeel is often explained as ‘the mingled experience deriving from
the sensations of the skin in the mouth during and/or after ingestion of a
food or beverage. It relates to density, viscosity, surface tension, and other
physical properties of the material being sampled’ (Bourne, 2002).
The literature on the subject is of little help, apart from the pioneer
work of Szczesniak (1979), who performed numerous studies in order to
define and measure mouthfeel for a vast range of food and beverages
(coffee included). In her survey, commonly used definitions for mouthfeel
are categorized. Starting from Szczesniak’s categorization, a set of
descriptors, which apply to espresso coffee, can be found. They are
divided into viscosity-related terms (thick, viscous), feel on soft tissue
surfaces (smooth, creamy), body-related terms (heavy, light, watery),
chemical effect (astringent), coating of oral cavity (mouth-coating),
resistance to tongue movement (syrupy), after-feel in the mouth
342 Espresso Coffee
(lingering). Body thus represents a category of mouthfeel, relating to
sensations apparently caused by the denseness of the rheological structure,
therefore purely tactile. However, most terms in this group can be
interpreted as having both flavour and mouthfeel characteristics
(Szczesniak, 1979).
When dealing with coffee in general many definitions are found, most
of them relating mouthfeel with olfactive properties connected to the
intensity of the aroma perceived (Pictet, 1989), and describing it with
rather vague terms such as ‘rich’ or ‘round’. But, according to the
International Coffee Organization, the descriptor mouthfeel comprises
only body ‘to describe the physical properties of the beverage’, and
astringency, ‘characteristic of an after-taste sensation consistent with a
dry feeling in the mouth’ (ICO, 1991).
With regard to espresso, little is found in the scientific literature, where
body is linked with physical parameters like viscosity or dry matter
(Petracco, 1989). The body of an espresso coffee is, in our opinion, better
described as a purely tactile sensation due to the interaction between the
beverage and the oral cavity.
To conclude, it must be pointed out that words that have different
meanings to different panellists (i.e., words that a panel might have
difficulty reaching agreement on) should be avoided at all costs.
Therefore, an accurate definition of the vocabulary must be developed,
on which every panellist should agree.
9.4.4 Psychophysical measurement: from stimulus
to perception
Psychophysical studies require essentially two parameters: the variation of
specific aspects of a physico-chemical property of the stimulus, and the
measurement of a dimension of the subject’s response. Therefore a
fundamental issue of sensory studies is the correct delivery of the stimulus.
Only in this way can we hope to get a reliable answer from an instrument
(in our case, a panel) that is characterized by a low signal-to-noise ratio
and presents problems with offset, gain and drift with time.
It is highly probable that different judges will have different thresholds of
perception for chemical compounds (both gustative and olfactive) (Bi and
Ennis, 1998; Bartoshuk, 2000; Linschoten et al., 2001); also, thresholds and
sensitivity change with age (Doty, 1989). The rating of two assessors on an
intensity scale will differ, even if the same stimulus is applied. Furthermore,
perception scales are non-linear, meaning that when the concentration of
Physiology of perception 343
the stimulus is doubled, a double intensity on a perception scale will seldom
be rated. Finally, the repetition of the delivery of the stimulus over time
(e.g., coffee brewed from the same blend in successive days) will not lead to
the same answer from the panel (Vanne et al., 1998).
These problems lead to the following considerations. The number of
panellists involved in an evaluation must be high enough to assure an
averaging of the responses, representative of the population. Relying on
the judgement of just one or a few assessors for determining the sensory
properties of one product can be misleading because of specific anosmias
(see 9.3.1) or hypersensitivity of the judge. The number can be determined
by simple algorithms, starting from a binomial distribution, in the case of
discriminative tests. Once the significance of the result is set; as far as
profiling is concerned, the assumption is that the scores given by the judges
would distribute around an average value with normal distribution. Thus,
analysis of variance (ANOVA) procedures can be followed to evaluate the
characteristics of the product. In this case the central limit theorem states
that the minimum number of measures for a distribution to be
approximated to a normal distribution should be 10. This requirement
can be met in a university laboratory performing basic tasks, but it is
seldom fulfilled in industry’s everyday experience. However, the risk of
reducing the number of assessors in a panel too far must be kept in mind. A
cost–risk analysis should be performed before planning the campaign.
In order to perform an analytical task, such as discrimination between
products or profiling, the panel must be recruited, checked for
sensitivities, then calibrated and trained, in order to share a common
vocabulary and provide homogeneous results. Continuous monitoring of
the panel should be done to check for possible outliers and to verify when
recalibration might be needed (McEwan, 2001; Stucky et al., 2001).
A set of ISO Standards has been developed in order to assure maximum
reliability of the results. They deal with the design of the test rooms (ISO,
1988a), the recruitment and training of the panel (ISO, 1992b, 1993,
1994a) and propose the descriptive and discriminative tests (ISO, 1983a,
1983b, 1985a, 1985b, 1987a, 1987b, 1988b, 1991a, 1991c, 1992a, 1994b,
1994c, 1999a, 1999b, 2002a) to be performed. As far as coffee is
concerned, there is one standard on sample preparation and one on
vocabulary (ISO, 1991b, 2002b).
9.4.5 Statistical analysis of sensory data
Once data have been collected, they have to be ‘transformed’ into results.
Because of the complexity of the stimulus and the low signal-to-noise
344 Espresso Coffee
ratio, sophisticated statistical techniques must be employed, in order to
obtain reliable results. Table 9.1 gives a short list of the most commonly
used tests with their application.
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Physiology of perception 351
CHAPTER10
Coffee
consumption
and health
M. Petracco and R. Viani
The history of coffee starts with legends about its discovery as a food. The
story is sometimes told of herdsmen in Abyssinia who, following the
example of their goats consuming the seeds of some plant, chewed either
raw cherries or cooked, smoked beans (Burton, 1961) (Figure 10.1). It is
difficult to imagine, for those who have tried to munch on a raw bean,
that the success of coffee, which is today one of the world’s most traded
food commodities, could ever have come about if such primitive habits
had persisted, without proper product segmentation.
It seems logical, on the one hand, that the attractiveness of coffee to
those early pioneers of food science derived basically from their
experiencing a stimulant condition that proved to be beneficial to their
activities (Ellis, 1824). Put differently, the primal cause for coffee
consumption must have been the physiological effects of caffeine on
the human organism, which are nowadays well substantiated (Weinberg
and Bealer, 2001).
On the other hand, it is fairly obvious why coffee has become so
popular, attaining the status of the second most largely consumed
beverage after water. As seen in Chapter 9, it is a question of flavour,
or better still, of overall impact on our senses. Sensory evaluation, which
used to be considered as in the realm of ‘magic’ because ‘taste is a matter
of taste’, is nowadays earning the status of a highly respected analytical
tool (cup-testing being its technical name), able to produce key
information with good reliability (Meilgaard et al., 1999).
Thus, it is no surprise that coffee is seen by many consumers as a mild
stimulant that additionally gives pleasure. In this perspective, coffee
drinking is often denigrated and deemed guilty by association with social
habits that give raise to harmful effects, as tobacco smoking and alcohol
drinking undoubtedly do. In fact, the medical community does not see
coffee as a real worry: only a couple of areas show some reasons for
concern that could advise moderation in coffee consumption. There are,
by contrast, several rational motives to consider coffee as beneficial to
health, acting as a protective agent against specific pathologies.
10.1 CONSUMPTION PATTERNS
An often-underestimated question should be addressed first when
designing a clinical or epidemiological experiment: ‘How much coffee
(and how much caffeine) do my subjects take in?’ Typically, just the
number of cups per day is reported, with no mention as to cup size and
coffee bean type. Of course, it is often difficult to make subjects
(especially patients) produce accurate reports of their drinking habits in
distant periods of their life (Bracken et al., 2002). It is yet more awkward
to arrive at an estimation of the quality of coffee they used to drink, with
its related chemical composition. Thus, most of the time, subjects are
simply categorized in two or three classes: abstainers, moderate drinkers
and heavy drinkers (Bunker and McWilliams, 1979).
The answer is only apparently simple when dealing with intervention
studies, where the actual food and beverages intake can be controlled, or
even prescribed (Lelo et al., 1986): the content of caffeine and other
coffee materials in a cup depends both on cup size and extraction
efficiency, and on coffee composition. As discussed in Chapter 3, large
Coffee consumption and health 353
Figure 10.1 Smoked-toasted coffee, as is still consumed in East Africa (Uganda);
each banana leaf pouch contains around 20 cherries
chemical variations are to be foreseen with different proportions of pure
varieties in the blend. Also the size of a single serving is highly variable,
ranging from 15 ml of concentrated Italian espresso (low in caffeine) to
over 250 ml in the English-speaking countries. Moreover, a coffee serving
can be derived from the more or less efficient brewing of a roasted ground
coffee portion as small as 5 g, up to 15 g or more, and the variation in
active constituents, such as caffeine and diterpenes, can be very high
depending on blending and brewing technique (Table 10.1).
In population studies, reference is usually made to average national
consumption. This is calculated from raw coffee production, import and
disappearance data, which may not be universally agreed upon and may
not represent an accurate portrait of the local coffee market situation
(just think about re-export streams and non-official parallel markets).
The most reliable statistics are those published on a quarterly basis by
the International Coffee Organization (ICO, 2003), along with those
occasionally produced by market research companies (for instance LMC,
2002).
As an international trade commodity, global coffee production ranks
second in value only to petroleum, and totals more than 6 billion kg/year.
Cultivation occurs in some 50 tropical countries (Figure 10.2). As many
of these are less developed countries, coffee often represents the chief
hard currency income.
Coffee consumption is widespread throughout the world, especially in
Europe, the USA and Japan. The largest coffee-consuming country is the
USA, at 16% of the world total, followed by Brazil (which is also the
largest producer country), with 11%. National associations keep track of
their domestic consumption trends, divided per outlet segment (home,
restaurant, bar) and product type – traditional, gourmet, espresso, etc. (for
instance NCA, 2003).
The type of related beverages and the pattern of consumption are
strictly associated with social habits and culture of the single countries.
Variations in raw bean composition, in roasting conditions and in the
extraction procedures used to prepare coffee brews result in a great
diversity of the final product, the cup of coffee. A remarkable difference
between coffee and all the other beverages is indeed the extraordinary
variety of brewing techniques that have been developed and used
traditionally in different countries: decoction methods (boiled coffee,
Turkish coffee, percolator coffee and vacuum coffee), infusion methods
(drip filter coffee and Napoletana), and the original Italian pressure
methods (Moka and espresso) (Petracco, 2001).
In Europe, the highest per capita consumption is found in the Nordic
countries (Figure 10.3), where mainly light-roasted wet-processed arabica
354 Espresso Coffee
Coffee consumption and health 355
Table 10.1 Caffeine1 and diterpenes2 content of various coffees
Brew Brewing technique3 Cup size (ml)
Caffeine
(mg/cup)*
Diterpenes
(mg/cup)
Boiled Coarsely ground roasted arabica coffee (50–70 g/l) is boiled in water 150–190 75–135 Very high (13)
Filter Boiling water is poured over finely ground light to dark roasted coffee
(30–80 g/l) in a paper filter or automatic drip machine
150–190 110–200 Negligible
Percolated Boiling water is re-circulated through coarsely ground light to medium
roasted coffee (30–60 g/l)
150–190 100–190 Not available
Plunger Hot water (875 ml) is poured onto coffee (50 g); brew and spent
grounds are separated by pushing down a perforated plunger
150–190 90–180 High
Instant Instant coffee is dissolved in hot water 150–190 45–180 Negligible
Moka Just overheated water is forced through very finely ground medium to
dark roasted coffee (5–8 g/cup)
40–50 50–190 Low (2)
Espresso Very finely ground medium to dark roasted mainly arabica coffee is
extracted with hot water under pressure (see Chapter 8)
25–35 60–80
(130)
Low (2)
Middle-
Eastern
Extremely fine ground medium to dark roasted coffee (4–6 g) in water
(50–60 ml) and sugar (5–10 g) is brought to a gentle boil
30–50 35–60 Very high (13)
Decaffeinated All brewing techniques Depending on
brewing technique
1–5 Depending on
brewing technique
*Caffeine content per cup can be estimated from:
Blend (arabica 1.3%, robusta 2.4%)
Ratio coffee grounds/brewing water (lowest in espresso and Middle-Eastern coffees)
Extraction time (shortest in espresso and Moka)
Physical separation of grounds (lacking in Middle-Eastern and boiled coffees)
It varies between 100% for soluble coffee, 90–98% for filter and percolated coffees, down to 70–80% for espresso.
Sources: 1Petracco, 1989; Peters, 1991. 2Gross et al., 1997; Urgert and Katan, 1997. 3D’Amicis and Viani, 1993; Petracco, 2001
coffee is consumed. Both robusta content in the blend and roasting degree
increase going southwest: medium in Germany, the Netherlands and
Austria, very dark in France, Spain and Portugal, darker in Italy.
East European countries drink mainly robusta, while countries with a
tea tradition, the UK, Ireland, the Russian Federation, Asian and South
356 Espresso Coffee
Figure 10.2 Major coffee producing countries in 2001 (ICO, 2003)
Figure 10.3 Major coffee consuming countries in 2000 (ICO, 2003)
Pacific countries, consume mostly soluble coffee. In the USA, where
brewing of thinner and thinner cups of poor quality blends had drastically
reduced consumption, the introduction of ‘gourmet’ blends, and fancy
preparations such as espresso and cappuccino, have lately begun to reverse
consumption trends.
10.2 COFFEE IS MORE THAN CAFFEINE
Not all is yet known with certainty about the physiological aspects of
coffee, but a bewildering array of health and mood effects are attributed to
it. For the reasons mentioned above, any figure quoted in quantitative
assessments of coffee consumption must be taken with due caution,
notwithstanding the plethora of epidemiological studies on the effect of
coffee that have been published in the past 20 years (Debry, 1994), whose
conclusions did not always lead to clear-cut answers.
Over 90% of the research carried out on the physiological properties of
coffee has been devoted to caffeine, an alkaloid that stimulates the central
nervous system (Stavric, 1992). Caffeine, the most used pharmacologically
active substance, is also present in other common beverages like tea,
colas and chocolate, besides both prescription and non-prescription drugs
(Spiller, 1998; Brice and Smith, 2002a). Its daily consumption by the vast
majority of the human population, while a generic habit, has never been
substantiated as imputable to any addictive properties (Nehlig, 1999),
even if, when caffeine intake is stopped abruptly, some individuals can
experience temporary headache, fatigue or drowsiness. In any case, coffee
is much more than caffeine. Its complex composition and the presence
of other substances as yet unidentified (see 10.3.1), but with evident
physiological effects, indicate that further research is needed to
demonstrate both the wholesomeness of coffee consumption as well as
the favourable effects this beverage can have in humans.
10.3 COFFEE IS BENEFICIAL TO HEALTH
Fruits and vegetables are often highlighted for their role in the
prevention, or delay, of chronic human diseases by protective mechanisms
that are not yet fully elucidated, albeit many plausible ones have been
proposed and are currently under investigation. In this context, coffee
appears to be a very interesting food plant, with a long history of use. In
contrast to most other traditional foods, it has been the subject of
extensive scientific research addressing its potential impact on human
health (Gray, 1998).
Coffee consumption and health 357
This section will review information extracted from the large body of
literature on coffee and health, giving evidence to support a direct link
between coffee intake and positive health effects. Human epidemiology
strongly suggests the possibility of coffee-mediated protective actions on
physiology, whose putative mechanisms are proposed in the light of recent
research conducted in experimental in vivo and in vitro models (Schilter et
al., 2001). Although further scientific confirmation is clearly needed, it
can be argued that, just as for other food plants, available data on the
health effects of coffee are compatible with a contribution of this common
beverage to a wholesome and balanced diet.
10.3.1 Effects generically linked with coffee
consumption
The quantity of coffee consumed is one of the questions commonly
considered in epidemiological and clinical studies and as a consequence a
mass of data has been generated (but see also 10.1). The presence in the
brew of various physiologically active compounds – mutagenic compounds
formed in trace amounts during roasting (hydrogen peroxide,
glyoxal, etc.) on the one hand and antioxidants like chlorogenic acids and
melanoidins on the other – is sometimes used to try and correlate the
findings of the epidemiological approach. Interesting effects of coffee
consumption have been reported on behaviour and brain activity, on
metabolic activity, on chemoprotection and related prevention of cancer
and of degenerative diseases, on protection of the digestive system and on
the alleviation of various miscellaneous symptoms.
10.3.1.1 Behaviour and brain activity
While a very large body of evidence shows the effects of coffee
consumption on behaviour, most of the experimental studies deal with
placebo-controlled administration of pure caffeine, and will be discussed
in 10.3.2.1.
A few researchers have investigated the activity of coffee beverages
per se on improving alertness and performance during the day and night
(Smith et al., 1993) and on the speeding of information processing in
adults (Ba¨ttig and Buzzi, 1986; Van Boxtel and Jolles, 1999) and older
people (Riedel et al., 1995a; Johnson-Kozlow et al., 2001). There is a
strong inverse correlation with tendency to suicide (Kawachi et al., 1996),
even though an increased risk has been observed among heavy coffee
drinkers (Tanskanen et al., 2000). An exciting possibility has been
358 Espresso Coffee
proposed by some scientists, who have observed a preventive activity of
coffee on depression and against addiction to heavy drugs (Boublick et al.,
1983; Wynne et al., 1987; Knapp et al., 2001).
10.3.1.2 Metabolic activity
Coffee acts on metabolism in different ways: it has long been used to
enhance endurance during exercise and has been shown to display a mild
anti-obesity activity. In sport, the ethical aspects may range from reducing
the athlete’s freedom of diet choice by limiting intake of cups of coffee, to
whole question of doping (IOC, 2000), recently solved by the elimination
of caffeine from the list of banned substances. The studies dealing with the
effects of caffeine on athletic performance will be discussed in 10.3.2.2.
A mild anti-obesity action may be ascribed to caffeine and explained by
its thermogenic effect, i.e. an increased expenditure of energy at doses
associated with moderate coffee consumption (Costill et al., 1978;
Acheson et al., 1980). Furthermore, coffee has been recently shown to
abate uricaemia, therefore protecting from gout (Kiyohara et al., 1999).
10.3.1.3 Chemoprotective activity
By chemoprotection we imply all phenomena linked with the capacity of
coffee to protect against the onset or aggravation of neoplastic (cancer) or
degenerative pathologies (Parkinson’s and Alzheimer’s).
Recent research suggests that coffee contains anti-toxic components,
displays anti-oxidant activity and may act as an anti-inflammatory. While
some of these studies are actually focused on caffeine (Devasagayam et al.,
1996; Varani et al., 1999, 2001; Foukas et al., 2002; Lu et al., 2002), much
interest is raised by the observation that several coffee components, other
than caffeine, display protective activity in vitro and in vivo.
From a biochemical point of view, several studies have investigated the
antioxidant activity of coffee, scavenging free radicals and protecting
against their neoplastic or degenerative action (Singhara et al., 1998;
Daglia et al., 2002; Natella et al., 2002). Special attention has been paid
to coffee polyphenols (Kato et al., 1991) and to other classes of
compounds present in coffee, like melanoidins (Del Castillo et al.,
2002; Morales and Babbel, 2002), or in tea (Hertog et al., 1993), or in
both (Lekse et al., 2001; Richelle et al., 2001). Recently, a new coffee
compound with activity on the detoxifying system has been identified as
methyl pyridinium (Somoza et al., 2003).
Other authors have studied the anti-genotoxic effects of coffee
(Abraham, 1996; Abraham and Singh, 1999) and of coffee diterpenes
Coffee consumption and health 359
(Kono, 1994; Huggett and Schilter, 1995; Poikolainen and Vartiainen,
1997; Honjo et al., 1999; Huber et al., 2002), where the finding that coffee
consumption enhances the level of the protective system of glutathione
and its transferase is particularly interesting (Scharf et al., 2001; Esposito
et al., 2003).
Much attention has been devoted to cancer onset, an area where the
protective action of coffee consumption may be inferred from statistical
data. Particularly evident is the case of colorectal cancer, where a large
body of evidence on its protective effect has now been collected. The
identified, or at least putative, effects of coffee on prevention of human
carcinogenesis are listed in Table 10.2.
Coffee consumption does not emerge as evidently linked with cancer;
however, a few cancer locations might still be adversely linked with coffee
intake. The weak association found with urinary tract and pancreatic
cancers might be due to residual confounding factors (Tavani and La
Vecchia, 2000).
Degenerative diseases like Parkinson’s and Alzheimer’s mainly affect
the elderly population, and regular coffee consumption may play a
preventive role, thus reducing their social burden. Parkinson’s disease has
recently been studied in some detail with respect to its correlation with
coffee, and preliminary conclusions indicate a neuroprotective effect of
caffeine (and nicotine) intake (Benedetti et al., 2000; Honig, 2000; Ross
et al., 2000; Ascherio et al., 2001; Ross and Petrovitch, 2001; Checkoway
et al., 2002; Herna´n et al., 2002; Ragonese et al., 2003), with proposed
models of action (Chen et al., 2001; Schwarzchild et al., 2002).
Alzheimer’s disease has also been the object of recent attention
(Lindsay et al., 2002), with studies on its relationship with caffeine
(Maia and de Mendonc¸a, 2002). The potential role of caffeine and
adenosine has been stressed in reducing inflammation (Montesinos,
2000; Dall’Igna et al., 2003), which may be beneficial in atherosclerosis
(Libby, 2002).
10.3.1.4 Digestive tract
In spite of popular belief in some countries that in some way coffee upsets
digestion, there is evidence that in reality its regular consumption may act
in a beneficial manner on various organs linked with food assimilation.
Updated research shows its helpful role on the liver, where the risk of
hepatic cirrhosis is reduced with increased quantity and length of coffee
consumption, while other caffeinated beverages have no effect (Klatsky
and Armstrong, 1992; Corrao et al., 1994, 2001; Gallus et al., 2002a;
Tverdal and Skurtveit, 2003). Moreover, it has been reported that coffee
360 Espresso Coffee
Coffee consumption and health 361
Table 10.2 Cancer protection by coffee
Cancer Findings/explanation Comments Reference
Bladder Small increased risk in heavy drinkers, but
unrelated with dose or length of exposure
Residual confounding by smoking? Sala et al., 2000;
Zeeger et al., 2001
Breast No link in females; protective effect in males
attributed to polyphenols
Johnson et al., 2002;
Michels et al., 2002
Colon and
rectum
Reduced risk/inhibition of bile acids by
diterpenes
Urgert and Katan, 1997;
Favero et al., 1998;
Antimutagenic compounds in coffee Giovannucci, 1998
Increased colon motility
Avoidance of coffee by high-risk individuals
Liver Reduced risk Gallus et al., 2002b;
Yagasaki et al., 2002
Lung Reduced risk in women Kubik et al., 2001
Oesophagus No or reduced risk Inoue et al., 1998
Ovary Inadequate evidence or no association Tavani et al., 2001
Pancreas Reduced risk at low doses; possible increased
risk at high doses for males
Confounding factors? Nishi et al., 1996;
Lin et al., 2002
Prostate No association or protection due to boron
content of coffee
Cui et al., 2004
Testicle Correlation with coffee and pork
consumption attributed to ochratoxin A
(OTA) contamination of coffee
Lack of correlation observed with cereal intake (the
major source of OTA) would exclude the link between
OTA and testicular cancer
Schwartz, 2002
Thyroid No association Mack et al., 2002
decreases the level of gamma-glutamyltransferase (GGT), an enzyme that
is a marker of liver damage induced by alcohol abuse (Sharp and
Benowitz, 1990; Casiglia et al., 1993). Unfortunately, an explanation for
both effects and the identity of the active compound are still missing.
Coffee consumption has been linked to a reduction of the risk of stone
formation in the gallbladder (Leitzmann et al., 1999, 2002) and in the
kidneys (Curhan et al., 1996, 1998): all these associations might,
however, be explained by a reduction of coffee drinking in subjects
with symptoms related to the pathology.
Furthermore, coffee seems to act on sugar metabolism, reducing the risk
of developing diabetes (Van Dam and Feskens, 2002; Rosengren et al.,
2004; Salazar-Martinez et al., 2004), which could be explained by the fact
that caffeine enhances patients’ perception of hypoglycaemia, helping
them to react accordingly (Kerr et al., 1993; Debrah et al., 1996; Watson
et al., 2000; Keijzers et al., 2002). Recently, the quinic fraction of coffee
has also been studied for its effects on glucose metabolism (Johnson et al.,
2003; Shearer et al., 2003). Lastly, there is even evidence of a preventive
action on dental caries (Daglia et al., 2002).
10.3.2 Effects associated with caffeine consumption
The amount of caffeine present in a cup depends on the type of coffee
used, with wide variation between arabica and robusta (see Table 2.5),
and on its mode of processing and preparation, varying between 1–5 mg
for decaffeinated coffee, up to 200 mg for filter coffee prepared from a
robusta blend. The amount of caffeine in a cup of espresso, which is
usually made from pure arabica or from a blend composed mainly of
arabica, is low, no more than 80 mg, usually less (see Table 10.1).
Within minutes after ingestion, caffeine is absorbed and distributed
throughout the body tissues. It is excreted, mostly metabolized, with a
half-life of 2–6 hours in healthy adults. Half-life increases in pregnant
women and in subjects with an impaired liver function, and is shortened
in smokers. Consumption of caffeine delays falling asleep and reduces
sleep duration.
10.3.2.1 Behaviour and brain activity
Caffeine has been known long since to modify human reactions, helping
to cope with awkward situations (Liebermann, 1987). Agreed beneficial
effects on behaviour associated with moderate caffeine intake are an
increase in alertness and reduction of fatigue, an improvement in
performance on vigilance tasks and tasks requiring sustained response
362 Espresso Coffee
(Smith and Brice, 2000; Smith, 2002), and on mood (Brice and Smith,
2002b). The area of prevention of driving accidents, where caffeine might
bring considerable alleviation, has been particularly investigated (Horne
and Reiner, 1996, 1999; Reyner and Horne, 1997, 1998, 2000; Brice and
Smith, 2001; De Valck and Cluydts, 2001).
Complex novel molecular mechanisms have been recently proposed to
explain the effect of caffeine on brain functions (Acquas et al., 2002;
Lindskog et al., 2002). The hypothesis that caffeine enhances short-term,
possibly also long-term memory, has been advanced by several authors
(Di Chiara et al., 2001; Nguyen Van Tam and Smith, 2001), and a
mechanism of action has been proposed (Riedel et al., 1995b).
While there is nowadays a large literature on the beneficial effects of
caffeinated coffee on performance and mood, a debate has started as to
whether the effects of caffeine merely reflect the removal of the negative
effects of caffeine withdrawal (James, 1994), or if they correspond to a real
benefit (Smith, 1994).
10.3.2.2 Metabolic activity
At doses associated with moderate coffee consumption, caffeine ingestion
has been found to increase energy expenditure (Hollands et al., 1981;
Astrup et al., 1990), which is confirmed in both younger and older women
(Arciero et al., 2000). The use of caffeine as a treatment for obesity
therefore appears very inviting, and has been much investigated
(Acheson et al., 1980; Dulloo et al., 1989; Daly et al., 1993; Yoshida et
al., 1994; Caraco et al., 1995). Unfortunately, thermogenesis and lipid
oxidation are less stimulated in obese than in lean subjects (Bracco et al.,
1995).
Caffeine enhances athletic performance in several sport disciplines
(MacIntosh and Wright, 1995; Pasman et al., 1995; Jackman et al., 1996;
Ferrauti et al., 1997); it has been found to increase the maximal anaerobic
power and to bring the effects of fatigue down, both by affecting byproducts
metabolism (Anselme et al., 1992) and by lowering its
perception threshold (Cole et al., 1996). Ergogenic effects of caffeine,
which occur at doses far lower than the limit accepted by the
International Olympic Committee Medical Code (12 mg/l of urine)
(IOC, 2000), cannot be satisfactorily explained just by thermogenesis and
lipid oxidation (Graham, 2001).
Another property of caffeine is its mild diuretic effect, which, however,
at moderate doses does not produce dehydration and fluid-electrolyte
imbalance during physical exercise (Stookey, 1999; Armstrong, 2002).
Further, caffeine appears to influence the resynchronization of hormonal
Coffee consumption and health 363
rhythms after rapid transmeridian travel (jet-lag), probably through a
modulation of melatonin activity (Hartter et al., 2003; Beaumont et al.,
2004).
10.3.2.3 Alleviation of symptoms
Various diseases have been investigated, both from a statistical and a
clinical standpoint, with regard to their link with coffee consumption. For
some there are hints that coffee, or caffeine, can alleviate the
symptomatic framework.
Caffeine has long since been known as a bronchodilator (mentioned by
Marcel Proust in one of his literary works early in the last century). In the
recent past, this simple treatment for soothing bronchial asthma
symptoms has again been given consideration (Pagano et al., 1988;
Kivity et al., 1990; Schwartz and Weiss, 1992; Bara and Barley, 2003;
Henderson et al., 1993).
The problem of hyperactive children (Schnackenberg, 1973) has seen
recent developments proposing caffeine, or coffee, treatment to help in
relaxing hyperkinetic behaviour in school age children (Stein et al., 1996;
Castellanos and Rapoport, 2002). Some reported side-effects of caffeine
administration could prove useful in clinical practice, such as an
inhibitory action on anaphylactic shock (Shin et al., 2000), enhanced
recovery after surgical intervention (Weber et al., 1997), reduction of
hypnotic effects of alcohol (El Yacoubi et al., 2003) and even a certain
amount of radioprotection (George et al., 1999).
Finally, a novel hope has been raised by the suggestion of a role for
caffeine as an adenosine receptor antagonist, to minimize manifestations
of Huntington’s chorea (Varani et al., 2001).
10.3.3 Effects attributed to specific constituents
other than caffeine
The coffee diterpene cafestol has been shown to raise serum total
cholesterol (Weusten-van der Wouw et al., 1994; Urgert and Katan, 1997;
De Roos et al., 2001); this reversible effect has been observed in subjects
consuming large amounts of the unfiltered beverage popular in Nordic
countries, the so-called ‘boiled’ coffee. Consumption of moderate
quantities of either espresso or Moka type brews has no effect on total
cholesterol (D’Amicis et al., 1996).
Together with kahweol, cafestol also possesses anticarcinogenic activity
(Cavin et al., 2002), and might play a role in the protective effects of
364 Espresso Coffee
coffee. However, animal data indicate that the level at which the coffee
diterpenes would be expected to exert a chemoprotective effect in
humans may be unrealistically high (Huggett and Schilter, 1995).
The importance as antioxidants of the chlorogenic acids, along with
their quinide derivatives formed on roasting, has lately been proposed
(Natella et al., 2002). Inasmuch as they are absorbed by humans (Olthof
et al., 2001; Nardini et al., 2002), their physiological value lies in possible
inhibition of the human adenosine transporter (Martin et al., 2001; de
Paulis et al., 2002), with consequences on brain functions (de Paulis and
Martin, 2003). Further work in this field would be welcomed.
10.4 COFFEE IS NOT HARMFUL TO HEALTH
Coffee is often attributed with the nature of a pleasant, yet malicious
beverage; this conviction originates perhaps from the darkness of the
beans, which are roasted on a fire – the mephistophelian element. Little
scientific proof is usually forthcoming to substantiate this vague negative
feeling about the beverage, with the finger of guilt most often being
pointed at the most physiologically active constituent in the cup –
caffeine. Nevertheless, the much investigated caffeine emerges from the
scrutiny of medical research as guiltless, particularly with respect to the
charge of being an addictive and procreation-jeopardizing substance.
10.4.1 Caffeine absolved from blames
10.4.1.1 Caffeine is not addictive
The question of a possible dependence on caffeine has been hotly
debated, particularly as it fulfils some of the established criteria for
dependence (APA, 1992; WHO, 1994). The social consequences of
caffeine are unknown and, if any, are certainly negligible when compared
to those of nicotine or alcohol. Caffeine, at doses typically consumed in
the diet, may lead to withdrawal effects and some physical dependence in
adults; the prevalence of such effects being variable, their intensity is
usually very low in most individuals. Caffeine withdrawal symptoms, such
as irritability, sleepiness, increased fatigue and headache, have been
observed in some habitual moderate coffee drinkers. They usually start
within 12–24 h, with a peak after one or two days, lasting for up to one
week (Van Dusseldorp and Katan, 1990; Lane, 1997). These symptoms
are not dose-dependent, i.e., they are unrelated to the quantity of caffeine
Coffee consumption and health 365
ingested. Non-pharmacological factors related to expectation might also
explain them (Dews et al., 2002). Further research is required to examine
whether similar withdrawal effects and physical dependence occur in
children.
In some subjects, the intake of more than 200 mg of caffeine
significantly prolongs sleep latency and shortens sleep duration; this
could be explained by differences in the rate of caffeine metabolism
among individuals. Blood pressure increases, which occur after acute
caffeine administration, show a rapid tolerance development.
Both low (75 mg or lower) and high (200 mg and higher) doses of
caffeine can be discriminated by some subjects against placebo with an
effect on mood. The mild reinforcing effect of caffeine varies with the
dose, with higher doses tending to reduce the choice or frequency of
caffeine self-administration, and very high doses leading to avoidance.
Animal studies confirm this low addictive potential of caffeine:
1 At 1 mg/kg body weight or lower – equivalent to one cup of espresso
– caffeine activates the nuclei involved in locomotion, mood and
sleep, without activating reward circuits in the brain (Nehlig and
Boyet, 2000).
2 Only at aversive doses of 10 mg/kg body weight (equivalent to 10
cups of coffee taken at one time) does caffeine induce a release of
dopamine and produce an increase in glucose utilization in the
nucleus accumbens, the area involved in addiction and reward with
drugs of abuse like amphetamines, cocaine and nicotine.
It may be concluded that ‘although caffeine fulfils some of the criteria for
drug dependence, the relative risk of addiction to caffeine is quite low’
(Nehlig, 1999). The controversy on the status of caffeine is created, rather
than by actual dependence, by the lack of quantitative criteria of abuse
potential and negative health consequences in the current classification
schemes (Fredholm et al., 1999).
10.4.1.2 Caffeine does not affect reproduction
Caffeine has been widely studied in relationship to reproductive
functions, without convincing evidence emerging for an increased risk
of reproductive adversity (Leviton and Cowan, 2002); contradictory
results may be explained by erroneous reporting of caffeine exposure
(Bracken et al., 2002).
Findings of a reduction in conception (infertility) in one study were not
confirmed by larger studies, and could be explained by the confounding
366 Espresso Coffee
effect of alcohol consumption and smoking (Wilcox et al., 1988; Olsen,
1991; Caan et al., 1998). In the issue of miscarriage, again, the small
increased risk of spontaneous abortion observed for women consuming
more than 100–150 mg/day during pregnancy could be due to confounders
(Fenster et al., 1998; Fernandes et al., 1998; Cnattingius et al., 2000; Wen
et al., 2001). A lowered birth weight was found in one study, but no
association in two others (Eskenazi et al., 1999; Grosso et al., 2001;
Clausson et al., 2002). Also, indication of a link between caffeine
consumption during pregnancy and sudden infant death has not been
confirmed by the most recent work (Ford et al., 1998; Alm et al., 1999).
10.4.1.3 Caffeine has minimal impact on other organs
Caffeine in coffee had in the past been seen as responsible for the gastric
discomfort known as ‘heart-burn’; recent work (Boekema et al., 1999,
2001) shows that caffeine has no effect on gastro-oesophageal sphincter
pressure, with the possible exception of patients suffering from reflux
disease, where a reduction in gastric reflux was observed after they
switched to decaffeinated coffee (Pehl et al., 1997).
Caffeine’s implication in coronary heart disease appears to be
unimportant, particularly when confounding factors like diet and
cigarette smoking, which often come in parallel with coffee consumption,
are taken into consideration. This applies to acute myocardial infarction,
where caffeine is unlikely to be a relevant factor even if there is an
increased risk at high consumption (D’Avanzo et al., 1993; La Vecchia et
al., 1993). Also, its effect on hypertension, if any, is minor, and a possible
increase in blood pressure at high consumption, particularly in hypertensive
persons, has been reported (Nurminen et al., 1999; Beilin et al., 2001;
Costa, 2002; Klag et al., 2002). Recently, a role of undefined components
other than caffeine has been attributed to coffee (Corti et al., 2002).
Finally, no influence at moderate doses on cardiac arrhythmias is to be
ascribed to caffeine (Rosmarin, 1989; Myers, 1991).
The contribution of caffeine to osteoporosis is still unclear. Very high
levels of caffeine – more than 450 mg/day – cause significant bone loss in
healthy women when limited to calcium intakes below the recommended
daily dietary allowance (800 mg) (Harris and Dawson-Hughes, 1994).
Such findings are, however, generally not supported by more recent
studies (Franceschi et al., 1996; Loyd et al., 1997, 1998; Huopio et al.,
2000), although acceleration in bone loss at the spine in elderly
postmenopausal women consuming more than 300 mg of caffeine per
day has been recently reported (Rapuri et al., 2001). Losses can be
overcome by daily milk intakes (Barrett-Connor et al., 1994).
Coffee consumption and health 367
10.4.2 Coffee itself does no harm . . . but spoiled
coffee might
The many favourable effects exhibited so far by a moderate and sustained
coffee consumption represent an important body of evidence. However,
this must not desensitize the scientific community to progress in analysing
possible contaminants that might be present in coffee, as in many other
commodities.
10.4.1.2 Contaminants from moulds
Mention has been made in other chapters (see 3.11.11.1 and 4.6.1) of
ochratoxin A (OTA), a fungal metabolite signalling past exposure to
mould attack. Its presence in coffee at levels of a few parts per billion (ng/
g) has long since been reported in the literature as occasional findings
linked with poor processing practices, particularly careless drying, in the
producer countries.
The noxiousness of OTA has been actively monitored: it has been
considered as a possible cause of Balkan nephropathy, an endemic disease
in some rural areas. Since it induces renal tumours in experimental
animals, the International Agency for Research on Cancer (IARC)
classifies it as possibly carcinogenic to humans (Plestina, 1996). However,
the genotoxicity of OTA is still under debate: recent studies have
observed that OTA is unlikely to form reactive intermediates capable of
binding to human DNA, because, unlike in animals, it is not metabolized
by cytochrome P450. A genotoxic carcinogenic effect is therefore
implausible (Zepnik et al., 2001, 2003).
It is therefore understandable that the scientific coffee community is
tackling the issue of OTA prevention in coffee very actively: in this, both
analytical aspects and correct sampling techniques are of great importance.
Destruction of up to 85% of the amount present in green beans has
been shown to occur during processing, most of it at roasting (Blanc et al.,
1998). OTA exposure from coffee consumption is estimated at around
9%, behind cereals (44%) and wine (10%), and followed by beer (7%)
(Miraglia and Brera, 2002).
10.4.2.2 Other contaminants
Another unpleasant travel companion of coffee is acrylamide (AA), a
compound recently discovered in various baked vegetal foods (Tareke et
al., 2002). Whilst initial reports on AA in food focused on cooked starch
products such as potato and cereal products, AA has also been detected
368 Espresso Coffee
at low levels in a number of other foods, including coffee. On the basis
of animal data, AA is considered to be a probable human carcinogen
(IARC, 1994). Nevertheless, there is no scientific evidence for this effect,
especially at the very low levels at which this compound is generally
present in cooked food and even more so in coffee. In the meantime, the
World Health Organization advises consumers that there is no reason to
alter their diets, and stresses that ‘People should eat a balanced and varied
diet, which includes plenty of fruit and vegetables, and should moderate
their consumption of fried and fatty foods’ (FAO/WHO, 2002). Coffee
drinkers can continue to enjoy their favourite beverage in moderate
amounts without health concerns.
10.5 CONCLUSIONS
In the scientific debate on the health aspects of coffee, which has been
conducted since at least the sixteenth century (Jussieu, 1715), the regular
scares about the supposed dangerous effects of coffee have each time been
counteracted by opposite indications. This suggests that the health effects
of coffee are, to say the least, subtle.
At the doses present in a cup of espresso, the main active constituent of
coffee – caffeine – has well-documented mild positive effects, which
themselves explain the reasons for coffee consumption. On the other
hand, the bonus constituted by other proven positive effects of clinical
relevance (such as, for instance, protection against colorectal cancer) has
yet to be perceived by the public. A sustained effort, founded on solid
scientific evidence, is needed to raise consumer awareness that coffee, as
any other nutrient, has a contribution to make to many facets of our wellbeing.
That said, it is doubtful that people will ever base their preference
for a cup of coffee on anything other than the sheer sensory pleasure they
find in it. And why not?
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Coffee consumption and health 383
Closing remarks
E. Illy
This book reminds me of a big tree with its beautiful foliage dancing in
the wind, representing the knowledge selected and made accessible by the
experts, and its invisible but equally imposing root system, constituted by
the broad literature which allows whosoever is interested in a deeper
vision to easily satisfy their curiosity.
This ‘tree of knowledge’ gives the reader a sense of the complexity of all
that concerns coffee; from genetics, to interaction with the environment,
processing of the cherries, to the transformation in the roaster and, finally,
to the delicious cup that delivers such a wonderful message to our senses,
followed by a mild stimulation of our brain and activation of the centres
of alertness, relaxation and well being, helping us to live better and even
possibly to live longer. This complexity is one of coffee’s major attractions
and the source of an interest that can sometimes evolve into a real love
for the product and the culture that surrounds its consumption.
The authors, the editors, all the people involved in the realization of
this work, have a common feeling. They love coffee and hope that their
efforts will contribute to a deeper understanding of this extraordinary
product and a profound respect for all those involved in the production,
industrialization and, finally, the preparation of a perfect cup of coffee –
the consumption of which is an unforgettable experience.
Meanwhile, the tree will continue to grow, and already we look forward
to engaging your attention with a future edition.
Index
Page numbers in italic refer to Tables and Figures.
Ababuna cultivar, 32
Absolute tests, 308
Acetic acid, 201
Acid flavour, 140
Acid phosphatase, 154
Acidity, 54, 302–3, 339
espresso brew, 297
Acrylamide, 368
Actinomycetes spp., 133
Active packaging, 250–2
Addiction to coffee, 365–6
Adsorption isotherms of roasted coffee,
239, 240
Aflatoxin B, 163
Africa
agronomic practices, 45
classification of coffee, 135
coffee berry disease, 39
coffee cultivars, 25, 26
coffee growing, 21–2
coffee production, 24
peasy beans, 133
shade trees, 36
see also Ethiopia; Kenya; Tanzania;
Uganda
Afrocoffea, 25
After-flavour, 341
After-taste, 303–4
Ageing, 230
‘Agobiado’ pruning, 35
Agobio cultivation, 44
Agronomy, 34–54
Africa, 45–7
Brazil, 41–3
Central America, 43–5
climate and soil, 34–5
diseases and pests, 39–40
green bean quality, 49–54
harvesting and yields, 40–1
India, 47–9
propagation and crop husbandry,
35–9
Air packaging, 247
Albizzia spp., 36
Alkaloid content, effect of roasting, 197
Aluminium, as packaging material, 246
Alzheimer’s disease, coffee in prevention
of, 360
Amino acids
changes during roasting, 193, 195, 197
content
arabica coffee, 52, 53
green beans, 153–5
green coffee, 153–5
robusta coffee, 53
Amplified fragment length
polymorphism, 70
Amylacetate, 332
Analytical methods
aroma extract dilution, 200
physical, 134–41
quantitative descriptive analysis, 336
see also Sensory analysis
Antestiopsis orbitalis ghesquierei, 134
Anthesis, 63, 64, 65
Anthocyanins, 66
Antioxidants, 207–8, 234, 358, 365
Apoata cultivar, 34
Appearance, 138
espresso brew, 339
green beans, 88, 109, 118, 122, 124
silverskin, 139
Arabica coffee, 21
acidity and aroma, 140
amino acid content, 52, 53
bean mass, 148
beverage analysis, 292
body, 306
caffeine content, 155, 298
characteristics, 53
climate and soil, 34
composition, 149, 194
acids, 54
carbohydrates, 151
carboxylic acids, 153
diterpenes, 158, 159
fatty acids, 157
minerals, 150
oils, 53
sterols, 161
Arabica coffee (continued)
disease resistance, 32
disease susceptible, 31
immature beans, 127
material data, 182
raw bean composition, 149
testing, 114
Arabinogalactan, 64, 152, 194
Arabusta coffee, 34
carboxylic acid content, 153
Arachidic acid, 162
Ardido beans see sour beans
Aroma, 54, 140, 316, 340
compounds, 200, 201
aldehydes and ketones, 241
phenols, 241
pyrazines, 200
pyridines, 197
pyrroles, 197
espresso brew, 298–300, 304
and glucose content, 151
Maillard reaction products, 233
robusta coffee, 198
volatile aroma compounds, 197–203
effects on cup impression, 200,
202–3
generation of roast aroma, 198
identification and characterization,
199–200
precursors of, 198–9
see also Odour
Aroma extract dilution analysis, 200
Aroma impact compounds, 201
Aroma index, 255
Aroma profiling, 308
Arome, 304
Arrhenius equation, 282
Ash content, 150
espresso brew, 292, 298
Ashy flavour, 128
Aspergillus spp., 112, 128, 129, 131,
133
Aspergillus carbonarius, 112, 163
Aspergillus flavus, 112, 133
Aspergillus fumigatus, 112, 131
Aspergillus niger, 112, 133, 163
Aspergillus ochraceus, 112, 133, 163
Aspergillus tamarii, 112, 133
Association Scientifique Internationale
du Cafe´, (ASIC), 163
Astringency, 306, 341
Atractylglycosides, 152
Attitudinal qualities, 2
Australia, coffee consumption, 356
Autotoxicity of caffeine, 59, 62, 63
5-avenasterol, 160
Bacchi’s type graphs, 110
Bacillus brevis, 129
Baracoffea, 25
Barista, 266, 271
Bayleton, 48
Bean colour/texture defects, 122–3
Bean shape
classification by, 135–6
defects in, 120–1
Bean size, 135–6
Beaters, 102
Beet army worm (Spodoptera exigua), 56
Behaviour, effects of coffee on, 358–9,
362–3
Behenic acid, 162
Bernoulli’s percolation, 260
Bernoulli’s piezometric energy, 17, 262
Big bags, 109
Biochemical ecology, 55–67
Bitterness, 304, 339
Black beans, 122, 128
Black rot (Koleroga noxia), 39, 49
Black-green/dark-green immature beans,
122, 127–8
Blending, 141–2
Blue Mountain cultivar, 31, 46
Body, 18, 149, 342
Bordeaux mixture, 48
Bourbon cultivar, 31
Bowl roaster, 186
BP series cultivars, 34
BR series cultivars, 34
Brain activity, effects of coffee on,
358–9, 362–3
Brazil
agronomy, 41–3
arabica cultivars, 31, 32
classification, 134, 141
coffee production, 24, 356
Brazilian Official Classification Table,
138
Breaking load, 216
Brewing, 17–18
rapid, 18–19
temperature, 264
Bruleries, 13
Bulk transport, 114–15
2,3-butanedione, 201
C R cultivar, 34
Cafe´ au lait, 312
Cafe´ cre`me, 312
Cafestal, 195
386 Index
Cafestol, 156, 157, 158, 364
effects of roasting, 195
Caffeic acid, 130, 133, 154, 196
Caffeicins, 196
Caffeine, 52, 56, 57, 155, 156, 265, 357
autotoxicity, 59, 62, 63
biosynthesis, 62
content of coffee, 355
content of leaves, 62
as defence compound, 63
in espresso brew, 298, 299
extraction yield, 265
fixation in coffee bean, 59
organism-related effects, 58
residue, 148
Caffeoyl acid, 153
5-caffeoylquinic acid, 57, 62, 126
see also Chlorogenic acids
Cahveh, 21
Cake, 18
definition of, 260–1
grinding force exerted against, 221
moistening of, 285–6
percolation, 270
porosity, 277–8
shape, 278–9
Canada, coffee consumption, 356
Cappuccino, 13, 273, 312
consumption of, 357
Caracoles, 103, 135
Caramelization, 193, 197, 198
Carbohydrates, 150, 151–2
content
arabica coffee, 151
espresso brew, 297
green coffee, 150, 151–2
robusta coffee, 151
effects of roasting, 192–3, 194
Carbon dioxide
escape during roasting, 181, 192, 193,
243
release during storage, 231, 232–3,
236, 237
supercritical, 148
decaffeination using, 143, 145, 146
Carbon dioxide scavengers, 251
Carbonic acid 5-hydroxytryptamides,
162
Carboxyatractyloglycosides, 194
Carboxylic acid content
arabica coffee, 153
arabusta coffee, 153
excelsa coffee, 153
green coffee, 152, 153
liberica coffee, 153
robusta coffee, 153
stenophylla coffee, 153
Casuarina spp., 36
Catadors, 103, 104
Catechins, 63
Category scale, 334
Category-ratio scale, 334
Catimor cultivar, 30, 32, 48
oil content, 53
Catuai cultivar, 31, 43
Caturra cultivar, 30, 31, 43
oil content, 53
soluble solids, 100
Cauvery, 48
Central America
agronomy, 43–5
arabica cultivars, 31
Central Coffee Research Institute, 48
Ceratitis capitata, 51, 130
Cereja descascado process, 94–5
Chatos, 135
Chemical changes during roasting, 192,
193–7
alkaloids, 197
carbohydrates, 192–3, 194
chlorogenic acids, 195, 196
minerals, 197
non-volatile lipids, 194–5
proteins, peptides and amino acids,
195
Chemical defence, 56–8
Chemoprotective effect of coffee,
359–60, 361
Cherries (coffee fruit), 64, 65, 66, 91
cleaning, 102
harvesting, 87, 88
hulling, 102
proportion of husk to green coffee,
103
storage, 101, 163
China, coffee consumption, 356
Chlorine taste, 280
Chlorogenic acids, 52, 56, 57, 62, 67,
133, 208
as antioxidants, 358, 365
effects of roasting, 195, 196
green coffee, 152–3, 154
Cholesterol, 160
Citric acid, 152, 153
Classification, 134, 135–41
appearance and humidity, 138
by colour, 138–9
by cup quality, 139, 140–1
by processing method, 139
by roast, 139
Index 387
Classification (continued)
by screen, 135–6
by species and varieties, 135
by type, 136, 137–8
Cleaning, 102–3
Climate, 34–5
Coffea, 25
Coffea arabica L., 21, 30, 31, 32–3
genome, 68–9
see also Arabica coffee
Coffea arabica var. arabica, 22
Coffea arabica var. bourbon, 22
Coffea canephora, 21
genome, 68–9
see also Robusta coffee
Coffea liberica, 21
see also Excelsa or Liberica coffee
Coffee bags, 109
big bags, 109
jute/sisal bags, 113
Coffee berry borer
Hypothenemus caffeicola, 59
Hypothenemus hampei, 40, 49, 120
Coffee berry disease, 30, 39
Coffee bricks, 248
Coffee cake see Cake
Coffee consumption patterns, 10–14,
353–4, 355, 356–7
Coffee fruit see Cherries
Coffee grinders, 215, 218–19
gap grinders, 219, 220
homogenisation, 221
impact grinders, 219
notch, 221, 226
Coffee leaf rust (Hemileia vastatrix), 22,
30, 39, 47
Coffee oil see Oil
Coffee plant, 21–86
agronomy, 34–54
biochemical ecology, 55–67
dimorphic branching, 26
diseases and pests, 39–40
growth, 25, 26, 27, 28–9
bud to leaf, 60–3
flower to fruit, 25, 26, 27, 28–9,
63–7
seed to plantlet, 58–60
molecular genetics, 67–75
origin and geographic distribution,
21–2
variety development, 29–34
Coffee production, 22, 23, 24, 356
Coffee wax, 161, 162
Colletotrichum kahawae (coffeanum), 39,
128
Colombia
arabica cultivars, 32
classification, 134, 136, 140
coffee production, 24, 356
Colour, 138–9
Colour sorters, 104, 106
Commercial quality, 3–6
historic evolution, 3–4
quality certification, 6
total quality, 4–6
Comminution, 215, 217, 224
Compacting, 278
Conformity certification, 5, 9
Congusta, 48
Conical cutters, 219, 220
Container shipment, 114–15
Contaminants, 163–6
effect of roasting, 209
from mould, 368
mycotoxins, 163, 164, 165, 209
pesticide residues, 166
polycyclic aromatic hydrocarbons,
165–6, 209
Costa Rica, 31
Cotesia marginiventris, 56
Cotyledons, 60, 61
p-coumaroyl acid, 153, 154
Country Quality System, 6
Crema, 141, 228, 291–2, 339
Crop husbandry, 35–9
fertilization, 37–8
irrigation, 38
organic and sustainable coffee, 38–9
propagation and planting, 35
pruning, 36–7
shade, 35–6
weed control, 37
Crop yields, 40–1
Cross-beater huller, 102
CROSSPY radical, 206
Cultural practices, 51
Cup impression, 200, 202–3
green coffee quality, 202
preparation, 202
roasting, 202
staling, 203
see also Espresso brew
Cup quality, classification by, 139,
140–1
Cupping technique, 307–8, 337, 338–43
after-flavour, 341
appearance, 339
astringency, 341
flavour, 340–1
gustatory sensations, 339
388 Index
olfactory sensations, 340
texture, 342–3
see also Degustation characteristics
Customer satisfaction, 5
Cutters, 219, 220
distance between, 225–6
Cyanobacteria spp., 133
b-damascenone, 200, 201
Darcy’s law, 267
Decaffeination, 142–8
caffeine residue, 148
capacity, 148, 149
conventional, 143, 144, 145
developments in, 147–8
direct solvent, 144
indirect adsorbent, 146, 147
indirect solvent, 144, 145
modern, 145, 146
organoleptic comparison, 146–7
regulatory aspects, 148
solvent residue, 148
Defects, 116–34
bean colour/texture, 122–3, 125
bean shape, 120–1, 125
black beans, 122, 128–9
black-green/dark-green immature
beans, 122, 127–8
characterization of, 117–19
earthy beans, 133–4
equivalency ratings, 137
foreign matter, 118, 119, 125
fruit parts, 119, 125
light-green immature beans, 122,
126–7
mouldy beans, 132, 133
odour thresholds, 117
off-tastes, 118, 124, 125
peasy bean, 134
Rioy beans, 113, 124, 130–1, 131–2
sour beans, 122, 127, 129
stinker beans, 93, 113, 124, 129–30
white beans, 123, 132
withered beans, 123
Definition of espresso coffee, 16–19,
310–11
Degassing, 228, 236, 237
Degree of excellence, 16
Degustation characteristics, 301–10
after-taste, 303–4
aroma, 304
astringency, 306
flavour, 305–6
mouthfeel, 306
taste, 302–3
Dehydrocafestol, 195
Dehydrokahweol, 195
Demineralizers, 281–2
Density of espresso brew, 293, 295
Derric¸adeiras, 90
Descriptive tests, 335–6
Destoning, 102
Detergents, machine cleaning, 286–7
a-dicarbonyls, 233
Dichlorogenic acids, 126
Dichloromethane, 144
Digestive tract, effects of coffee on, 360,
362, 367
Diketopiperazines, 204
Dimethyldisulphide, 129
Dimethylsulphide, loss on storage, 242
Direct solvent decaffeination, 144
Discoloration of green beans, 109–10
Discrete numerical scale, 334
Discriminative tests, 335
Disease resistant cultivars, 32
Disturbances, 226–7
Diterpenes, 157, 158, 159
content of coffee, 355
effects of roasting, 195
DNA polymorphism, 69–70, 71
Dry fermentation, 93
Dry processing, 92, 139
Drying, 96–101
contamination during, 163–4
mechanical drying, 99
natural versus washed coffee, 99, 100,
101
patio drying
natural coffee, 97–8
pulped natural and washed coffees,
98
Duo tests, 307
Earthy beans, 133–4
EC Council Regulation n. 823/87, 10
EC Regulation n. 852/04, 9
EC Regulation n. 2081/92, 11
EC Regulation n. 2092/91, 11
Economy of quality, 4
Effective quality, 5
EisKaffe, 12
El Salvador, coffee cultivars, 31
Electron spin resonance, 255
Electronic sorting, 104, 105–8
bichromatic, 107
ejection devices, 104–5
feeding systems, 104
infrared, 108
monochromatic, 106
Index 389
Electronic sorting (continued)
optical sorting with lasers, 108
optical system, 104
processing unit, 105, 106
trichromatic, 107
UV fluorescence, 107–8
Emulsification, 265–6
Endocarp, 28, 57, 59, 64, 66, 119
Endosperm, 60, 61, 64, 100
Enterobacteriaceae spp., 134
Environment, 50
Enzymes, 154–5
Equivalency ratings, 137
Erythrina spp., 43
Erythrina indica, 36
ESBAD beverages, 13
Espresso brew, 290–315
definition of, 16–19
degustation characteristics, 301–10
after-taste, 303–4
aroma, 304
astringency, 306
flavour, 305–6
mouthfeel, 306, 341, 342–3
taste, 302–3
foam, 141, 228, 291–2
liquid, 292–8
acids, 297
carbohydrates, 297
density, 293, 295
lipids, 296–7
minerals, 298
nitrogen compounds, 298
refractive index, 295
surface tension, 295–6
total solids, 296
viscosity, 295
quality, 15–16
sensory evaluation, 307–10
cupping practice, 307–8, 337,
338–43
instrumental testing, 308–10
visual characteristics, 301, 302
volatile aroma, 298–300
Espresso machine, 270, 271–4
cleanliness of, 286–7
extraction chamber, 273
heat exchanger, 272–3
pump, 272
water pre-treatment, 273–4
Espresso-milk mixes, 311–13
Ethiopia
arabica cultivars, 31, 32
coffee production, 24, 356
2-ethyl-3, 5-dimethyl-pyrazine, 201
Ethyl-3-methylbutyrate, 197
Ethylacetate, 144
4-ethylguaiacol, 201
European Authority for Food Safety
(EFSA), 10
European Directive 85/374, 9
European Regulation 178/2002/EC
(General food law), 9
Eurotium spp., 112, 129, 133
Excelsa coffee, 21
carboxylic acid content, 153
Expected quality, 4
Fast roasting, 190, 191
Fatty acid esters, 159
Fatty acids, 156–7
Fertility, effects of coffee on, 366–7
Fertilization, 37–8
Feruloyl acid, 153, 154
Fick’s law of diffusion, 264
Filter perforation, 279
Fineness, 221–4, 277
imaging and sensing zone, 222–3
laser diffractometry, 223, 224
particle size, 224
sifting, 222
Fines, 268
Finishing, 220
Finite element method, 217
Fixed drum roaster, 186
Flat cutters, 219, 220
Flats, 135
Flavour, 305–6, 316, 340–1
acid, 140
ashy, 128
espresso brew, 305–6
mouldy, 305
off-flavours, 94, 233, 244
‘onion’, 94
peasy, 198, 305
robusta coffee, 140
stinker, 94, 305
see also taste
Flavour dilution factors, 200
Flexible polymer aluminium multi-ply,
246
Floaters, 91, 97
Fluidized bed roaster, 186
Foam, 141, 228, 291–2, 301, 339
Foam index, 292
Foam stability, 194
Foamability, 195
Foreign matter defects, 118, 119, 125
Foul/dirty beans, 124
Fraction void, 260
390 Index
Fracture mechanics, 216–18, 217, 218
France, coffee consumption, 13–14, 356
Frappuccino, 312
Free fatty acids, 157
Free radicals, 234
Freshness, 17
Friction hullers, 102
Frost damage, 123
Fungibility to use, 4, 5
Furfurylthiol, 199, 200, 201
Fusarium spp., 131, 133
Fusarium concolor, 51
Fusarium wilt disease (Fusarium
xylarioides), 39–40
G-proteins, 319
Gaggia coffee machine, 271–2
Galactomannan, 64, 152, 194
a-galactosidase, 154
Gamma irradiation, 113
Gap grinders, 219, 220
Gas chromatography, 199
Gas chromatography-mass spectrometry,
199
Gas chromatography-olfactometry, 199
Gel filtration chromatography, 205
Gene expression, 72, 73
General food law, 9–10
Genetic transformation, 73–4
Genetically modified organisms, 9
Genetics, 51–4, 53, 54
Genome, 68–9
Geosmin, 117, 133
Geotrichum spp., 133
Germany, coffee consumption, 12
Germination, 56–8, 61
Gimbi cultivar, 31
Glass, as packaging material, 246
Glycosides, 152
Good manufacturing practice, 148
Grading, 103
Greece, coffee consumption, 356
Green coffee (beans), 87–178
appearance, 88, 109, 118, 122, 124
colour/texture, 122–3
shape, 120–1, 135–6
composition, 148, 149–66
amino acids, peptides and proteins,
153–5
ash and minerals, 150
carbohydrates, 150, 151–2
carboxylic acids, 152, 153
chlorogenic acids, 153, 154
contaminants, 163–6
glycosides, 152
lipids, 156–62
minerals, 150
non-protein nitrogen, 155–6
volatile substances, 162
water content, 150
defects in see Defects
discoloration during storage, 109–10
drying, 96–101
harvesting, 87–91
processing, 102
quality, 49–54, 202
cultural practices, 51
environment, 50
genetic aspects, 51–4, 53, 54
size, 135–6
storage, 113–14
Green scale (Coccus viridis), 40
Grevillea spp., 43
Grevillea robusta, 36
Grinding, 16, 215–29
coffee grinders, 218–21
gap grinders, 219, 220
homogenisation, 221
impact grinders, 219
fracture mechanics theory, 216–18,
217, 218
ground product fineness, 221–4, 224
parameters affecting, 224–7
cutters’ distance, 225–6
disturbances, 226–7
moisture of roasted beans, 225
roasting degree, 225
variability of coffee blend, 225
physicochemical modifications due
to, 227–8
Ground coffee
expansion during percolation, 279
fineness, 221–4, 277
portion size, 275–6
Guaiacol, 198, 199, 201
Guatemala, coffee production, 24, 356
Gustation, 317–25
olfaction, 325–32
cross-cultural differences, 332
number of detectable odours,
331–2
odours, 326–7
olfactory receptor genes, 328,
329–30
physiology, 327–8
transduction and processing, 330–1
6-N-propylthiouracil sensitivity,
324–5
taste buds, 318–20
taste coding, 320
Index 391
Gustation (continued)
tongue map, 324
transduction, 318, 320–4
bitter and sweet tastes, 322–3
salty and acid tastes, 321–2
umami taste, 323–4
Gustatory senses, 339
after-taste, 303–4, 341
taste, 302–3, 340–1
Gustducin, 319
Hand-picking, 89
Haptic senses
astringency, 306
mouthfeel, 306
Harar cultivar, 31
Harvesting, 40–1, 87–91
hand-picking, 89
mechanical, 89, 90
selective picking, 90–1
stripping, 88, 89
Hawaii, coffee cultivars, 31
Hazard Analysis Critical Control Point
(HACCP), 9
Health aspects of coffee consumption,
352–83
consumption patterns, 353–4, 355,
356–7
health benefits, 357–65
alleviation of symptoms, 364
behaviour and brain activity,
358–9, 362–3
chemoprotective activity, 359–60,
361
digestive tract, 360, 362
metabolic activity, 359, 363–4
lack of harmful effects, 365–9
addiction, 365–6
infertility, 366–7
physiological effects, 357
Heat transfer during roasting, 187, 188,
189
Hedonic reactions, 316
Hedonic tests, 336
Hemicelluloses, 64
Cis-4-heptenal, 127–8
High vacuum distillation, 199
High yield roasting, 190, 191
Holocelluloses, 152
Homogenisation, 221
Hooke’s law, 216
Hulling, 102
beaters, 102
catadors, 103, 104
cross-beater huller, 102
friction hullers, 102
Humidity, 96, 138
during container shipment, 115
during storage, 109
Hummingbirds, pollination by, 55
Husk contamination, 164
Hydrostatic pressure, 263
Hygroscopicity, 110, 111
Hypocotyl, 61
Icatu cultivar, 32
oil content, 53
soluble solids, 100
IF series cultivars, 34
Immature beans, 139
black-green/dark-green, 122, 127–8
light-green, 122, 126–7
Impact grinders, 219
India
agronomy, 47–9
arabica cultivars, 31, 32
coffee production, 24, 356
Indirect adsorbent decaffeination, 146,
147
Indirect solvent decaffeination, 144, 145
Indonesia
classification, 135
coffee production, 24, 356
Inert gas packaging, 248
Infusion, 308
Inga spp., 36, 43
Insect pests, 40
Instrumental testing, 308–10
Internally transcribed sequence, 70
International Coffee Organization
(ICO), 343, 354
International Organization for
Standardization see ISO
International Trade Centre, 140
Interspecific hybrids, 33, 34
Invasion, 261
Irrigation, 38
ISO 4149-1980 (Green Coffee)
Olfactory and Visual Examination
and Determination of Foreign
Matter and Defects, 134
ISO 5492 regulations, 8
ISO 9000 regulations, 5
ISO 10470-1993 (Green Coffee) Defect
Reference Chart, 118, 134
3-isobutyl-2-methoxypyrazine, 197
2-isobutyl-3-methoxypyrazine, 201
Isobutylmethoxypyrazine, 198
2-isopropyl-3-methoxypyrazine, 201
2-isopropyl-methoxypyrazine, 134
392 Index
Israel, coffee consumption, 14
Italian espresso, 18–19
Italy, coffee consumption, 12–13, 356
Ivory Coast, coffee production, 24,
356
Jamaica, coffee cultivars, 31
Japan, coffee consumption, 356
Jimma cultivar, 31
K7 cultivar, 31
Kaffeeklatsch, 12
Kahweol, 156, 157, 158, 195, 364
effects of roasting, 195
Kent cultivar, 31
Kenya
agronomy, 45–7
arabica cultivars, 31, 32
classification, 135, 140
Koffie verkeerd, 11
Kona cultivar, 31
Kouilou cultivar, 34
KP423 cultivar, 31
Labdane, 326
Lactobacillus spp., 131
Lamellae, 291, 301
Laser diffractometry, 223, 224
Latte, 13, 312
Laurina cultivar, oil content, 53
Leaf miners (Perileucoptera coffeella), 40
Leucaena spp., 36
Liberica coffee, 21
carboxylic acid content, 153
Liberine, 57
Light, auto-oxidation by, 244–5
Light-green immature beans, 122, 126–7
Lignoceric acid, 162
Limu cultivar, 31
Lipids, 156, 157–62, 294
coffee wax, 162
diterpenes, 157, 158, 159
effects of roasting, 194–5
in espresso brew, 296–7
fatty acids, 156–7
oxidation during storage, 243, 244
sterols, 159, 160, 161
tocopherols, 161
Liquid endosperm, 64
Macchiato, 312
Maillard reaction, 193, 197, 204, 205
Maillard reaction products, effect on
roasted coffee stability, 232–6
Main olfactory epithelium, 316
Malate dehydrogenase, 155
Malic acid, 152, 153
Maragogype cultivar, 30
soluble solids, 100
Mbuni coffee, 46, 47
Measurable qualities, 2
Mechanical drying, 99
Mechanical harvesting, 89, 90
Mechanical sorting, 103–4
Melanoidins, 193, 195, 204–8, 233
chemistry, 204–5, 206–7
molecular weight, 234
oxidation stability of coffee lipids, 235
physiological effects, 207–8
Meloidogyne spp., 40
Meloidogyne exigua, 74
3-mercapto-3-methyl-butylformate, 201
3-mercapto-3-methylbutanol, 201
3-mercapto-3-methylbutylformate, 200
Mercaptopentanone, 201
Mesocarp, 66, 100
Metabolism, effects of coffee on, 359,
363–4
Metallic taste, 126
Methanethiol, 201, 233, 331
loss on storage, 242
Methional, 200, 201
3-methyl-2-buten-1-thiol, 201
2-methyl-3-furanthiol, 201
5-methyl-5H-6,7-dihydrocyclopentapyrazine,
201
2-methylbutanal, 201
2/3-methylbutanoic acid, 201
16-O-methylcafestol, 156, 157, 158
24-methylenecholesterol, 160
2-methylisoborneol, 52, 117, 133
16-O-methylkahweol, 158
2-methylpropanol, 201
loss on storage, 242
Methyltransferases, 62
Mexico, coffee production, 24, 356
Microsatellites, 70
Milk, effect on cup impression, 202
Mineral content, 101, 150
effects of roasting, 197
of espresso brew, 298
Mocas, 103, 135
Mochaccino, 312
Moisture, and staling, 238, 239, 240, 241
Moka, 271, 354
soluble solids, 100
Molecular diversity, 69–70, 71
Molecular genetics, 67–75
DNA polymorphism and molecular
diversity, 69–70, 71
Index 393
Molecular genetics (continued)
expressed genes, 72, 73
genome, 68–9
introduction of new genes, 73–4
Monochlorogenic acids, 126
Mould, 111–12, 115
contaminants from, 368
Mouldy beans, 132, 133
Mouldy flavour, 305
Mouthfeel, 306, 341, 342–3
Mucilage removal, 94
Mundo Novo cultivar, 30, 31
oil content, 53
soluble solids, 100
Mycotoxins, 163, 164, 165, 209
N39 cultivar, 31
Napoletana, 354
Natural (dry) process, 92
Navellier method, 100
Near infrared transflectance, 309
Nematodes, 40
Netherlands, coffee consumption, 10–12
New York Coffee and Sugar Exchange,
136
Nicotinic acid, 207
Non-protein nitrogen, 155–6
caffeine, 155, 156
trigonelline, 155
(E)-2-nonenal, 201
Nordic countries, coffee consumption,
13, 354–5, 356
Notch, 221, 226
Nutritional quality, 7
Nyctinasty, 57
Ochratoxin A, 163, 368
Odour, 326–7, 340
number of detectable odours, 331–2
see also Aroma
Odour activity value, 200
Odour thresholds, 117
Off-flavours, 94, 233, 244
Off-tastes, 118, 124, 125
Oil, 228
content, 53
droplets, 18
size distribution, 294
migration, 244
Ojo de Gallo (Mycena citricolor), 39, 44
Oleic acid, 157
Olfaction, 305–6, 325–32
cross-cultural differences, 332
number of detectable odours, 331–2
odours, 326–7
olfactory receptor genes, 328, 329–30
physiology, 327–8
transduction and processing, 330–1
Olfactory epithelium, 327
Olfactory examination, 136
Olfactory receptor genes, 328, 329–30
Olfactory senses, 340
flavour, 305–6
odour, 304
Olfactory sensory neurons, 327
‘Onion’ flavour, 94
Organic coffee, 38–9
Organic products, 11
Organoleptic characteristics, 140–1
decaffeinated coffee, 146–7
Over-extraction, 265, 301, 302
Oxidation reactions, 240–1
Pacas cultivar, 30, 31, 33, 43
Packaging
control parameters, 252, 253–5
conditioning, 252–3
logistics, 254
testing of finished products, 254–5
green coffee, 109, 113
roasted coffee, 245–55
Packaging materials, 245, 246–7
choice of, 252
Packaging techniques, 247–52
active packaging, 250–2
air packaging, 247
barrier properties of, 249
choice of, 252–3
inert gas packaging, 248
pressurization, 249–50
vacuum packaging, 248
Paraxanthine, 155, 156
Parchment coffee, 57, 59, 64
cleaning, 102
hulling, 102
proportion of husk to green coffee, 103
storage, 101
Parkinson’s disease, coffee in prevention
of, 360
Parra plant architecture, 44
Particle microstructure, 269
Particle size, 224
Particle size distribution, 276–7
Patio drying
natural coffee, 96–8
pulped natural and washed coffees, 98
Peaberries, 103, 135
Peasy beans, 134
Peasy flavour, 198, 305
Penicillium spp., 112, 129, 131
394 Index
2,3-pentanedione, 201
loss on storage, 242
Peptides
effects of roasting, 195
in green coffee, 153–5
Perceived quality, 5
Percolation, 259–89
definition of, 259–60
espresso machine, 270, 271–4
extraction chamber, 273
heat exchanger, 272–3
pump, 272
water pre-treatment, 273–4
modelling of, 266, 267–70
parameters affecting, 274–87
cake moistening, 285–6
cake porosity, 277–8
cake shape, 278–9
ground coffee portion, 275–6
machine cleanliness, 286–7
particle size distribution, 276–7
percolation time, 285
pressure, 283–4
temperature, 282–3
water quality, 280–2
physical description, 261–3
physicochemical description, 263,
264–6
see also Cake; Espresso machine
Pericarp, 65, 66, 67
Perisperm, 64, 65, 102
Peroxidase, 155
Pesticide residues, 166
pH
effects of roasting on, 193
of water, 282
2-phenylacetaldehyde, 201
Phenylalanine ammonia lyase, 62
Pheromones, 316
Phosphoric acid, 153
Physical analysis, 134–41
Physicochemical changes
grinding, 227–8
percolation, 263, 264–6
storage, 230, 231–45
Phytochemicals, 56–8
Phytosanitary certificate, 115
Plantvax, 48
Polishing, 102, 139
Pollination, 55–6
Polycyclic aromatic hydrocarbons,
165–6, 209
Polyphenol oxidase/tyrosinase/catechol
oxidase, 155
Potassium content, 101
Potential quality, 5
Pratylenchus spp., 40
Pre-breaking, 220
Pre-cleaning, 102
Prenylalcohol, 198
Pressure, 18, 262
hydrostatic, 263
of percolation, 283–4
Pressurization packaging, 249–50
Processing, 91–6
classification by, 139
environmental impact, 95–6
natural (dry) process, 92
pulped natural process, 94–5
washed (wet) process, 93, 94, 102
Promised quality, 4–5
Propagation, 35
6-N-propylthiouracil sensitivity, 324–5
Protected designation of origin, 11
Protected geographical indication, 11
Proteins, 153–5
effects of roasting, 195
in green coffee, 153–5
Proust effect, 326
Pruning, 36–7
Pseudomonas spp., 133
Psilanthus, 25
Psychophysical scales, 333–4
Pulped natural process, 94–5
Qawha, 21
Quality, 1–20
commercial, 3–6
cup, 139, 140–1
definition of, 2–3
effective, 5
espresso coffee, 15–16
expected, 4
of food products, 7–10
and general food law, 9–10
green beans, 49–54, 202
origins and meanings, 1–2
nutritional, 7
perceived, 7
potential, 5
promised, 4–5
of water, 280–2
Quality certification, 6
Quality management, 4
Quantitative Descriptive Analysis, 336
Quinic acid, 152, 153,154
Quinides, 196
Random amplified polymorphic DNA,
69
Index 395
Ratio scale, 334
Raw coffee beans see Green coffee
Red Bourbon, 43
Red Catuai, oil content, 53
Refractive index of espresso brew, 295
Restriction fragment length
polymorphisms, 69
Rewetting, 164, 165
Rhizopus spp., 131
Rioy beans, 113, 124, 130–1, 131–2
Ristretto, 13, 299
Roast, classification by, 139
Roasted coffee
adsorption isotherms, 239, 240
effect of light on degradation, 244–5
moisture content, 225
packaging, 245–55
storage
formation of volatile compounds,
241, 242, 243
Maillard reaction products
affecting stability, 232–6
physicochemical changes induced
by, 230, 231–45
Roasting, 179, 180–4, 185–214, 225
bean profile during, 181
chemical changes, 192, 193–7
alkaloids, 197
carbohydrates, 192–3, 194
chlorogenic acids, 195, 196
minerals, 197
non-volatile lipids, 194–5
proteins, peptides and amino acids,
195
contaminants, 209
melanoidins, 204–8
physical changes, 191, 192
techniques, 184, 185, 186–91
bowl, 186
energy balance, 187, 188, 189
fixed drum, 186
fluidized bed, 186
heat transfer, 187, 188, 189
high yield or fast roasting, 190, 191
rotating cylinder, 186
spouted bed, 186
swirling bed, 187
temperature, 183, 184
volatile aroma compounds, 197–203
effects on cup impression, 200,
202–3
generation of roast aroma, 198
identification and characterization,
199–200
precursors of, 198–9
volume expansion, 182
Robusta coffee, 21, 33
amino acid content, 53
aroma, 198
bean mass, 148
beverage analysis, 292
body, 306
caffeine content, 155, 298
characteristics, 53
climate and soil, 34–5
composition, 149
acids, 54
carbohydrates, 151
carboxylic acids, 153
diterpenes, 158
fatty acids, 157
minerals, 150
sterols, 161
flavour, 140
hulling, 102
removal of 2-methylisoborneol, 133
testing, 114
Roller cutters, 219
Rotating cylinder roaster, 186
Rotten fish taste, 127
Ruiru II cultivar, 30, 32, 46
Rume Sudan cultivar, 32
Russia, coffee consumption, 356
S274 cultivar, 34
S795 cultivar, 32
S2828 cultivar, 32
SA series cultivars, 34
Saccaromyces cerevisae, 130
San Ramon cultivar, 30
Sarchimor cultivar, 30, 32
Secondary metabolites, 56–8
Seedlings, 35
Selective picking, 43, 90–1
Sensorial quality, 2, 8
Sensory analysis, 134–41, 307–10,
332–45
cupping technique, 307–8, 337,
338–43
after-flavour, 341
appearance, 339
astringency, 341
flavour, 340–1
gustatory sensations, 339
olfactory sensations, 340
texture, 342–3
instrumental testing, 308–10
psychophysical measurement, 343–4
psychophysical scales, 333–4
statistics, 344, 345
396 Index
stimuli-reaction categorization, 334–6
analytical tests, 335–6
hedonic tests, 336
Service content, 8–9
Shade trees, 36, 43
Shading of coffee plants, 35–6
Sifting, 222
Silverskin, 64, 65, 102, 128, 217
appearance, 139
Simultaneous distillation extraction,
199
b-sitosterol, 160
SL28, 31
SL34, 31
Slurping technique, 337, 340
Smoked-toasted coffee, 352, 353
Soil, 34–5
Solid phase micro extraction, 199
Soluble solids, 100
Solvent residue, 148
Solvent-assisted flavour extraction, 199
Sorting
electronic, 104, 105–8
mechanical, 103–4
Sour beans, 122, 128, 129, 139
Sour taste, 129
Sourness, 339
Spain, coffee consumption, 356
Species, classification by, 135
Spongy beans, 123
Spouted bed roaster, 186
Staling, 203, 232, 236–41
degassing, 236, 237
and moisture content, 238, 239, 240,
241
oxygen uptake, 240–1
temperature, 238
Stearic acid, 157
Steh-kaffee, 12
Stem borers (Xylotrechus quadripes), 40
Stenophylla coffee, carboxylic acid
content, 153
Sterigmatocystine, 163
Sterols, 159, 160, 161
Stinker beans, 93, 113, 124, 129–30
causes of, 130
Stinker flavour, 94, 305
Stipules, 60
Stir bar sorptive extraction, 199
Storage, 101, 108–14, 111, 112, 113–14
coffee bags, 109
contamination during, 163–4
degradation during, 111–12
discoloration during, 109–10
gamma irradiation, 113
green coffee, 113–14
humidity, 109
hygroscopicity, 110, 111
intergranulate space, 110
roasted coffee
formation of volatile compounds,
241, 242, 243
Maillard reaction products
affecting stability, 232–6
physicochemical changes induced
by, 230, 231–45
temperature, 112
Strecker aldehydes, 233
Streptococcus spp., 131
Stripping, 88, 89
Sumatra, 30
Supercritical carbon dioxide, 145, 146
Surface tension of espresso brew, 295–6
Sustainable coffee, 38–9
Swallow breath, 337, 338
Sweepings, 89
Sweetness, 302, 339
Swirling bed roaster, 187
System certification, 5
Tanzania, 31
classification system, 135
Taste, 302–3, 316
acid, 302–3, 321–2, 339
bitter, 304, 322–3, 339
chlorine, 280
metallic, 126
rotten fish, 127
salty, 321–2
sweet, 302, 322–3, 339
unami, 323–4
see also Flavour
Taste buds, 318–20
Taste coding, 320
Taste receptor cells, 318
Taxonomy, 24–5
Tazzulella, 13
Tekesik cultivar, 31
Temperature
brewing, 264
effects on staling, 238
percolation, 285
roasting, 183, 184
storage, 112
Texture, 342–3
Theacrine, 57
Theobromine, 56, 63, 155, 156
Theophylline, 56, 63, 155, 156
Thermoperiod, 50
Tiger skin effect, 301
Index 397
Tinplate, as packaging material, 246
Tocopherols, 160, 161
Tokyo, coffee consumption, 14
Tongue, 319–20
papillae circumvallatae, 320
papillae foliatae, 320
taste buds, 318–20
Tongue map, 324
Total quality, 4–6
Total quality control, 4
Total solids content of espresso brew,
296
Transduction, 318, 320–4, 330–1
bitter and sweet tastes, 322–3
salty and acid tastes, 321–2
umami taste, 323–4
Transport of coffee, 114–15, 116
2,4,6-trichloroanisole, 117, 130–1
Trigeminal system, 327
Trigonelline, 56, 57, 155, 208
effects of roasting, 197, 198
2,3,5-trimethylpyrazine, 201
Trio tests, 307
Tubette seedlings, 42
Tulhas, 101
Typica cultivar, 31
Uganda, coffee production, 24, 356
UK, coffee consumption, 12, 356
Umami taste, 318, 323–4
Under-extraction, 265, 285, 302
USA, coffee consumption, 13, 14, 356
Vacuum packaging, 248
Vanilline, 201
Variability of coffee blend, 225
Variety, classification by, 135
Variety development, 29–34
arabica, 30, 31, 32–3
breeding strategies, 29–30
interspecific hybrids, 33, 34
robusta, 33
Varric¸ao, 89
Vegetative propagation, 35
Velum palatinum, 337
Vietnam, coffee production, 24, 356
Villa Sarchi cultivar, 31
2-vinyl-5-methyl-pyrazine, 201
Vinylguaiacol, 198, 201
Viscosity of espresso brew, 295
Visual analogue scale, 334
Visual characteristics of espresso brew,
301, 302
Volatile aroma compounds, 197–203
effects on cup impression, 200, 202–3
generation of roast aroma, 198
identification and characterization,
199–200
extraction procedures, 199
instrumental sensory analysis,
199–200
precursors of, 198–9
Volatile substances, 162
loss during storage, 241, 242, 243
Vomeronasal system, 316, 327
Wallemia sebi, 112
Warehouses, 108–9
Washed (wet) process, 93, 94
Water
chlorine taste, 280
hardness, 280–1
pH, 282
pre-treatment, 273–4
quality, 280–2
Weed control, 37
Wet fermentation, 93
Wet processing, 93, 94, 102, 139
White beans, 123, 132
White stem borer, 49
Whitening, 109–10
factors affecting, 110
Winnowing, 89
Withered beans, 123
Xylella fastidiosa, 39
Yellow Bourbon, soluble solids, 100
Yirga Chefe cultivar, 31
Young’s modulus, 216
398 Index
