Western and Central Africa
Eastern and Southern Africa
Eastern Europe and Central Asia
Coffee 'quality' is the human sensory experience of the
inherent flavour compounds present in the coffee bean. Processing
and roasting bring chemical change that accentuates some of these
and reduces others. However, the chemical composition of
coffee beans is a scientific subject that is beyond the scope and
objectives of www.thecoffeeguide.org.
This handbook and its Q&A service deal with coffee trade
matters that, of course, include 'quality' but, only in as far as
it impacts on the commercial value of green coffee.
Nevertheless it is of interest to provide a minor introduction
to the subject of coffee's chemical composition and, more
importantly, to indicate where interested parties may learn more
about the huge field of coffee science.
Green coffee beans are the seed contained in coffee
cherries. Three main processes exist to separate bean from
cherry: Natural process = ripe cherries are dried after which the
beans are removed mechanically by hulling; Pulped natural = ripe
cherries are pulped, then dried into parchment followed by hulling;
Wet process = cherries are pulped, the resultant parchment is then
fermented, washed and hulled. For more on coffee processing and
quality see Chapters 11 and 12 of the Guide.
Coffee quality is determined by chemical composition
which itself is influenced by plant variety, altitude, soil
composition, climatic conditions and processing. Ripening
and processing produce chemical change that in turn determines the
final 'quality' which is experienced through human sensory
evaluation of the different flavour components that together form
the taste sensation. Chemical change during green bean processing
is most pronounced in the wet process that tends to accentuate
certain components and reduces others through leaching.
Whilst each of these three processes results in chemical change
in the green bean and in so doing produce a different end result,
it is the roasting process that generates hundreds of aroma and
flavour compounds through a cascade of chemical reactions. The
release of these flavour compounds produces the human sensory
quality experience or 'aroma'.
As the bean temperature rises so the bean expands and literally
begins to crack. The sugars start to undergo a caramelisation
process that produces flavour volatiles including such chemical
compounds as alcohols, furans and enols. As the temperature rises,
further reactions take place between sugars and amino acids, the
so-called Maillard process common to all heated and roasted foods.
Typical chemicals generated by this process are pyrazines and
various sulphur-containing compounds commonly responsible for
roasted aroma notes. Rapid roasting at very high temperatures tends
to accelerate these processes in different ways, leading to varying
proportions of the different aroma compounds. This in turn impacts
on the human sensory perception, both in strength and quality.
If the roasting time and temperature are not properly controlled
then 'over roasting' occurs - the beans literally begin to
burn and many flavour compounds are lost by volatilisation or are
destroyed, leaving a dull and burnt taste experience.
Coffee science plays an important part in the coffee
industry and there is an enormous and constantly expanding
body of published research that deals not only with different
aspects of coffee production, processing and roasting but also with
coffee's inter-action with human health issues.
www.ico.org provides a good
overview of the coffee world generally whereas a helpful
introduction to a number of subjects is easily found at www.coffeechemistry.com
that offers a number of excerpts and quotes from both articles and
publications in the field of coffee chemistry, mostly on the
roasting process however. Access is free by registration.
More in-depth material in the form of a very large number of
research papers, also covering green bean processes, is available
at www.asic-cafe.org, the
web site of the Association for the Science and Information on
Coffee, an association founded in 1963 with the objective of
organising ca. every 2 years an international conference on coffee
science. The lectures and communications presented at the 22 ASIC
conferences have been published in the form of Proceedings,
cumulating today into over 16,000 pages. Paper abstracts are freely
accessible and searchable on ASIC web site. Free access to full
text papers is reserved to ASIC members, while non-members can
purchase them at a nominal € 10 fee per paper. Further information
can be obtained by contacting ASIC's Scientific Secretary at firstname.lastname@example.org.
The ASIC web site also offers a Coffee Science Alerts service that
lists the most recent scientific papers related to coffee and
caffeine. In addition it provides a very comprehensive list of
links to institutions and other organizations that concern
themselves with matters of coffee science, including a number of
research institutes in coffee producing countries.
Material related to coffee science and human health issues is also
available at www.cosic.org ,
website of The Coffee Science Information Centre. Finally, the
following websites are also useful sources: www.teaandcoffee.net; www.coffeeandcocoa.net; www.roastmagazine.com.