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 coffee-science@asic-cafe.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.