• QA 129
    How do bean density, bean processing and bean size affect roast characteristics?
    I am interested in finding resources to help me understand how bean density, bean processing, bean size etc all affect roast characteristics. I would like to use this knowledge in order to properly roast each type of bean I receive and create useful roast profiles.
    Asked by:
    Roaster - USA

    Each individual coffee has its own, inherent flavour profile. This profile can be exploited but also reduced, or even destroyed, by the roasting process. It is therefore indeed very important to know the optimal roast profile of a coffee. 

    The subject is very large and cannot be discussed in detail here but useful information sources are quoted at the end of this text. Briefly however, flavour is the result of a number of factors:

    • Inherent characteristics: variety, altitude, soil and climatic conditions;
    • The processing method used to produce the green bean;
    • The moisture content;  
    • The roasting process;

    Basic flavour profiles standardize certain flavour objectives. For example

    • For a fresh, clean cup with some acidity: use washed or mild arabicas;
    • For a more full-bodied cup, add natural (dry-processed or sun dried) arabicas; 
    • To increase the yield (more cups) and lower the cost price: add robustas;

    In terms of processing this makes it clear that washed or wet-processed coffees offer more delicate flavours whereas natural (dry-processed or sun dried) arabicas offer more body. Robustas, provided neutral in the cup, add yield. (But, good wet-processed robustas also show 'quality' and can be used for other purposes). Clearly all these coffees have different roasting profiles, irrespective of whether one produces blends or single origin specialty coffee. There are substantial quality and roasting differences between dry and wet processed coffees, also when originating from the same source.

    Bean density is the result of numerous factors. Basically, the denser (harder or solid) the bean, the more likely it came from well-nourished coffee trees, not under stress, and was grown at altitude. Better quality is to be expected, provided primary processing and drying were carried out correctly. Softer beans may reflect less favourable growing conditions and/or coffees grown at low altitudes where bean development is more rapid and 'quality' as such is less. Excessive moisture content also makes for softer beans and reduced quality.

    Bean size is an indicator but does not guarantee quality. For example, Kenya AA grade is often considered amongst the best coffees around. Usually correctly so but, in fact AA simply means a size grade and it is possible that poor growing conditions or incorrect processing produced an AA of poor quality. Bold beans may be soft (see previous paragraph) in which case again quality is not guaranteed by size. Dense, bold beans take longer to roast than do soft or small beans. Very small or light beans reduce the flavour, acidity and body of a coffee, and can turn a potentially fine cup into a mediocre one. Proper size and density separation of the green coffee is therefore essential.

    For more on how green bean aspects influence coffee quality we recommend to read chapter 11 (Coffee Quality) of the Coffee Guide. In terms of technical, scientific information on the roasting process itself we suggest to consult the following publications - of course this list is not all-inclusive.

    Coffee Technology, book by Michael Sivetz and Norman Desrosier, AVI Publishing Co, Westport, Connecticut.  1979. ISBN 0870552694. This remains a classic reference book for the roasting industry…

    Coffee Technology, book by R J Clarke and R Macrae, Elsevier Applied Science, London and New York. 1987. ISBN 1-85166-034-8

    Espresso Roasting, book by Andrea Illy and Rinantonio Viani, Academic Press, San Diego, California. 1995. ISBN 0-12-370670-X

    When it comes to articles and presentations then www.teaandcoffee.net offers a number, for example by Dr Terry Mabbett, free of charge. Another useful source is www.freshcup.com but by subscription. At www.coffeereview.com by Kenneth Davids one finds descriptions of individual coffees that often include details of the roasting process used. The SCAA's Resource Center (www.scaa.org) offers publications on the subject as well - the SCAA also conducts roasting workshops as does the Specialty Coffee Association of Europe, the SCAE. See www.scae.com.  Individual manufacturers of roasting equipment are a further useful source of information  For example www.neuhaus-neotec.de; www.probat.com; and www.diedrichroasters.com.

    Posted 10 January 2007


    Related chapter(s):
    Related Q & A:
    QA=&A 083, Q&A 111