• QA 093
    What causes phenolic taste in green coffee beans and how to combat it?
    What is phenol or phenolic taste, what causes it and how to combat it?
    Asked by:
    Consultant - Germany

    As far as we know the occurrence of phenolic taste, like a number of other off-tastes, is linked to the chemical composition of the bean. Such beans cannot be detected by visual inspection nor is there any recognised method for combating their occurrence. 

    Different sources offer differing causes but it should be understood that not everyone understands the same by phenolic taste. For example, some cuppers wrongly identify certain types of over-fermentation* as phenolic taste. Others believe it can be caused by poor sanitation or mould infestation during wet processing or drying but, although these may be variations on the same taste theme, the causes are not necessarily the same. * See 11.07.03.

    True phenolic beans, according to some, are more likely to be produced by drought and heat affected trees. That is to say, the bean's chemical composition changes as a result of extreme growing conditions and so therefore does the taste. If so then the chemical change might in fact represent some kind of natural reaction, in response to the unfavourable environment. This appears to be entirely logical since healthy, vigorous trees always produce better quality than do stressed trees. The most likely remedy would therefore appear to be the application of at least a minimal level of irrigation, assuming of course this resource is available.

    Other beans that cause equally unpleasant off-tastes include what is known as invisible stinkers; beans that have been affected by chemical substances as carbolic acid for example; or beans that have suffered bacterial infection during the growing stage. These off-tastes may in some cases be mistaken for phenolic taste but it is important to recognise that the cause is different.

    Invisible stinkers: beans that have been over-fermented during wet processing but not to the point where actual decomposition sets in, i.e. they maintain a bluish-green appearance and are hard to spot. Or, beans that have suffered insect stings or minute cracks that allow fermentation water to enter and so continue the process….  Or, beans that have been affected by unsanitary conditions or mould infestation during processing and drying.

    Bacterial infections can occur when coffee cherries are stung by insects whilst on the tree with the sting damage allowing bacterial infection to take place, for example producing potato flavour or peasiness. This is fairly prevalent in certain countries.

    These three groups of off-beans share one common trait: their chemical composition is different from that of sound beans and, in most instances, they can only be recognised and removed through Ultra Violet Sorting. 

    The background to this is that the question of why beans of good green appearance nevertheless sometimes produce off-tastes has always been of interest because such beans can cause unexpected problems for roasters. This is particularly so for gourmet/specialty roasters who normally roast smallish batches that offer little chance of the offending bean being dispersed over a large quantity.

    Already in June 1975 at the 7th International Scientific Colloquium on Coffee in Hamburg, the East African Industrial Research Organization in Nairobi presented a paper dealing with the identification of over-fermented beans (stinkers) through exposure to ultra-violet light that made such beans fluoresce because their chemical composition was different from that of sound beans. Yet such beans were often unrecognizable with the naked eye: an important finding.

    To note though that as coffee ages so its chemical composition changes as well: the resultant woody or old taste is in fact the result of chemical change. And this means that as the beans age, so most or all of them begin to fluoresce. This then makes it impossible to select the offending beans that were the original target. Therefore, as we understand it, for the ultra-violet sorting process to work well it should only be used for fresh coffee, promptly after milling. Also, the coffee should not be overly coated, i.e. not too much silver skin remains attached to the beans. Within these limitations we estimate that for certain producers UV sorting equipment may be of interest. For further information we suggest to visit www.Satake-USA.com 

    For more scientific questions and discussion we suggest visiting www.asic-cafe.org to make contact with the Association Scientifique Internationale du Café (ASIC).

    NB: Synonyms for phenol include Carbolic Acid and Hydroxybenzene

    Posted 03 May 2006

    Related chapter(s):
    Related Q & A: