New aspects of coffee processing: How do the different post harvest treatments influence the formation of potential flavour precursors?

This paper represents a brief overview on coffee processing with special emphasis on the physiology and biochemistry of the coffee beans. Green coffee is obtained by submitting the harvested coffee cherries to either wet or dry processing. It is well accepted that wet processed coffees evolve much better quality. The decisive quality criterion of coffee as a beverage is its aroma, being composed of more than 800 compounds. Surprisingly, only about 30 of these contribute significantly to the specific coffee aroma. These aroma impact compounds are suitable indicators to estimate objectively the aroma differences resulting from different processing. Up to now, the reasons for the quality differences of technologically distinctively produced coffees are unknown. In this context, the biochemical and physiological processes which occur in the living coffee bean during post harvest processing, and which are related to quality, must be taken into consideration. The coffee plant reveals some particularities which are relevant for the generation of aroma components but which, so far, hardly any attention has been paid to: the coffee bean stands intermediately between recalcitrant and orthodox seeds. It does not undergo a resting period induced by maturation drying, and seed germination is initiated while the fruit is still in the final stages of development. In consequence, coffee beans that are submitted to processing do not represent resting seeds but rather developing seedlings. Substantial features of a seed in state of germination are an increasing rate of respiration and the mobilization of storage compounds. These physiological processes must have an impact on the concentration of aroma precursors in green coffee beans and thus on coffee quality. Accordingly, the reported discrepancies between differently processed coffees appear to be the consequence of distinct developmental stages in the course of the coffee seed germination, since the different processing procedures (dry or wet) are suitable to favour germination to an unequal extent.