The influence of the wet-processing of Arabica coffee upon the characteristics of the coffee bean

Abstract

The presence of galacturonic acid within the reported pectic component of coffee mucilage has been confirmed. The insoluble mucilage material (protopectin) contains arabinose, glucose and xylose units in addition to the uronic acid. Most of the arabinose is probably associated with the galacturonic acid within the middle lamella whereas the glucose and xylose units predominate in the cell wall. The structures containing a high proportion of glucose and xylose are particularly resistant to degradative reactions. Earlier studies of mucilage composition have failed to distinguish between the insoluble protopectin fraction and the water-soluble material. An awareness of this distinction serves to clarify many of the problems of fermentation. The sugars sucrose, fructose and glucose are dominant components of the soluble fraction. Fermentation Reactions In coffee fermentation, it might be expected that one result of the pulping operation, involving as it does cellular rupture and exposure of mucilage to oxygen, is to initiate a number of reactions dependent upon enzymes present within the fruit flesh. Apart from the polyphenoloxidase system known to occur in coffee, however, no evidence exists to show that such enzyme reactions are of great importance to coffee fermentation. The great majority of the pectic enzyme activity has been shown recently to be elaborated by micro-organisms. The open, cellular, physical structure of the mucilage of freshly pulped coffee beans is very retentive of water; the retained liquor is at a pH of about 5.5 and contains simple sugars. Such a system forms an attractive substrate to a wide variety of micro-organisms. Microbial activity is largely responsible for the essential reaction during fermentation, the degradation of the insoluble protopectin material left adhering to the endorcarp after pulping. Free arabinose is released early on during fermentation; free galacturonic acid is not normally observed at any stage except at very low concentrations. The insoluble residual material present at the end of fermentation, detached from the endocarp, is composed predominantly of glucose and xylose units. The sugars sucrose, fructose and glucose, constituents of the cherry juice, are also attac~d by micro-organisms. Since these sugars are soluble, their initial concentrations, besides being dependent upon the - 3 - maturity of the fruit, are greatly affected by the degree of water-washing involved during o~ immediately after the pulping stage. The more important products of the degradation of the sugars are acetic and lactic acids. Under the conditions of a very heavy initial w3shing, the concentrations of sugars are very low and the production of acetic and lactic acids is correspondingly low; after forty hours or so of undisturbed development, these acids can be accompanied by butyric and propionic acids, the lactic acid already formed being destroyed. A close correlation exists between the level of propionic acid and the incidence of a taint known as "onion flavour". The acids produced Lower the pH of the fermenting aqueous mass which in turn slows the rate of mucilage degradation; evidence obtained from buffered micro-fermentations is presented to demonstrate this important pH effect. The efficiency of acid production from the sugars is greater if fermentation proceeds under water insteud of under the normal conditions of free drainage; the low pH attained under these circumstances probably partly accounts for the relatively slow rate of mucilage decomposition observed. The overall pattern of change is complex and incorporates numerous trivial reactions, one of which leads to the evolution of hydrogen gas as a result of bacterial attack upon sugars. Another releases an unidentified compound very similar to galacturonic acid. Effects upon the Coffee Bean During the fermentation process, it can be expected that diffusion of material into and out of the bean will occur. Overall, there is a net loss of bean dry matter of the order of one per cent during a normal fermentation, much of which is in the form of simple sugars. It is shown that the quality of the bean improves during fermentation and, in particular, the bean loses much of its potential to undesirable brown colorations which become evident when dry. The improvement of coffee bean quality is particularly evident in the raw appearance of the bean itself. Statistical evidence is available to demonstrate the closeness of the correlations of raw appearance with both the liquor quality and the overall quality assessment of coffee. It is evident that two desirable changes are proceeding concomitantly during a normal fermentation process, the degradation of mucilage and the diffusion effect which improves quality. It can be reasoned that these two changes can be carried out more effectively if they are encouraged to proceed independently. A rapid removal of mucilage followed by a simple immersion of the clean bean in water forms the basis of a procedure which produces excellent coffee successfully and efficiently.