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Coffee has an unusual carbohydrate storage material, a poly-mannan, that distinguishes it from staple seeds, which have a poly-glucan, starch, and is unlikely to be readily available to most organisms. It may be that the small proportion of mono- and disaccharides present in the beans provide most of the carbon nutrition for any non-specialist organisms associated with coffee. The fruit tissue presents a different set of nutritional conditions in close proximity to the seed, different in composition and more readily available than much of the nutrients found in the seed.
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Associated with the fruit in the orchard, Cladosporium, Cryptococcus, Aureobasidium and Fusarium stilboides are ubiquitous but only the latter species is, for all practical intents and purposes, restricted to coffee. Ochre aspergilli are documented to occur in freshly harvested beans and species of the niger complex are especially associated with robusta coffee. Results of recent trials and surveys carried out under the global coffee project have demonstrated that there is sometimes OTA accumulation in samples fresh coffee cherry. Further work is needed to investigate this and find effective means of control at production level.If we consider fresh coffee being dried, within the first range, 1.0 to 0.95, bacteria, yeast, yeast-like fungi as well as Zygomycetes and species of Fusarium and Botrytis will predominate. In the second range, 0.95 to 0.78, mesophiles and xerophiles, if present, will grow well and, as discussed above, some OTA-producers can be expected to produce OTA down to an Aw of 0.82 to 0.80. In the driest range, only xerophiles are capable of growth but if present could outgrow mesophiles from an Aw as high as 0.90. Any particular outcome will also necessarily depend on the composition of the microbial population present when drying conditions are imposed. The significance of the presence/absence of certain key species such as the OTA-producers is obvious but the density and make-up of potential competitors/symbionts must also be appreciated. This would include not just species that share the mesophilic character of the OTA-producers but also hydrophilic species and certain fast-growing xerophiles. Coffee processing methodologies must be understood in terms of the effect on the microbial populations directly, such as when microbial load is reduced by the elimination of fruit tissue in a pulping step, and indirectly through the imposition of selective conditions such as fermentation or drying.
