The Problem

OTA-producers in coffee

Mould growth in coffee in the field

All plants contain a large amount of fungi eliciting no discernable disease or 'immune' reaction and a given plant will have associated with it certain generalist fungi and certain specially adapted fungi.

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 A. niger complex are especially associated with robusta coffee. However, the conditions for the growth of these fungi in the orchard are not optimal, and there are not yet any convincing reports of OTA-accumulation in fresh coffee derived from this source. These fungi can 'infect' and survive as a part of the normal course of events but, as far as can be understood, do not make much growth whilst in the field in healthy coffee.

On fresh coffee being dried, within the first range of 1.0 to 0.95 A_w, 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 A_w mesophiles and xerophiles, if present, will grow well and, as discussed above, some OTA-producers can be expected to produce OTA down to an A_w of between 0.82 and 0.80.

In the driest range, only xerophiles are capable of growth but, if present, could outgrow mesophiles from an A_w 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. However, 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.

Therefore, coffee processing methodologies must be understood in terms of both the direct effect on microbial populations (e.g. reducing the microbial load by the elimination of fruit tissue during pulping), and the indirect effect (e.g. introducing selective conditions such as fermentation or drying).