The Problem

OTA-producers in coffee

OTA-producing fungi in the lab

Fungal growth is a time-dependent function so the extent of growth will be related to time spent in good conditions for growth. Mycotoxins are produced during fungal growth, though they are not inevitably a by-product of growth.

All organisms are adapted to certain ranges of factors required for growth. In conditions where an organism finds itself moving toward the edge of its range of one or more of these factors (which include temperature, pH, oxygen tension, water availability, nutrition and others) toxin production will cease before growth ceases.

Broadly defined, micro organisms can be considered to fall into one of three groups with respect to their requirement for water. In A_w terms, the thresholds for growth can be taken to lie around the points 0.95, 0.78 and 0.68 for hydrophilic, mesophilic and xerophilic organisms respectively.

A. ochraceus is a mesophilic organism which shows fastest growth in laboratory culture at A_w levels slightly reduced from pure water corresponding to solutions of about 20% sugar. Toxin production is related to growth, but the limits for expression of this biochemical phenotype are narrower than those of growth per se. Laboratory studies have shown that the limiting A_w for OTA production of this species is 0.82 to 0.80, and for growth it is about 0.78.

Minimum A_w for OTA production by A. carbonarius is about 0.92, though growth continues down to about 0.85. Studies of A. niger isolates of this kind have not been undertaken, due to the rarity of producing isolates and their generally feeble ability to produce OTA.

The constant temperature maximum for OTA-production of A. ochraceus is near to 40^oC though the species is capable of growth to 42^oC; that for A. carbonarius is about 35^oC and the maximum temperature for growth is correspondingly lower.

There are many physiologically active compounds in coffee, some of which have documented effects on OTA production. Studies have variously shown caffeine causing both increases and decreases in OTA-production. Phenyl-propanoids and their glycosides are active and common, but their effect has not been evaluated. Robusta and arabica coffees inherently have slightly different concentrations of caffeine (approx. 2.5% and 1.5% respectively). There are also differences in other small, active, compounds, notably phenolics, and differences between varieties and origins.

Laboratory-based studies are important sources of information about the behaviour of OTA-producers, but field conditions are often so complex (e.g. the presence of competing organisms and distinct micro-environments) that it is very difficult to predict accurately the behaviour of real systems. Surveys and extensive field trials have been necessary to establish 'safe' recommendations for general production and post-harvest practices.