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 growth, though they are not usually a necessary by-product of growth as is CO2 or organic acid production, for example. 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.
A. ochraceus is a mesophilic organism which shows fastest growth in laboratory culture at Aw 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 Aw for OTA production of this species is 0.82 to 0.80, and for growth it is about 0.78. Minimum Aw 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 do not appear to have been done, probably 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°C though the species is capable of growth to 42°C; that for A. carbonarius is about 35°C and the maximum temperature for growth is correspondingly lower. It is not clear what response the diurnally cyclic temperature regime that is characteristic of sun drying might elicit.
There are many physiologically active compounds in coffee, some of which have documented effects on OTA production. Studies have variously shown increases and decreases in OTA production by caffeine. Phenyl-propanoids and their glycosides are active and common but evaluation of their effect is lacking. robusta and arabica inherently have slightly different concentrations of caffeine (about 2.5% and 1.5%, respectively) and other small, active compounds, notably phenolics, as do different varieties and different origins.
Laboratory-based studies are important sources of information about the behaviour of OTA-producers, but the field conditions are often so complex (competing organisms, distinct micro-environments, etc) that it is very difficult to predict the behaviour of real systems. Surveys and extensive field trials are required to establish "safe" production and post-harvest practices.
