Evaluating the combined effect of water activity, pH and temperature on ochratoxin A production by Aspergillus ochraceus and Aspergillus carbonarius οn culture medium and Corinth raisins

The objectives of this study were: (a) to evaluate the potential of the ELISA method in the determination of the produced OTA by Aspergillus ochraceus and Aspergillus carbonarius in malt extract agar (MEA) at different pH (3.9, 5.1, 5.9, 6.8), water activity (aw) (0.87, 0.93, 0.99), and temperature (10, 15, 20, 25, 30, 40 °C) levels, providing a rapid screening for the optimum and marginal conditions of OTA production, (b) to comparatively evaluate the performance of ELISA and HPLC method, and (c) to evaluate the ability of A. ochraceus to produce OTA in rehydrated Corinth raisins during storage for 36 days. Two independent experiments were carried out to estimate OTA production on MEA and Corinth raisins. The produced OTA was evaluated qualitatively by the ELISA method and selected cases were verified by HPLC. The levels of OTA decreased with water activity, whereas pH seemed to have no specific effect. Furthermore, A. ochraceus produced maximum amounts of OTA on raisins at the 24th day of incubation, indicating that the endogenous microflora may restrictively affect OTA production. The knowledge of optimal and marginal levels of ecological factors in order to optimise post-harvest and storage of food products may significantly affect the production of OTA. Moreover, endogenous microflora of certain foodstuffs may cause OTA detoxification and consequently reduction of OTA levels; a fact that has to be taken into account in food commodities such as raisins, grapes, and wine.