Impact of water activity and temperature on growth and ochratoxin A production of two Aspergillus carbonarius isolates from wine grapes in Greece

The aim of this study was to determine the effects of water activity (a(w); 0.85 to 0.98) and temperature (10 to 40 degrees C) on the radial growth rate and ochratoxin A (OA) production of two Aspergillus carbonarius isolates in vitro. The isolates were obtained from wine grapes cultivated in Greece, and the trial was conducted with a synthetic grape juice medium similar in composition to grapes between veraison (beginning of color change) and ripeness. Fungal growth and OA production data were collected for 55 days. Response surface curves and cardinal values for a. and temperature were obtained using multiple regression analysis. The lag phase lasted from less than 1 to 10 days. Both isolates grew optimally at 30 to 35 degrees C and 0.96 a(w), but maximum OA production occurred under suboptimal growth conditions (15 to 20 degrees C and 0.93 to 0.96 a(w)). Growth also was observed at 0.85 a(w) and 25 degrees C, however at this same aw the fungus failed to produce mycelium at any other temperatures tested. The isolates produced OA at 15 to 30 degrees C and 0.90 to 0.98 a(w). Maximum OA production was detected after 25 days of incubation at 20 degrees C and 0.96 a(w) and was 3.14 and 2.67 mu g g(-1), respectively, for the two strains. The isolated strains used in this study were more xerotolerant than others from the Mediterranean basin. These data will allow producers to identify and thus monitor critical environmental conditions effectively in wine grapes. These data also increase the knowledge base concerning the ability of A. carbonarius to grow and produce toxin under different ecological conditions and can contribute to the development of secondary models for the prediction and risk assessment of OA in wine production.