Incubation time, temperature and water activity effects on Aspergillus carbonarius germination on grape skin and flesh.

Aspergillus carbonarius is confirmed as the main species responsible for ochratoxin A (OTA) in grapes and products based on vine fruits. Several studies have provided information on the ecology of A. carbonarius, in particular growth and OTA production. To date, no detailed studies have been addressed to quantify the germination rate on grape skin and flesh. The objective of this study was to compare the ability of A. carbonarius spores to germinate on grape skin and flesh in relation to temperature (15-40°C) and relative humidity (85-100% RH), in time course studies (4-36 hours). A spore suspension of A. carbonarius (106 spores ml-1) was inoculated onto the skin and on the flesh of cut berries of white organic grapes. For comparison, spores were spread plated onto an artificial grape juice medium. Time to 5% germination was significantly shorter on grape flesh than in vitro and on grape skin (6 vs 9 vs 24 hours in optimal conditions). Fifty percent germination was reached on grape skin at 25-30°C and RH ≥ 90% and on grape flesh in almost all the tested conditions. This suggests that damaged skin may be the main pathway which is conducive to A. carbonarius spore germination providing an easier and rapid infection route. Data on the combined effect of temperature and aw on spore germination were fitted with Bete and polynomial equations providing a good fit of the biological processes (R2 ≥ 0.98). These functions can contribute in a predictive model to estimate the risk of OTA occurrence in grapes.