Effect of temperature and water activity on growth and ochratoxin A production by Australian Aspergillus carbonarius and A-niger isolates on a simulated grape juice medium

The effect of water activity (0.92, 0.95, 0.965 and 0.98) and temperature (15 degrees C, 25 degrees C, 30 degrees C and 35 degrees C) on growth rate and ochratoxin A (OA) production by five strains of Aspergillus carbonarius and two strains of A. niger isolated from Australian vineyards was characterised on a synthetic grape juice medium. Maximum growth for A. carbonarius occurred at ca 0.965 a(w) and 30 degrees C, and for A. niger, at ca 0.98 a(w) and 35 degrees C. The optimum temperature for OA production was 15 degrees C and little was produced above 25 degrees C. The optimum a(w) for toxin production was 0.95-0.98 for A. carbonarius and 0.95 for A. niger. Toxin was produced in young colonies after and, typically, did not continue to accumulate the entire surface area of the plate was colonised. Rather, the amount decreased as colonies aged. Trends for growth and OA production were similar among Australian isolates and those from European grapes, as reported in the literature. (c) 2006 Published by Elsevier B.V.