Effect of water activity and temperature on the germination and growth of Aspergillus tamarii isolated from "Maldive fish"

Abstract

Germination times and radial growth rates of cyclopiazonic acid producing strains of Aspergillus tamarii isolated from a smoked dried fish product were studied over water activities (a(w)) ranging from 0.99 to 0.79 at 25°C, 30°C, 35°C and 40°C on two laboratory media. The a(w) of the media was controlled by either NaCl or a mixture of glucose and fructose. The optimum germination and growth were observed at temperatures between 30°C and 35°C. Germination was favored at the highest a(w) of 0.99 under all conditions. Growth however was dependent on the media and temperature with a lower optimum a(w) of 0.95 for NaCl media and 0.95 to 0.92 a(w) on media containing glucose/fructose. The minimum a(w) for growth was often higher than for germination while both parameters were influenced by temperature and media type. Germination on NaCl media was prevented at a(w) values below 0.82 at 25°C and 30°C, 0.85 at 35°C and 40°C. However, growth did not occur at a(w) <0.85 at 25-35°C. At those temperatures on glucose/fructose media, growth was observed at the lowest a(w) tested (0.79). On both media, the restrictive effect of lowered water activity was more pronounced at 40°C than at 25-35°C. Delays in germination increased and growth rates decreased with marginal a(w) and temperature conditions. The fungi displayed better tolerance on glucose/fructose media than on NaCl media on which it was partly inhibited by the NaCl. The information obtained here could be used to develop strategies for the control of this xerophilic fungus on smoked dried fish and other tropical foods on which it predominates.