This study examined the effect of water activity (0.85-0.995 aw) on growth rate and asexual spore production for four mycotoxigenic strains (Aspergillus flavus, Aspergillus ochraceus, Aspergillus carbonarius and Penicillium verrucosum) on Malt Extract Agar (MEA). The water activity levels of MEA media were modified ionically (NaCl) and non-ionically (glycerol). Results showed that the optimum aw for growth was at 0.98-0.995 for all species using both solutes. However, when water stress was inflicted, there was a slower growth for all species. The limit for growth of the strains was at 0.85-0.9 aw, there was no growth at 0.9 aw for A. carbonarius using NaCl solute and 0.85 aw for A. carbonarius and A. flavus using glycerol solute. A. ochraceus and P. verrucosum had a higher tolerance to lower water activity than A. carbonarius and A. flavus when modified with NaCl. There were significant differences in sporulation between species on glycerol and NaCl-amended media. The optimum conditions for production of asexual spores is often very different from that for growth. Little amount of conidial spore occurred at 0.93-0.95 aw modified with NaCl in cultures of A. carbonarius and P. verrucosum but high amounts were produced by A. ochraceus and A. flavus. Optimum water activity for spore production was 0.995 aw for A. carbonarius, 0.98 aw for P. verrucosum, 0.95 aw for A. flavus and 0.85 aw for A. ochraceus on modified media with glycerol. This is the first detailed study to examine the similarities and differences in growth and sporulation in response to the change of water activity level of important mycotoxigenic species. This study can help in understanding why these species are varied in mycotoxin production, so the results obtained in this study may be useful for application in systems of food safety management.
Key words: Food fungi, osmotic, abiotic parameters, conidiospores
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