Evaluation of the Effect of Water Activity and Temperature on Lag Phase and Growth Rate of Aflatoxigenic Aspergillus Section Flavi Strains Isolated from Stored Rice Grain

Abstract

Abstract This study examined the effect of environmental factors water activity (a w ) and temperature on lag phase prior to growth and growth rate of six aflatoxigenic fungi, Aspergillus section Flavi , strains were isolated from stored paddy rice grain in Thailand. Statistical analysis indicated that both studied stress factors significantly affected lag phases and radial growth rates at the a w and temperature regimes studied (P w ranges but optimal condition was found to be around 0.95 a w and 30°C. Under severe a w stress (0·90 a w ) with elevated temperature (40°C), differences were observed in growth responses, with some strains unable to grow. The lag phases were significantly increased at marginal temperature and a w levels. The combined factors showed statistical interaction for growth rate (P 0.05). Growth rate under more freely available water conditions (>0.95 a w ), the disordinal interaction was observed when strains were cultured at higher temperatures (>30°C). In contrast, growth rate with cooler temperatures at w and 30°C. However, this was not significantly different from that at 0.98 and 0.90 a w (P>0.05).