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

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<0.05). Overall, the growth of each strain was similar over the 20-40°C and 0.90-0.98 a_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), while there was no evidence of statistical interaction effect on the lag phases prior to growth (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 <30°C showed ordinal interaction. Growth rates were fastest at 0.95 a_w and 30°C. However, this was not significantly different from that at 0.98 and 0.90 a_w (P>0.05).