Effect of water activity and temperature on the growth of Aspergillus flavus, the expression of aflatoxin biosynthetic genes and aflatoxin production in shelled peanuts

Abstract

The contamination of peanuts with Aspergillus flavus and subsequent aflatoxins is considered to be one of the most serious safety problems in the world. Water activity (aw) and temperature are limiting factors for fungal growth and aflatoxins production during storage. To optimize the practical storage parameter, the effect of aw (0.85–0.99) and temperature (15–42 °C) on fungal growth, aflatoxin production and the expression of aflatoxin biosynthetic and regulatory genes in shelled peanuts was investigated. A. flavus grew at a lower rate when temperature ≤20 °C or aw ≤ 0.85. For the growth of A. flavus in shelled peanuts, the optimum conditions were aw was 0.98, and the optimum temperature was 37 °C. The maximum amount of AFB1 in peanuts was obtained at 28 °C and aw 0.96. Real-time analysis showed that 16 of 25 genes had highest expression levels at 28 °C under aw 0.92, while 9 genes had highest expression levels at 37 °C under aw 0.92. Compared with 37 °C, all aflatoxin biosynthetic pathway genes were down-regulated at 42 °C. All the pathway genes and laeA were up-expressed at aw of 0.96 under 28 °C, compared to aw 0.99. Furthermore, there was a good positive correlation between the ratio of aflS/aflR and AFB1 production. The expression of laeA was also positively correlated with AFB1 production while the expression of brlA was correlated with the A. flavus growth. The results of this study suggest that AFB1 production in peanut kernels can occur over a wider range of aw × temperatures levels compared to formula media and peanut media. Previous studies have showed that AFB1 could not be produced on formula media at 37 °C without the expression of most aflatoxin structural genes. But, in the un-autoclaved shelled peanuts, high concentration of AFB1 was produced at 37 °C with up-regulation of some aflatoxin biosynthetic genes. From a food safety point of view, the results can be used to optimize certain food technological processes and develop prevention strategies to control such carcinogenic natural metabolites in grains (such as peanuts, maize and rice) and derived products.