Impact Of Water Activity And Temperature On Aspergillus Flavus Growth And Aflatoxin Production In Stored Peanuts

Peanut is an important oil and economic crop and worldwidly cultivated, but peanut was easily infected by Aspergillus flavus and Aspergillus parasitic during planting, storage and processing,which can produce carcinogens- aflatoxin（AFT）. The problem that AFT beyond the standards seriously occurred frequently, which have been a biggest obstacle to China agricultural exports, and leading to enormous economic losses for peanut processing and export enterprises,seriously restricting the development of peanut industry in China. Therefore, according to the national major requirements of “Prevention and control of aflatoxin in peanuts”, this study investigate the Aspergillus flavus（A.flavus） infection and aflatoxin B1（AFB₁） contamination in peanut kernels from sowing to storage by analysis the occurrence of fungi and A.flavus in soil and peanut kernels and the concentration of AFB₁, and to reveal the key period and the main influencing factors of AFB₁ contamination in peanut kernels. The impact of peanut water activity（aw） and temperature on the growth of Aspergillus flavus and aflatoxin production was studied, illuminating the molecular mechanism of aw and temperature on AFB₁ production. All research provided the theoretical basis and technical support to control the AFT contamination in stored peanuts, which improved the quality and safety of peanuts to protect the human health. The specific conclusions are as following:1. Two different peanut fields（Zhanhong2 and Zhanyou75） were selected to estimate the varition in AFB₁ contamination in peanut kernels from sowing to storage.The result showed that A.flavus infection mainly occurred at the mature period of peanuts with the contamination rate <8%, and low level of AFB₁ was determined. The number of A. flavus in Zhanyou75 field soils was significantly lower than that of Zhanhong2, while contamination rate A.flavus in peanuts was higher. The result indicated that Zhanhong2 peanuts may be resistant to A.flavus infection. During storage, A.flavus and AFB₁ were undetected in peanuts kernels. It may be due to the low aw（<0.70） of peanuts kernels after drying, which was not suitable for A. flavus growth and aflatoxin biosynthesis. It was very important to remain low aw during storage for the prevention and control of AFB₁ contamination.2. To research the impact of aw on the growth of A.flavus and aflatoxin production, unshelled pranuts were used as training matrix. The results showed that the optimal conditions for growth of A.flavus and aflatoxin production were some different. When T<33 °C, A.flavus grows best in peanut under aw 0.99, while when T>37 °C, A.flavus grows slowly under high aw（0.98-0.99）.The optimal aw for aflatoxin production were also different with different temperature. At low temperature（T<28 °C）, the optimal aw is 0.96, while at high temperature（T>33 °C）, it dropped to 0.92.3. To research the impact of temperature on the growth of A.flavus and aflatoxin production, unshelled pranuts were used as training matrix. When temperature below 15 °C or above 42 °C,it is not suitable for the growth of A.flavus and aflatoxin production. When aw ≤ 0.92, A.flavus can grow well at the range of 28 to 40 °C, ant optimal growth temperature is 37 °C. While aw ≥ 0.96, the suitable temperature for A.flavus growth is from 25 to 40 °C, the optimum temperature is different under different aw. A.flavus can produce much AFB₁ at 25-33 °C, with the optimal temperature is 28 °C.The suitable temperature range of aflatoxin production in peanuts was significantly wider than that in medium, when aw.under 0.90, the amount.of AFB₁ produced by A.flavus in peanuts at 37 °C is about 61.2 % of that at 28 °C.While in the culture medium, A.flavus will not produce AFB₁ at 37 °C.4. To detect the effects of water activity and temperature on the expression of aflatoxin biosynthetic genes, revealing the mechanism of regulating AFB₁ production. The results showed that at 28°C, expression of most aflatoxin biosynthetic genes were significant down-regulated at aw 0.99 compared with 0.96. There was a positive correlation between aflatoxin biosynthetic genes expression and AFB₁ production as well as between growth genes espression and A.flavus growth mass. The impact of temperature on AFB₁ production mainly through the regulation of gene expression of afl R and afl S, with the change of the ratio of afl R and afl S expression. Especially at 42 °C, gene afl S expression was too low that no enough AFLS to prevent the conbintion of inhibitor and AFLR, leading to a significant reduction in the expression of downstream genes.At 37, 42 °C, key aflatoxin biosynthetic gene expression were downregulated compared with 28 °C, but gene afl R, afl S, afl F, afl I, afl M, afl U, nad A were up-regulated at 37 °C, and gene afl I, nad A were up-regulated at 42 °C. Gene aba A was up-regulated at 37 and 42 °C, while gene brl A was up-regulated only at 37 °C. Thus, high water activity and temperature inhibited aflatoxin production in peanut mainly through key gene expression significantly down-regulated, leading to the decrease.of its corresponding transcription product.