Effects of Water Activity and Temperature on Growth and Toxin-producing of Aspergillus flavus MI18-S1 on Tartary Buckwheat Grains

  Objective  Effects of water activity (a_w) and temperature on the growth and toxicity of Aspergillus flavus on mature grains of tartary buckwheat were studied to determine the safety conditions for harvest, processing, transportation, and storage of the crop.

  Method  Varied medium a_w (i.e., 0.87, 0.90, 0.93, 0.95 and 0.99) were created by additions of glycerol to culture A. flavus MI18-S1 isolated from the infected grains for a 7-day test at 15, 21, 28, 35 or 42 ℃. Colony diameter on petri dish was measured, spore count calculated from the blood cell counter board, and aflatoxin content determined by HPLC.

  Result  The fungus was found to produce aflatoxins B₁ and B₂, predominately B₁, but not G₁ or G₂. The highest spore count and toxicity were generated at 28 ℃ and a_w=0.93. When the temperature was above 28 ℃, the colony could grow better under high water activity. At a temperature either below 15℃ or above 42 ℃, the fungal growth and toxin-production slowed or ceased. Within the range between these temperatures, the aflatoxin content increased at first and followed by a decline. A multivariate linear regression analysis showed no significant correlation between the colony diameter and the toxin production. Instead, the amount of aflatoxin generated in the culture was mainly affected by the a_w and temperature of the environment, and most critically, sporulation of the fungus. And, sporulation of the fungus was significantly affected by the a_w, temperature of the environment, the amount of aflatoxin generated in the culture and the colony diameter.

  Conclusion  The growth and aflatoxin production of A. flavus on buckwheat grains were at peak when stored at 28 ℃ on a medium with a_w of 0.93. Therefore, in many aspects of the buckwheat production chain, especially during the storage stage, it is advisable to avoid these optimal conditions in order to control the spore and toxin production of buckwheat, ensuring food safety. This study can provide a theoretical basis and data support for the development of preventive and control measures against Aspergillus flavus contamination during buckwheat harvesting, processing, transportation, and storage.