Effect of temperature, water activity and other toxigenic mold species on growth of Aspergillus flavus and aflatoxin production on corn, pinto beans and soybeans

Abstract

Portions of corn, a commodity in which aflatoxin is frequently found, were held at 16, 26 and 32°C after the moisture contents were adjusted to achieve water activities (a(w)) ranging from too low to ample for support of mold growth. Suspensions of mold spores from toxigenic cultures of Aspergillus flavus, A. ochraceus, Penicillium citrinum, P. cyclopium and P. urticae were added to the test portions, either as A. flavus alone, as A. flavus with one of the other molds or as a mixture of all 5 species. Additional water was used to obtain the proper moisture levels. A temperature of 16°C was generally too low for aflatoxin production by either the added or native strains of A. flavus, although the mold was able to grow at 16°C at a(w) values as low as 0.80, 0.77 and 0.85 on corn, soybeans and pinto beans, respectively. Aflatoxin production was essentially the same at 26 and 32°C with limiting a(w) values in the range of 0.85-0.89. Limiting a(w) values for mold growth at 26 and 32°C were 0.73, 0.69 and 0.75 for corn, soybeans and pinto beans, respectively. This study provided no evidence that substrate suitability at limiting temperatures and a(w) levels is a factor in the observed differences in the risk of alfatoxin contamination for these commodities. The study did indicate that the associated mold flora, when the seed is exposed to mold invasion, is a risk determinant.