Response of toxigenic and non-toxigenic strains ofAspergillus flavusto irradiation

Abstract

Condidia-bearing cultures of 3 aflatoxin producing and 3 non-toxigenic strains of A.flavus were irradiated 16 times in 6 months at a dosage of 160 -240 krad each time. Subcultures were grown between irradiations and about 70% of 60 subcultures survived all irradiations. Survival was approximately the same in both groups.Surviving subcultures were refrigerated and transferred to fresh medium each 6 months. One, 2 and 3 years after the multiple radiation treatment the subcultures were grown on rice. Of the 3 toxigenic strains, 0, 30 and 50%, respectively, were able to produce aflatoxins after I year; 50, 30 and 60%, respectively, after 3 years. Often the aflatoxin yield was much lower in irradiated subcultures than in the parent strain but sometimes it was higher.Two of the 3 non-toxigenic strains remained aflatoxin-negative, while of 14 subcultures of the third strain, 2 were positive after 2 years, and 4 more after 3 years.These results are not due to loss or gain mutations or to repair processes. A. flavus is heterocaryotie and an accumulation of nuclei possessing or not the genetic information for attatoxin synthesis appears possible in the course of many transfers to fresh medium. This may lead to loss, recovery or gain of the ability to produce aflatoxins.

Scientific Opinion on the efficacy and microbiological safety of irradiation of food

“…Experiments carried out under conditions simulating practical application have either found unchanged or, more frequently, decreased production of mycotoxins in irradiated foods (Chang and Markakis, 1982;Chiou et al, 1990;Ito et al, 1994). Results of many repeated cycles of sublethal irradiation and growth of various strains of A. flavus showed that such recycling treatment more frequently led to a decrease or complete loss of aflatoxin production more frequently than to an increase (Frank et al, 1971). According to Diehl (1995) "there are no indications that any food irradiated and stored under conditions prevailing in practice would be at risk of increased formation of mycotoxins".…”

Effects of irradiation on fungi and fumonisin B1in corn, and of microwave-popping on fumonisins in popcorn

“…Studies on the effect of irradiation on the reduction and elimination of fungi that might produce additional mycotoxins in foods and feeds have been reported since the late 1950s (Yen et al 1956;Webb et al 1959), with the first known study of the affect of gamma-irradiation on toxigenic A. flavus reported in 1971 (Frank et al 1971). A. flavus isolates irradiated in growth media in tubes with 1.6-2.4 kGray of irradiation showed only a 30% reduction in viability.…”