It was because strains that didn't have aflatoxin biosynthesis function were chosen when high-quality Aspergillus oryzae strains were gained from natural Aspergillus oryzae, or aflatoxin biosynthesis function was assumed to be lost resulting from selection and breeding over hundreds years.
麹菌のアフラトキシン生合成能は、天然の麹菌から良質の麹菌株を得た時、もとからアフラトキシン生合成能を持たない株を選抜したか、数百年にわたる選抜および育種の結果、アフラトキシン生産能が欠けてしまったものと考えられる。