Effects of Substrate and Temperature on Aflatoxin Production by Aspergillus parasiticus and Aspergillus flavus

The effects of substrates (synthetic and semisynthetic broths and several foods) on aflatoxin production by Aspergillus parasiticus and Aspergillus flavus were studied at different temperatures. The addition of calcium lactate as a carbon source to synthetic and semisynthetic broth media was evaluated for its effect on mycelial growth and aflatoxin production. At temperatures of 15 and 25°C, lactate did not support either mycelial growth or aflatoxin production in a basal salts broth, but did support mycelial growth, though not aflatoxin production, by both molds in a semisynthetic (YEL) broth. No growth of either mold or aflatoxin production was observed at 5°C on any of the liquid media employed. Little or no aflatoxin was detected on high protein/low carbohydrate foods such as Cheddar cheese, cottage cheese, yogurt and summer sausage inoculated with A. parasiticus and held at 15°C, but substantial quantities of aflatoxins were produced on yogurt and summer sausage by A. flavus at 15°C. Cheddar and cottage cheeses were poor substrates for aflatoxin production by A. flavus at 15°C. Cheddar cheese was a favorable substrate for aflatoxin production by both molds at 25°C. Cottage cheese was a poor substrate for aflatoxin production by both organisms at 25°C. Yogurt and summer sausage were poor substrates for A. parasiticus , but favorable substrates for A. flavus for aflatoxin production at 15 and 25°C. Trace levels of aflatoxins were produced on whole soybeans by A. parasiticus , whereas high levels of aflatoxins were produced by A. flavus on soybeans at 15 and 25°C. No growth or aflatoxin production occurred on soybean blocks (Meju) at 15°C, but high levels of aflatoxins were produced at 25°C by both molds. Aflatoxins did not diffuse into the second 1-cm layer of process cheese at 15°C; however, diffusion occurred at 25°C. Process cheese appeared to be a poor substrate for aflatoxin production by both molds at 15 and 25°C. Trace levels of aflatoxins (10 to 60 ppb of aflatoxin B₁) were detected on some samples of yogurt and summer sausage at 5°C, although no growth of either mold was observed.