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Abstract
The effects of cycling temperature, temperature and substrate were studied on aflatoxin production by Aspergillus parasiticus NRRL 2999 and Aspergillus flavus NRRL A16100.
Cycling temperatures (5 C for 12 hr and 25 C for 12 hr) after preincubation at 25 C for 1, 2 and 3 days resulted in a greater increase in maximum aflatoxin production by A. parasiticus than did the constant temperature incubation at either 25 C or 15 C in YES medium. Cultures that were not preincubated at 25 C but subjected to constantly fluctuating temperatures produced maximum amounts of aflatoxin equivalent to cultures incubated at a constant 25 C.
Cycling temperatures resulted in more aflatoxin production by A. parasiticus than when incubated at 25 C, 18 C or 15 C on rice, while A. flavus was found to produce less aflatoxin under cycling temperatures than when incubated on rice at constant temperatures. On Cheddar cheese, cycling temperatures resulted in much less aflatoxin production by both molds than at 25 C but much higher levels of aflatoxin were produced under cycling temperatures than at 15 C. Preincubation of cultures at 25 C for 2 days prior to onset of temperature cycling did not increase aflatoxin production on rice but increased production on cheese by 2-3 times, compared with no preincubation.
Lactate, as a carbon source, did not support either mycelial growth or aflatoxin production in a basal salts broth at 15 C and 25 C; however, it did support mycelial growth, though not aflatoxin production, in a semisynthetic (YEL) medium for both molds. Little or no aflatoxins were detected on high protein/low carbohydrate foods inoculated with A. parasiticus at 15 C. Cheddar cheese and Meju were favorable substrates for aflatoxin production by both molds at 25 C, but not at 15 C. Cottage cheese and processed cheese were poor substrates for aflatoxin production by both molds at 15 C and 25 C. Yogurt, summer sausage and whole soybeans were poor substrates for A. parasiticus but were favorable for A. flavus at 15 C and 25 C. Aflatoxins did not diffuse into the second 1 cm layer on processed cheese at 15 C; however, diffusion was found at 25 C. Processed cheese containing sorbate did not support growth or aflatoxin production in up to 100 days of incubation. Neither mold growth nor aflatoxin production was observed at 5 C on liquid media or the various food substrates.
