Influence of temperature, water activity, and oil content on growth and aflatoxin production on oil seeds by Aspergillus flavus and A. parasiticus
Aflatoxins (AFs) are highly toxic second metabolites produced by Aspergillus flavus and A. parasiticus. They are widely detected in cereals, spices, and drinks worldwide. Aflatoxin contamination of foods and crops poses a high health risk for humans and livestock. It is well known that environmental conditions and substrates could influence fungal growth and aflatoxin production. This study tested the effect of water activity (0.82, 0.86, 0.90, 0.94, and 0.98 aw) and incubation temperatures (20°, 27°, and 35°C) on the growth and aflatoxin production of A. flavus and A. parasiticus on ground flax seeds and ground niger seeds. The effect of oil contents of ground niger seeds on fungal growth and aflatoxin production was also investigated in this study.
These two fungal species could not grow on any of the tested substrates with 0.82 aw at 20°, 27°, or 35°C. Aspergillus flavus grew most rapidly on flax seeds with 0.90 aw at 27°C and also 0.94 aw at 27° or 35°C. However, on niger seeds, A. flavus grew best at 0.90 or 0.94 aw incubated at 35°C as well as at 0.94 or 0.98 aw incubated at 27°C. Aspergillus parasiticus showed the optimum growth on flax seeds with 0.90 aw at 35°C, whereas on niger seeds, the optimum occurred on seeds with 0.90 aw at 35°C and also on seeds with 0.94 aw at 27° or 35°C. The optimum conditions for A. flavus to produce high levels of aflatoxins (270-299 μg/kg) on flax seeds were 0.90 aw at 35°C; whereas, the optimum conditions for A. flavus to produce aflatoxin (203-278 μg/kg) on niger seeds were 0.90 or 0.98 aw at 27°C and also 0.90 aw at 35°C. Aspergillus parasiticus produced high levels of aflatoxins (284-365 μg/kg) on flax seeds under the following three conditions, 0.86 or 0.98 aw at 35°C and 0.94 aw at 27°C; A. parasiticus produced 200-384 μg/kg of aflatoxins on niger seeds under nine out of 12 tested incubation conditions.
Reducing mean oil contents from 35.2 to 10.5% of ground niger seeds had very little effect on the growth of the two fungi but significantly decreased their aflatoxin production under certain incubation conditions. On de-oiled niger seeds inoculated with A. flavus, only 13μg/kg of AFB1 was found on seeds with 0.94 aw at 27°C; whereas, on de-oiled niger seeds inoculated with A. parasiticus, high levels of aflatoxins (245-345 μg/kg) were only detected under the three following incubation conditions, 0.90 or 0.94 aw at 27°C, and 0.86 aw at 35°C.This study showed that the optimum growth and aflatoxin production by A. flavus and A. parasiticus were not identical and influenced by incubation conditions, including temperature, water activity, and growth substrates. The results of this study could help establish guidelines for post-harvest and storage conditions for oil seeds to prevent fungal growth and aflatoxin formation.