Aflatoxin B1 (AFB1) production by Aspergillus flavus and Aspergillus parasiticus on ground Nyjer seeds: The effect of water activity and temperature.

Nyjer oil seed cake supports high levels of aflatoxin B1 (AFB1) production. AFB1 is a secondary metabolite of Aspergillus flavus and A. parasiticus, classified as a Class 1A carcinogen. The aim of this study was to determine the effects of temperature (20, 27, and 35 °C) and water activity (0.82, 0.86, 0.90, 0.94, and 0.98 aw) on fungal growth and AFB1 production of A. flavus and A. parasiticus on ground Nyjer seeds over a 30-day incubation period. Linear regression models indicated that both fungal growth and AFB1 production were significantly influenced by water activity of Nyjer seeds and incubation temperature. The two fungi did not grow on Nyjer seeds at 0.82 aw at the three incubation temperatures. The most favorable growth conditions for both fungi were 0.90-0.98 aw at 27 °C or 0.90-0.94 aw at 35 °C. The optimum temperature for AFB1 production was 27 °C for both A. flavus and A. parasiticus (with regression coefficients of 6.01 and 9.11, respectively). Both fungi were likely to produce high levels of AFB1 at 0.90 aw (with regression coefficients of 3.56 for A. flavus and 7.17 for A. parasiticus). Aspergillus flavus only produced AFB1 on seeds with 0.90-0.98 aw at 27 °C (in the range of 203-282 µg/kg) and on seeds with 0.90 aw at 35 °C (212 µg/kg). No detectable AFB1 was produced by this fungus in any other culture conditions that were studied. Aspergillus parasiticus, in contrast, was able to produce AFB1 under all of the growth conditions. At 20 °C, this fungus produced the highest level of AFB1 (212 µg/kg) at high water activity (0.98 aw). At 27 °C, A. parasiticus produced high levels of AFB1 (in the range of 209-265 µg/kg) at a wide range of water activities (0.86-0.98 aw). In the entire study, the highest AFB1 concertation for A. parasiticus was detected on seeds incubated at high temperature (35 °C) and low water activity (0.86 aw). The findings of this study could help optimize the storage conditions of Nyjer oil seeds to reduce aflatoxin contamination.

Effect of water activity, temperature, and incubation period on fungal growth and ochratoxin A production on Nyjer seeds.

Aspergillus fresenii and Aspergillus sulphureus produce ochratoxin A (OTA), which is a secondary metabolite of Aspergillus and Penicillium species, with nephrotoxic effects and potential carcinogenic activity. The aim of this study was to determine the effects of temperature (20, 30, and 37 °C), water activity (0.82, 0.86, 0.90, 0.94, and 0.98 aw), incubation period (5, 10, 15, and 30 days) on fungal growth, and OTA production by A. fresenii and A. sulphureus on Nyjer oil seeds. There was no fungal growth at 0.82 aw. The two fungal species were able to produce OTA from the fifth day of incubation from 0.86 to 0.98 aw and temperature 20 to 37 °C. Aspergillus fresenii produced the highest concentration of OTA (643 µg/kg) at 0.90 aw and 37 °C within 15 days, while A. sulphureus produced the highest level of OTA (724 µg/kg) at 0.98 aw and 20 °C within 10 days. The optimum water activity and temperature for the growth of both fungi were similar at 0.94 aw and at 30 °C. There was statistically significant difference between the levels of OTA production among the two fungi. Overall, A. sulphureus produced significantly higher levels of OTA (p < 0.05). Higher temperature (37 °C) and 0.90-0.94 aw were optimal for OTA production by A. fresenii. Our results show that Nyjer seeds can support the growth of A. fresenii and A. sulphureus and OTA production, and the two species had similar temperature and water activity requirements for growth but different requirements for OTA production on Nyjer seeds.