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 a w ), 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 a w . The two fungal species were able to produce OTA from the fifth day of incubation from 0.86 to 0.98 a w and temperature 20 to 37 °C. Aspergillus fresenii produced the highest concentration of OTA (643 μg/kg) at 0.90 a w and 37 °C within 15 days, while A. sulphureus produced the highest level of OTA (724 μg/kg) at 0.98 a w and 20 °C within 10 days. The optimum water activity and temperature for the growth of both fungi were similar at 0.94 a w 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