Effect of Water Activity and Temperature on Growth and Ochratoxin A Production by Aspergillus fresenii and Aspergillus sulphureus on Niger Seeds

Introduction : Niger seeds are used for extracting cooking oil and used as cattle and bird feed. When stored improperly, Niger seeds support mold growth and production of mycotoxins. Ochratoxin A (OTA) is a ubiquitous mycotoxin produced by Aspergillus and Penicilliumspecies. It has been found in a variety of food and feed worldwide.

Purpose : The purpose of this study was to evaluate the effect of water activity and temperature on growth of A. fresenii and A. sulphureuson Niger seeds and resulting OTA production.

Methods : The two fungi were cultivated on Czapek Yeast Extract agar (CYA) individually at 30°C for 5 days. Sterilized ground Niger seeds (2.5g) with different water activity (a_w) values (0.98, 0.94, and 0.90) were placed in separate petri plates. Each plate was spot inoculated with 10 μl spore suspension containing10⁴-10⁵conidia. Plates were then sealed with parafilm and incubated in a closed glass jar at 20 or 30°C for 10 days. Growth on each plate was determined by measuring the area covered by mycelium. OTA production in each inoculated plate was determined using enzyme linked immunosorbent assay after 5 or 10 days of incubation.

Results : Niger seeds with a_w from 0.90 to 0.98 supported both fungal growth and OTA production at both 20 and 30°C. Highest growth rates of the two fungi were observed on seeds with 0.94 a_w and incubated at 30 °C for 10 days. It was also found that OTA formed more rapidly on seeds incubated at 30°C than at 20°C. Seeds with 0.98 and 0.94 a_w yielded higher OTA concentrations than did seeds with 0.90 a_w.

Significance : The data collected from this study can be used to establish storage standards of Niger seeds.