This research investigated the feasibility of utilizing rice bran as feedstock for submerged fermentation of Pythium irregulare to produce fungal lipids, which contain polyunsaturated fatty acids (PUFAs) including arachidonic acid (AA, C20:4n6) and eicosapentaenoic acid (EPA, C20:5n3).
The fungal nutrient requirements, temperature characterization and growth modeling were determined in glucose-yeast extract media. Sixteen media combinations based on a 24 factorial design with four levels of glucose and yeast extract ranged from 1% to 4% and 0.25% to 1%, respectively, and corresponding carbon/nitrogen ratio (C/N) ranged from 4 to 64, were examined. The suitable C/N for Pythium irregulare growth ranged from 12 to 24, while that for fungal lipid accumulation as well as EPA and AA synthesis was 32. The optimal medium for EPA and AA production was composed of 2% glucose and 0.25% yeast extract, with addition of 0.1% KH2PO4. Three constant incubation temperatures, 13 °C, 19 °C and 25 °C, were then tested for increasing the EPA and AA production with this medium. Biomass, substrate utilization, lipid and individual fatty acid yields during incubation were determined and the fungal growths were modeled. The optimal incubation temperature was 25 °C for EPA and AA production.
To meet the fungal nutrient requirements, a 5% (w/v) rice bran media was made and the cultures were incubated at 25 °C to produce fungal lipids containing EPA and AA. Biomass, substrate utilization, lipid, and individual fatty acid yields during incubation were determined. The results indicated rice bran could be utilized to produce EPA and AA by Pythium irregulare. After 7 days of fermentation, EPA and AA were synthesized by the fungus with yields of 207 mg/L and 70 mg/L, respectively. The percentages of bioconversion from rice bran to EPA and AA by the fungus were 0.42% and 0.14%, respectively. Meanwhile, more than 73% COD in the rice byproduct medium was removed by the fungus.