The use of agricultural by-product as feed stock and co-culture fermentation is a good strategy for improving the efficiency of fermentation and ethanol production. Most rice husks have low protein and nitrogen content and need to be supplemented with nitrogen for fermentation process. This research sought to determine the optimal supplementation of rice husk stream–based fermentation medium with nitrogen and molasses sources, initial pH and incubation time for maximizing ethanol production by co-culturing Saccharomyces cerevisiae with Candida tropicalis. Urea, sodium nitrate and ammonium nitrate were used as nitrogen sources and molasses was used as carbon sources. Co-cultures of S. cerevisiae and C. tropicalis can use different nitrogen sources and molasses for growth and ethanol production. Molasses supplemented with rice husk hydrolysate medium, initial pH and incubation period significantly influenced ethanol yield and content of nitrogen and carbon in distillers grains (DDG). Maximum ethanol yield (20.32 ± 0.42%) with nitrogen (4.40 ± 0.11%) and carbon (9.20 ± 1.01%) content of DDG were obtained in the rice husk hydrolysate medium containing 16.0 g/l urea, 12.0 g/l NaNO3, 12.0 g/l NH4NO3, 1.0 g/l KH2PO4, 0.7 g/l MgSO4•7H2O, 20 ml/l molasses, 1.0 g/l KH2PO4 and 0.7 g/l MgSO4•7H2O with initial pH 5.5 and 6 days incubation period at 28 to 29°C, 50% relative humidity in the dark for 5 d in a rotary incubator at 60 rpm.
Key words: Rice husk, Saccharomyces cerevisiae, Candida tropicalis, co-culture, ethanol yield, nitrogen and molasses.
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