Ethanol production by Mucor indicus and Rhizopus oryzae from rice straw by separate hydrolysis and fermentation

Rice straw was successfully converted to ethanol by separate enzymatic hydrolysis and fermentation by Mucor indicus, Rhizopus oryzae, and Saccharomyces cerevisiae. The hydrolysis temperature and pH of commercial cellulase and β-glucosidase enzymes were first investigated and their best performance obtained at 45 °C and pH 5.0. The pretreatment of the straw with dilute-acid hydrolysis resulted in 0.72 g g−1 sugar yield during 48 h enzymatic hydrolysis, which was higher than steam-pretreated (0.60 g g−1) and untreated straw (0.46 g g−1). Furthermore, increasing the concentration of the dilute-acid pretreated straw from 20 to 50 and 100 g L−1 resulted in 13% and 16% lower sugar yield, respectively. Anaerobic cultivation of the hydrolyzates with M. indicus resulted in 0.36–0.43 g g−1 ethanol, 0.11–0.17 g g−1 biomass, and 0.04–0.06 g g−1 glycerol, which is comparable with the corresponding yields by S. cerevisiae (0.37–0.45 g g−1 ethanol, 0.04–0.10 g g−1 biomass and 0.05–0.07 glycerol). These two fungi produced no other major metabolite from the straw and completed the cultivation in less than 25 h. However, R. oryzae produced lactic acid as the major by-product with yield of 0.05–0.09 g g−1. This fungus had ethanol, biomass and glycerol yields of 0.33–0.41, 0.06–0.12, and 0.03–0.04 g g−1, respectively.