Lime pretreatment, enzymatic saccharification and fermentation of rice hulls to ethanol

Rice hulls used in this study contained 35.6±0.1% cellulose and 12.0±0.7% hemicellulose. The maximum yield of monomeric sugars from rice hulls (15.0%, w/v) by lime pretreatment (100 mg g−1 hulls, 121 °C, 1 h) and enzymatic saccharification (45 °C, pH 5.0, 72 h) using a cocktail of three commercial enzyme preparations (cellulase, β-glucosidase and hemicellulase) at the dose level of 0.15 ml of each enzyme preparation g−1 hulls was 154±1 mg g−1 (32% yield). The lime pretreatment did not generate any detectable furfural and hydroxymethyl furfural in the hydrolyzate. The concentration of ethanol from lime-pretreated enzyme-saccharified rice hull (138 g) hydrolyzate by recombinant Escherichia coli strain FBR5 at pH 6.5 and 35 °C in 19 h was 9.8±0.5 g l−1 with a yield of 0.49 g g−1 available sugars. The ethanol concentration was 11.0±1.0 g l−1 in the case of simultaneous saccharification and fermentation by the E. coli strain at pH 6.0 and 35 °C in 53 h.