Dilute acid pretreatment, enzymatic saccharification, and fermentation of rice hulls to ethanol

Rice hulls, a complex lignocellulosic material with high lignin (15.38 ± 0.2%) and ash (18.71 ± 0.01%) content, contain 35.62 ± 0.12% cellulose and 11.96 ± 0.73% hemicellulose and has the potential to serve as a low‐cost feedstock for production of ethanol. Dilute H2SO4 pretreatments at varied temperature (120–190 °C) and enzymatic saccharification (45 °C, pH 5.0) were evaluated for conversion of rice hull cellulose and hemicellulose to monomeric sugars. The maximum yield of monomeric sugars from rice hulls (15%, w/v) by dilute H2SO4 (1.0%, v/v) pretreatment and enzymatic saccharification (45 °C, pH 5.0, 72 h) using cellulase, β‐glucosidase, xylanase, esterase, and Tween 20 was 287 ± 3 mg/g (60% yield based on total carbohydrate content). Under this condition, no furfural and hydroxymethyl furfural were produced. The yield of ethanol per L by the mixed sugar utilizing recombinant Escherichia coli strain FBR 5 from rice hull hydrolyzate containing 43.6 ± 3.0 g fermentable sugars (glucose, 18.2 ± 1.4 g; xylose, 21.4 ± 1.1 g; arabinose, 2.4 ± 0.3 g; galactose, 1.6 ± 0.2 g) was 18.7 ± 0.6 g (0.43 ± 0.02 g/g sugars obtained; 0.13 ± 0.01 g/g rice hulls) at pH 6.5 and 35 °C. Detoxification of the acid‐ and enzyme‐treated rice hull hydrolyzate by overliming (pH 10.5, 90 °C, 30 min) reduced the time required for maximum ethanol production (17 ± 0.2 g from 42.0 ± 0.7 g sugars per L) by the E. coli strain from 64 to 39 h in the case of separate hydrolysis and fermentation and increased the maximum ethanol yield (per L) from 7.1 ± 2.3 g in 140 h to 9.1 ± 0.7 g in 112 h in the case of simultaneous saccharification and fermentation.