Submitted to: Biotechnology Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 25, 2005
Publication Date: June 1, 2005
Citation: Saha, B.C., Iten, L.B., Cotta, M.A., Wu, Y. 2005. Dilute acid pretreatment, enzymatic saccharification, and fermentation of rice hulls to fuel ethanol. Biotechnology Progress. 21:816-822. Interpretive Summary: Rice hulls, an abundant waste byproduct from rice processing, contain 50% carbohydrate that has the potential to be converted to fuel ethanol. Research needs to be carried out to develop an efficient pretreatment method which can help enzymes break down the carbohydrate to simple sugars without generating inhibitors to fermentation. Lignocellulose containing feedstock produces a mixture of sugars. The efficient utilization of the sugars is essential for cost-effective production of ethanol from rice hulls. Research has been conducted to break down rice hulls using dilute acid as a pretreatment option and commercially available enzymes followed by fermentation of the sugar mixture by an ethanol producing recombinant bacterium. The results are important for developing bioprocess technologies for conversion of rice hulls to fuel ethanol.
Technical Abstract: Rice hulls, a low bulk density 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 an abundant low cost feedstock for production of fuel ethanol. Dilute H2SO4 pretreatments at varied temperature (120-190 deg C) and enzymatic saccharification (45 deg 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 deg C, pH 5.0, 72 h) using cellulase beta-glucosidase, xylanase, esterase, and Tween 20 was 287 ± 3 mg/g (60% yield). Under this condition, no furfural and hydroxymethyl furfural were produced. The yield of ethanol per L by the mixed sugar utilizing ethanologenic 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, and 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 hull) at pH 6.5 and 35 deg C. Detoxification of the acid and enzyme treated rice hull hydrolyzate by overliming (pH 10.5, 90 deg C, 30 min) reduced the time of maximum ethanol production (17 ± 0.2 g from 42.0 ± 0.7 g sugars per L) by the E. coli strain from 64 h 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.