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Dilute Acid Pretreatment, Enzymatic Saccharification, and Fermentation of Rice Hulls to Ethanol

Authors

  • Badal C. Saha,

    Corresponding author
    1. Fermentation Biotechnology Research Unit, National Center for Agricultural Utilization Research, USDA-ARS, Peoria, Illinois 61604
    • Fermentation Biotechnology Research Unit, National Center for Agricultural Utilization Research, USDA-ARS, Peoria, Illinois 61604. Phone: (309) 681–6276. Fax: (309) 681–6427
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  • Loren B. Iten,

    1. Fermentation Biotechnology Research Unit, National Center for Agricultural Utilization Research, USDA-ARS, Peoria, Illinois 61604
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  • Michael A. Cotta,

    1. Fermentation Biotechnology Research Unit, National Center for Agricultural Utilization Research, USDA-ARS, Peoria, Illinois 61604
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  • Y. Victor Wu

    1. New Crops and Processing Technology Research Unit, National Center for Agricultural Utilization Research, USDA-ARS, Peoria, Illinois 61604
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  • Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture

Abstract

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.

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