ARS | Publication request: Lime pretreatment, enzymatic saccharification, and fermentation of rice hulls to ethanol

Submitted to: Biomass and Bioenergy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 18, 2008
Publication Date: July 1, 2008
Citation: Saha, B.C., Cotta, M.A. 2008. Lime pretreatment, enzymatic saccharification, and fermentation of rice hulls to ethanol. Biomass and Bioenergy. 32(10):971-977.

Interpretive Summary: Rice hulls, a complex lignocellulosic material, contain 47% carbohydrates that have the potential to be converted to fuel ethanol. Research needs to be carried out to develop an efficient pretreatment method which can facilitate the enzymatic breakdown of the carbohydrates to simple sugars without generating compounds inhibitory to the fermentative microorganism. The efficient utilization of all sugars is essential for cost-effective production of ethanol from rice hulls. Native rice hulls are very resistant to breakdown by enzymes. Research has been conducted to evaluate various pretreatment options for rice hulls. In this research, inexpensive lime pretreatment has been evaluated for its effectiveness in the enzymatic breakdown of the carbohydrate to sugars and fermentability of the sugars to ethanol by an ethanol producing recombinant bacterium. The results are important in choosing a suitable pretreatment option for developing bioprocess technologies for conversion of rice hulls to fuel ethanol.

Technical Abstract: 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 deg C, 1 h) and enzymatic saccharification (45 deg C, pH 5.0, 72 h) using a cocktail of three commercial enzyme preparations (cellulase, beta-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 deg 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 deg C in 53 h.