Submitted to: Journal of Biobased Materials and Bioenergy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/7/2008
Publication Date: 7/1/2008
Citation: Saha, B.C., Biswas, A., Cotta, M.A. 2008. Microwave Pretreatment, Enzymatic Saccharification, and Fermentation of Wheat Straw to Ethanol. Journal of Biobased Materials and Bioenergy. 2(3):210-217. Interpretive Summary: Ethanol is a renewable oxygenated fuel. Wheat straw contains about 70% complex carbohydrates that can be used as a low cost feedstock for conversion to fuel ethanol. Research needs to be carried out to develop an efficient pretreatment method which can help enzymes breakdown the carbohydrates to simple sugars without generating compounds inhibitory to fermentative microorganism. The efficient utilization of all these sugars is essential for cost-effective production of ethanol from wheat straw. Research was conducted to evaluate various pretreatment options for wheat straw. In this research, microwave pretreatment was evaluated for its effectiveness in the enzymatic breakdown of the carbohydrates 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 wheat straw to fuel ethanol.
Technical Abstract: Wheat straw used in this study contained 38.8±0.5% cellulose and 31.0±0.3% hemicellulose. The effects of temperature (160-240 deg C, 5 min) and duration (5-20 min at 200 deg C) of microwave pretreatment of wheat straw (8.6%, w/v, in water) on its enzymatic saccharification to fermentable sugars were evaluated. The yield of monomeric sugars from microwave (200 deg C, 10 min) pretreated wheat straw (8.6%, w/v, in water) after enzymatic saccharification (45 deg C, pH 5.0, 120 h) using a cocktail of 3 commercial enzyme preparations (cellulase, beta-glucosidase, and hemicellulase) at the dose level of 0.15 ml of each enzyme preparation per g substrate was 544±7 mg/g straw (glucose, 320±14 mg; xylose, 189±7 mg; arabinose, 21±1 mg; galactose, 10±0 mg; 79% yield). In addition, the hydrolyzate contained 2.9±1.6 mg furfural, 0.3±mg hydroxymethyl furfural, and 60±8 mg acetic acid per g of straw. The maximum release of sugars (651±7 mg/g straw, 93% yield) from wheat straw after enzymatic hydrolysis was obtained by microwave pretreatment in dilute acid (0.5% H2SO4, w/v,) at 160 deg C for 10 min. Microwave pretreatment of wheat straw with lime (0.1 g/g straw) at 160 deg C for 10 min released 604±30 mg total sugars/g straw (87% yield) after enzymatic hydrolysis. The concentration of ethanol from microwave pretreated (200 deg C, 10 min, in water) and enzyme saccharified (45 deg C, pH 5.0, 120 h) wheat straw (78 g/liter) hydrolyzate after overliming by recombinant Escherichia coli strain FBR5 at pH 6.5 and 35 deg C in 40 h was 16.8±0.1 g/liter with a yield of 0.49 g/g available sugars (0.22 g/g straw). The ethanol concentration was 16.7±1.5 g/liter with a yield of 0.21 g/g straw in the case of simultaneous saccharification and fermentation of microwave pretreated (200 deg C, 10 min, in water) wheat straw (78 g/liter) after overliming by the E. coli strain at pH 6.0 and 35 deg C in 69 h.