Comparison of Pretreatment Strategies for Enzymatic Saccharification and Fermentation of Barley Straw to Ethanol

Authors: Saha, Badal; Cotta, Michael

Submitted to: New Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/20/2009
Publication Date: 2/1/2010
Citation: Saha, B.C., Cotta, M.A. 2010. Comparison of Pretreatment Strategies for Enzymatic Saccharification and Fermentation of Barley Straw to Ethanol. New Biotechnology. 27(1):10-16.

Interpretive Summary: Ethanol is a renewable oxygenated fuel. Barley straw contains about 60% complex carbohydrates that can be used as a low cost feedstock for conversion to fuel ethanol in certain regions of the U.S. 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 microorganisms. The efficient utilization of all these sugars is essential for cost-effective production of ethanol from barley straw. Research has been conducted to evaluate various pretreatment options (dilute acid, lime, and alkaline peroxide) for barley straw, enzymatic saccharification, and fermentation of the generated sugars to ethanol. In this paper, data has been presented that shows that barley straw can be converted to ethanol with a very good yield. The results are important for developing bioprocess technologies for conversion of barley straw to fuel ethanol.

Technical Abstract: Barley straw used in this study contained 34.1±0.6% cellulose, 22.6±0.4% hemicellulose, and 13.3±0.2% lignin (moisture, 6.5±0.0%). Several pretreatments (dilute acid, lime, and alkaline peroxide) and enzymatic saccharification procedures were evaluated for the conversion of barley straw to monomeric sugars. The maximum release of sugars (glucose, 395±0 mg; xylose, 176±0 mg; arabinose, 32±0 mg; total sugars, 604±0 mg; 94% yield per g) from barley straw (10%, w/v) was obtained by alkaline peroxide (2.5% H202, pH 11.5) pretreatment (35 deg C, 24 h) and enzymatic saccharification (45 deg C, pH 5.0, 120 h) after diluting 2 times prior to adding a cocktail of 3 commercial enzyme (cellulase, ß-glucosidase, and hemicellulase) preparations each at the dose level of 0.15 ml per g of straw. Dilute acid and lime pretreatments followed by enzymatic saccharification generated 566±8 mg (89% yield) and 582±38 mg (92% yield) total sugars, respectively per g of barley straw. The yield of ethanol from the dilute acid pretreated and enzymatically saccharified barley straw hydrolyzate (23.7±0.1 g/L) was 11.4±0.1 g/L (0.48 g/g available sugars, 0.26 g/g straw) by a mixed sugar utilizing recombinant Escherichia coli strain FBR5 in 17 h. The ethanol yields were 11.4±0.1 and 11.9±0.6 g/L from 24.4±0.4 and 26.2±0.1 g sugars/L obtained from lime and alkaline peroxide pretreated barley straw, respectively. No inhibition of fermentation occurred by any of the three pretreatments under the conditions used.