Microwave Pretreatment, Enzymatic Saccharification and Fermentation of Wheat Straw to Ethanol

Wheat straw used in this study contained 38.8 +/- 0.5% cellulose and 31.0 +/- 0.3% hemicellulose. The effects of temperature (160-240 degrees C, 5 min) and duration (5-20 min at 200 degrees 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 degrees C, 10 min) pretreated wheat straw (8.6%, w/v, in water) after enzymatic saccharification (45 degrees 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 wheat straw was 544 7 mg/g straw (glucose, 320 +/- 14 mg; xylose, 189 +/- 7 mg; arabinose, 21 +/- 1 mg; galactose, 10 +/- 0 mg; 70% yield). In addition, the hydrolyzate contained 2.9 +/- 1.6 mg furfural, 0.3 +/- 0.0 mg hydroxymethyl furfural, and 60 +/- 8 mg acetic acid per g of straw. The maximum release of sugars (651 7 mg/g straw, 84% yield) from wheat straw after enzymatic hydrolysis was obtained by microwave pretreatment in dilute acid (0.5% H2SO4, w/v) at 160 degrees C for 10 min. Microwave pretreatment of wheat straw with lime (0.1 g/g straw) at 160 degrees C for 10 min released 604:E 30 mg total sugars/g straw (78% yield) after enzymatic hydrolysis. The concentration of ethanol from microwave pretreated (200 degrees C, 10 min, in water) and enzyme saccharified (45 degrees 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 degrees 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 degrees C, 10 min, in water) wheat straw (78 g/liter) after overliming by the E coli strain at pH 6.0 and 35 degrees C in 69 h.