Submitted to: Process Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/27/2005
Publication Date: 8/1/2005
Citation: Saha, B.C., Iten, L.B., Cotta, M.A., Wu, Y. 2005. Dilute acid pretreatment, enzymatic saccharification and fermentation of wheat straw to fuel ethanol. Process Biochemistry. 40:3693-3700.
Interpretive Summary: Ethanol is a renewable bio-based oxygenated fuel. It is currently produced from corn derived glucose by fermentation. Wheat straw contains about 70% carbohydrates (cellulose and hemicellulose) that can serve 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 complex carbohydrates to simple sugars without generating compounds inhibitory to fermentative microorganisms. Further, lignocellulose containing feedstocks generally generates mixed sugars. The efficient utilization of all these sugars is essential for cost-effective production of ethanol from these feedstocks. Research has been conducted to breakdown wheat straw using dilute acid as pretreatment option and commercially available enzymes followed by fermentation of the sugars by an ethanol producing recombinant bacterium. The results are important for developing cost-effective bioprocess technologies for conversion of wheat straw to fuel ethanol.
Technical Abstract: Wheat straw consists of 48.57 +/- 0.30% cellulose and 27.70 +/- 0.12% hemicellulose which make up the total carbohydrate content of 76.27 +/- 0.42% on dry solid (DS) basis and has the potential to serve as a low cost feedstock for production of fuel ethanol. Dilute acid (0.0-4.0%, v/v) pretreatment at varied temperature (120-180 deg C) and enzymatic saccharification were evaluated for conversion of wheat straw cellulose and hemicellulose to monomeric sugars. The maximum yield of monomeric sugars from wheat straw (7.83%, w/v DS) by dilute H2SO4 (0.75%, v/v) pretreatment and enzymatic saccharification (45 deg C, pH 5.0, 72 h) using cellulase, beta-glucosidase, xylanase, and esterase was 565 +/- 10 mg/g (74% of the theoretical yield). Under this condition, no measurable quantities of furfural and hydroxymethyl furfural were produced. The yield of ethanol (per L) from acid pretreated enzyme saccharified wheat straw (78.3 g) hydrolyzate by recombinant Escherichia coli strain FBR5 was 19 +/- 1 g with a yield of 0.24 g per g DS (0.32 g per g total sugars present in wheat straw). Detoxification of the acid and enzyme treated wheat straw hydrolyzate by overliming (pH 10.5, 90 deg C, 30 min) reduced the fermentation time from 118 h to 39 h in the case of separate hydrolysis (45 deg C, pH 5.0, 72 h) and fermentation (35 deg C, pH 6.5), and increased the ethanol yield from 13 +/- 2 to 17 +/- 0 g/L and decreased the fermentation time from 136 h to 112 h in the case of simultaneous saccharification and fermentation (35 deg C, pH 6.0).