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Title: Enzyme characterization for hydrolysis of AFEX and liquid hot-water pretreated distillers' grains and their conversion to ethanol

item Dien, Bruce
item O Bryan, Patricia
item Li, Xin Liang
item Cotta, Michael

Submitted to: Bioresource Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/17/2007
Publication Date: 11/8/2007
Citation: Dien, B.S., Ximenes, E.A., O'Bryan, P.J., Moniruzzaman, M., Li, X., Balan, V., Dale, B., Cotta, M.A. 2008. Enzyme characterization for hydrolysis of AFEX and liquid hot-water pretreated distillers' grains and their conversion to ethanol. Bioresource Technology. 99(12):5216-5225.

Interpretive Summary: This paper describes important technology for processing the animal feed product distillers dried grains with solubles (DDGS) into ethanol. DDGS is the major co-product of fuel ethanol; 17 lb are produced for each gallon of ethanol when using ground corn for fermentation. As ethanol continues to grow, DDGS has likewise grown, and now there are concerns that supply of DDGS may get ahead of market demand. DDGS prices have been volatile within general downward price pressures. DDGS was converted to ethanol by pretreating the material followed by enzymatic conversion of the fibers into sugars. The biomass was pretreated either using liquid hot-water or ammonia fiber expansion (AFEX), both of which are leading technologies for pretreating biomass. The focus of this paper is on what enzymes are required to convert the pretreated fibers into sugars. We determined a combination of commercial cellulase, beta-glucosidase, pectinase, and feruloyl esterase enzyme preparations was sufficient for releasing the sugars. Furthermore, the sugars produced were readily fermentable to ethanol by Saccharomyces yeast. We estimated that converting DDGS to ethanol using this technology would yield an additional 0.22 gal/bu of corn.

Technical Abstract: Dried distillers grains with solubles (DDGS), a co-product of corn ethanol production, was investigated as a feedstock for additional ethanol production. DDGS was pretreated with liquid hot water (LHW) and ammonia fiber explosion (AFEX) processes. Cellulose was readily converted to glucose from both LHW and AFEX treated DDGS using a mixture of commercial cellulase and beta-glucosidase; however, these enzymes were ineffective at saccharifying the xylan present in the pretreated DDGS. Several commercial enzyme preparations were evaluated in combination with cellulase to saccharify pretreated DDGS xylan, and it was found that adding commercial grade (e.g., impure) pectinase and feruloyl esterase (FAE) preparations were effective at releasing arabinose and xylose. The response of sugar yields for pretreated AFEX and LHW DDGS (6% wt/solids) was determined for different enzyme loadings of FAE and pectinase and modeled as a response surfaces. Arabinose and xylose yields rose with increasing FAE and pectinase enzyme dosages for both pretreated materials. When hydrolyzed at 20% wt/solids with the same blend of commercial enzymes, the yields were 278 and 261 g sugars (i.e., total of arabinose, xylose, and glucose) per kg of DDGS (dry basis, db) for AFEX and LHW pretreated DDGS, respectively. The pretreated DDGS’s were also evaluated for fermentation using Saccharomyces cerevisiae at 15% wt/solids. Pretreated DDGS were readily fermented and were converted to ethanol at 89-90% efficiency based upon total glucans; S. cerevisiae does not ferment arabinose or xylose.