Submitted to: American Chemical Society Symposium Series
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 15, 2003
Publication Date: December 1, 2004
Citation: Dien, B.S., Nichols, N.N., Iten, L.B., Bothast, R.J. 2004. Enhancement of ethanol yield from the corn dry grind process by fermentation of the kernel fiber fraction. In: Nelson, W.M., editor. Agricultural Applications in Green Chemistry. Chap. 6. Washington, DC:American Chemical Society. p. 63-77. Interpretive Summary: The U.S. produced 1.77 billion gallons of fuel ethanol in 2001 using 690 million bushels of corn. Ethanol production is expected to triple in the next 10 years. Corn is converted to ethanol, in equal amounts, by wet milling and dry grinding. Dry grinding is the preferred process for smaller ethanol plants that are owned by farmer cooperatives. Besides ethanol, these plants also produce an animal feed (distillers dry grains with solubles (DDGS)) from the leftover fermentation stillage. Tripling ethanol production is expected to increase DDGS production 3 fold or more, which may saturate the DDGS market. However, DDGS contains fiber, which is a good feedstock for ethanol. DDGS has been successfully converted to ethanol using industrial yeast and an in-house patented recombinant bacterium, which has been engineered to produce ethanol. These results suggest that ethanol yield could be increased by 0.1 gallons per bushel or a maximum of 30 million gallons per year.
Technical Abstract: Approximately 50% of the corn processed for fuel ethanol in the U.S. is dry grinded. Currently, dry grind yields 2.7-2.8 gal of ethanol per bushel of corn. This ethanol yield could be increased if the fiber component of the corn kernel was also converted into ethanol. Currently, the kernel fiber in a dry grind plant is collected after the fermentation in a solids cake by centrifugation and termed distillers wet grains (DWG). Three different samples of DWG were analyzed and found to contain 14.7-18.1% glucans and 34.9-40.5% total carbohydrates. We have successfully converted the fiber component of DWG into ethanol using either an industrial Saccharomyces cerevisiae strain (Y-2034) or ethanologenic Escherichia coli strain FBR5. For the S. cerevisiae fermentation, DWG was pretreated with dilute acid and simultaneously saccharified and fermented (SSF) by adding cellulase, beta-glucosidase, and glucoamylase along with the yeast. The ethanol yield was 7.52 g ethanol per 100 g DWG (dry basis), and the fermentation was completed within 30 hr. For the bacterial fermentation, DWG was first treated with dilute acid and the syrup, containing the hydrolyzed pentose and starch components, separated from the residual solids. Fermentation of this hydrolysate was completed within 30 hr, and the final ethanol concentration was 2.12% w/v. The ethanol yield for the bacterial fermentation was 0.49 g ethanol per g sugar(s) initially present in the hydrolysate, which is 96% of the theoretical yield. DWG is normally used as an animal feed. Pretreating DWG significantly increased the percent of crude protein that was soluble from 4 to 29%, which would have significant impact on the animal nutritional properties of the modified DWG.