Submitted to: Applied Biochemistry and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/3/2003
Publication Date: 3/1/2004
Citation: Dien, B.S., Nagle, N., Singh, V., Moreau, R.A., Nichols, N.N., Johnston, D., Cotta, M.A., Hicks, K.B., Nguyen, Q., Bothast, R.J. 2004. Fermentation of "quick fiber" produced from a modified corn milling process into ethanol and recovery of corn fiber oil. Journal of Applied Biochemistry and Biotechnology. 115:937-950.
Interpretive Summary: The U.S. produced over 2.0 billion gallons of fuel ethanol in 2002 using 740 million bushels of corn. Ethanol production is expected to continue to grow in the next 10 years until production totals 10 billion gallons. Most corn is converted to fuel ethanol using the dry grind method. In this process, corn is hammer milled, treated with enzymes to break down the corn starch, and fermented to ethanol by yeast. As the market expands for ethanol, it is expected that further co-products will need to be developed for this process. Researchers at the University of Illinois have developed a modified milling process that allows for recovery of the fiber and corn oil fractions from the corn prior to fermentation. Not only does this allow for recovery of the valuable corn oil, but it also raises the value of the animal feed co-product. In this work, we examined what could be done to add further value to the recovered fiber fractions. We found that the fiber could be successfully converted to ethanol and that depending upon the specific process used, the ethanol yield could be increased approximately 3-4% per bushel of corn. Furthermore, the fiber was also found to be a rich source of corn fiber oil, which has potential for being marketed as a nutraceutical for lowering cholesterol.
Technical Abstract: Approximately 7% of the 9.5 billion bushels of corn harvested in the U.S. is used for fuel ethanol production (2001), half of which is prepared for fermentation by dry grinding. The University of Illinois has developed a modified dry grind process that allows for recovery of the germ and pericarp fractions prior to fermentation. They estimate the process has the potential to generate an additional 5-7 cents income per gallon ethanol produced because it allows for recovering corn oil from the germ and for a more efficient ethanol fermentation. Additional value might be realized if the pericarp, termed "quick fiber," was also converted to ethanol. In this paper, we report on the use of Quick Fiber (QF) as a feedstock for ethanol production. Quick fiber was analyzed and found to contain 32% (w/dw) glucans and 65% (w/dw) total carbohydrates. We have successfully converted QF into ethanol using an industrial Saccharomyces cerevisiae strain (Y-2034) and ethanologenic Escherichia coli strain FBR5. For the S. cerevisiae fermentation, QF 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 0.096 gal/bu of processed corn assuming a QF yield of 3.04 lb/bu. The fermentation was completed within 72 hr. For the bacterial fermentation, QF was first treated with dilute acid and the syrup, containing the hydrolyzed pentose and starch components, was separated from the residual solids. Fermentation of this hydrolysate gave a final ethanol concentration of 30 g/l. The ethanol yield for the bacterial fermentation was 0.44 g ethanol per g sugar(s) initially present in the hydrolysate, which is 85% of the theoretical yield. The residuals from the fermentations were also evaluated as a source of corn fiber oil. Corn fiber oil, which is extracted from the pericarp fiber fraction, is a nutraceutical being developed by the Agricultural Research Service that has cholesterol lowering properties. Corn fiber oil yields were 1.12% w/w for the residual material from the SSF process. For the FBR5 fermentations, the residual fiber was recovered after hydrolysis and prior to fermentation. The corn fiber oil yield was 8.28% w/w.