Enhancement of Ethanol Yield from the Corn Dry Grind Process by Conversion of the Kernel Fiber Fraction

Abstract

Approximately 50% of the corn processed for fuel ethanol in the U.S. is dry grinded. Dry grinding yields 2.7-2.8 gal of ethanol per bushel of corn. This ethanol yield could be increased if the fiber c omponent of the cornkernel 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 p entose 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.