Ethanol Fermentation of AFEX-Pretreated Corn Stover by Three Microbes Compared

Scientists from the (United States) DOE (Department of Energy) Great Lakes Bioenergy Research Center, Michigan State University, report the side-by-side comparison of cellulose-ethanol fermentation performance of two bacterial strains (Escherichia coli and Zymomonas mobilis), and a yeast strain of Saccharomyces cerevisiae, on a second generation biofuel feedstock. The substrate used was the liquid hydrolyzate from AFEX (ammonia fiber expansion)-pretreated corn stover, supplemented by corn steep liquor (CSL). S. cerevisiae is the commonly used ethanol-fermenting organism in both the beverage and fuel ethanol industries. The bacteria, E. coli and Z. mobilis have recently been considered as potential alternatives to S. cerevisiae, due to their faster growth rates. In contrast to previous comparisons on ethanol fermentation, this study is probably one of the first to compare ethanol fermentation performance of bacteria and yeast, from a second generation biofuel feedstock (i.e. lignocellulosic biomass). Results showed that the three microorganisms were able to ferment the CSL-supplemented AFEX-pretreated corn-stover- hydrolyzate, at a metabolic yield, final concentration, and ethanol productivity of 0.42 g ethanol per g sugar consumed, 40 g ethanol per liter, and 0.7 g ethanol per liter per hour (7 to 48 hours), respectively. The results also showed that glucose fermentations for the three organisms were effective at high solids loading (18% by weight), but xylose consumption in the corn-stover-hydrolyzate is seen as the major bottleneck affecting overall yield and rate of the process. Saccharomyces cerevisiae was gauged as the most "relevant strain for industrial production" for its ability to ferment both glucose and xylose from AFEX-pretreated corn-stover-hydrolyzate. The complete study is published in the open access journal, Biotechnology for Biofuels (URL above).

Related information on AFEX (Ammonia Fiber Expansion) Pretreatment http://www.biotechnologyforbiofuels.com/content/3/1/1
http://aiche.confex.com/aiche/2006/techprogram/P58580.HTM