|Skory, Christopher - Chris|
Submitted to: World Journal of Microbiology and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/23/1996
Publication Date: N/A
Interpretive Summary: Corn fiber obtained as a byproduct of the corn milling process represents an abundant renewable resource that can be used for the manufacture of fuel ethanol. However, the corn fiber must be first broken down into simple sugars that yeast can then ferment to ethanol. Saccharomyces cerevisiae or baker's yeast, which is most often used for alcohol fermentations, is unable to utilize one of the predominant sugars, xylose, obtained in this breakdown step. In order for the conversion of corn fiber to ethanol to be economically feasible, all of the sugars including xylose, must be fermented by the organism. This yeast was genetically modified with xylose utilizing genes from another organism so that the recombinant Saccharomyces strain could now use xylose for the fermentation of ethanol. In this study, the modified yeast strain was examined under many different growth condition (e.g., aeration, pH, inoculum level) to determine how to obtain optimal ethanol production from sugars found in the corn fiber. In one experiment, sugars derived from corn fiber were fermented to ethanol with approximately 98% theoretical efficiency. This study clearly demonstrates the efficacy of xylose utilization and fermentation for this recombinant Saccharomyces strain. With slight modifications to improve the genetic stability of introduced genes, this strain could represent a significant advancement for the biofuels industry.
Technical Abstract: This study examined the ability of a recombinant Saccharomyces yeast strain to ferment the sugars glucose, xylose, arabinose, and galactose which are the predominant monosaccharides found in corn fiber hydrolysates. Saccharomyces strain 1400 (pLNH32) was genetically engineered to ferment the pentose sugar xylose by expressing genes encoding a xylose reductase, a xylitol dehydrogenase and a xylulose kinase. The recombinant was shown to efficiently ferment xylose alone or in the presence of glucose. Xylose grown cultures had very little difference in xylitol accumulation, with only 4-5 g/l accumulating, in aerobic, micro-aerated and anaerobic conditions. Highest production of ethanol with all sugars was achieved under anaerobic conditions. From a glucose (80 g/l) and xylose (40 g/l) mixture, this strain produced 52.0 g/l ethanol, equivalent to 85% of theoretical yield, in less than 24 h. Using a mixture of glucose (31 g/l), xylose (15.2 g/l), arabinose (10.5 g/l) and galactose (2 g/l), all of the sugars except arabinose were consumed in 24 h with an accumulation of 22 g/l of ethanol, or 90% yield (excluding the arabinose in the calculation since it is not fermented). Approximately 98% theoretical yield, or 21.0 g/l ethanol, was achieved using an enzymatic hydrolysate of AFEX pretreated corn fiber containing an estimated 47.0 g/l mixed sugars. In all mixed sugar fermentations, less than 25% of the arabinose was consumed and converted into arabitol.