Fermentation of corn ®bre sugars by an engineered xylose utilizing Saccharomyces yeast strain

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 ®bre hydrolysates has been examined. Saccharomyces strain 1400 (pLNH32) was genetically engineered to ferment xylose by expressing genes encoding a xylose reductase, a xylitol dehydrogenase and a xylulose kinase. The recombinant ef®ciently fermented xylose alone or in the presence of glucose. Xylose-grown cultures had very little difference in xylitol accumulation, with only 4 to 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 mixture of glucose (80 g/l) and xylose (40 g/l), this strain produced 52 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 ethanol/l, a 90% yield (excluding the arabinose in the calculation since it is not fermented). Approximately 98% theoretical yield, or 21 g ethanol/l, was achieved using an enzymatic hydrolysate of ammonia ®bre exploded corn ®bre containing an estimated 47.0 g mixed sugars/l. In all mixed sugar fermentations, less than 25% arabinose was consumed and converted into arabitol.