FERMENTATION OF CORN FIBER 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 predomi nant monosaccharides found in corn fibre hydrolysates has been examine d. Saccharomyces strain 1400 (pLNH32) was genetically engineered to fe rment xylose by expressing genes encoding a xylose reductase, a xylito l dehydrogenase and a xylulose kinase. The recombinant efficiently fer mented xylose alone or in the presence of glucose. Xylose-grown cultur es 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 ana erobic 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 theoret ical yield, in less than 24 h. Using a mixture of glucose (31 g/l), xy lose (15.2 g/l), arabinose (10.5 g/l) and galactose (2 g/l), all of th e sugars except arabinose were consumed in 24 h with an accumulation o f 22 g ethanol/l, a 90% yield (excluding the arabinose in the calculat ion since it is not fermented). Approximately 98% theoretical yield, o r 21 g ethanol/l, was achieved using an enzymatic hydrolysate of ammon ia fibre exploded corn fibre containing an estimated 47.0 g mixed suga rs/l, In all mixed sugar fermentations, less than 25% arabinose was co nsumed and converted into arabitol.