BIOCHEMISTRY AND PHYSIOLOGY OF XYLOSE FERMENTATION BY YEASTS

The race of ethanol production and the ethanol concentrations attained by the most promising xylose-fermenting yeasts, Pichia stipitis, Cand ida shehatae, and Pachysolen tannophilus, compare poorly with that of commercial ethanol fermentation by non-xylose-fermenting Saccharomyces cerevisiae using glucose-based substrates. The oxygen requirement for efficient fermentation by the xylose-fermenting yeasts and the lack o f such a general requirement by S. cerevisiae indicates basic underlyi ng differences in their physiological relations to oxygen. The redox i mbalance in the initial conversion of xylose to xylulose, sensitivity to high concentrations of ethanol differences in the respiratory pathw ay and sensitivity to microbial inhibitors, particularly those liberat ed during pretreatment and hydrolysis of lignocellulose substrates, ha ve been identified as major factors limiting ethanol fermentation by t he xylose-fermenting yeasts. Recombinant S. cerevisiae, containing fun ctional xylose reductase and xylitol dehydrogenase, grows on, but poor ly ferments, xylose. The unfavorable kinetic properties of these enzym es and an inadequate pentose phosphate pathway apparently limit the ab ility of the recombinant yeast to ferment xylose.