Effect of Product Inhibition on Xylose Fermentation to Ethanol by Saccharomyces cerevisae 424A (LNH-ST)

Commercially viable manufacturing of cellulosic ethanol requires high titers of product from both hexose and pentose fermentation. This article reports inhibition of initial specific xylose consumption rates and ethanol generation in batch fermentations of xylose using S. cerevisiae 424A (LNH-ST), a genetically modified strain capable of co-fermenting glucose and xylose. The fermentations were carried out in yeast extract peptone (YEP) medium in the presence of varying concentrations of ethanol (0% to 9% w/v) either added in a single dose or generated in situ by the yeast from glucose prior to xylose fermentation. The Levenspiel inhibition function was used to model inhibition of initial specific xylose consumption rates by ethanol. When ethanol was added in a single dose, the yeast ceased xylose fermentation when ethanol concentration reached 90 g L-1. However, when ethanol was generated in situ from glucose, the maximum final titer of ethanol was 110 g L-1. Comparing the effect of ethanol on xylose fermentation to the effect of ethanol on HXT transport of xylose in non-xylose-fermenting yeast suggests that inhibition of xylose transport into the cell is rate-limiting for fermentation.