Production of xylitol by yeast fed with formic/acetic acid organosolv treated wheat straw

The C5 fraction generated from formic and acetic acid organosolv processes provides a good source of xylose for xylitol production, but the high formic acid content (e.g. 50–100 g L⁻¹) which remains in the fraction is particularly challenging for microbial processes. Although Saccharomyces cerevisiae and Pichia kudriavzevii were able to grow and produce some xylitol in medium containing ~20 g L⁻¹ formate at pH 6, this required dilution of the C5 fraction and limited the amount of xylitol produced. By expressing both xylose reductase and formate dehydrogenase genes in S. cerevisiae or P. kudriavzevii, and providing C5 fraction only in the feed, the strains tolerated C5 fraction containing up to 65 g L⁻¹ formic acid in fed batch culture at pH 5.5 to 6.0 and produced 53 to 112 g L⁻¹ xylitol. Formate was oxidized by the strains producing formate dehydrogenase as it was fed into the cultures so that formate concentrations remained low and the cells continued to produce xylitol. The production of xylitol was dependent on the concentration of xylose in the feed, which needed to be >200 g L⁻¹ in order to achieve high (> 100 g L⁻¹) xylitol concentrations. S. cerevisiae H4031 was more productive than P. kudriavzevii H4496 in the C5 fraction, although H4496 had high titre (155 g L⁻¹) and production rate (1.6 g L⁻¹ h⁻¹) when provided pure xylose.