Fermentation of Glucose/Xylose/Xylulose in the Presence of Furfural by Yeast for Ethanol Production

Abstract. Yeast xylose utilization could be increased by isomerization of xylose to xylulose. In experiments using synthetic media containing glucose, xylose, and xylulose, xylose utilization was prevented and as a result xylitol production was very low. Glucose and xylulose utilization were 99.95% and 40.65%, respectively, for Saccharomyces cerevisiae and 100% and 49.15%, respectively, for Schizosaccharomyces pombe in the absence of furfural. Ethanol yield on sugars was higher at 0.5 g furfural L^-1 (0.487 g ethanol g^-1 sugars) compared to 0.0 g L^-1 (0.450 g ethanol g^-1 sugars) and 2.5 g L^-1 (0.452 g ethanol g^-1 sugars) furfural concentration for S. cerevisiae. However, for S. pombe, a xylose-consuming yeast, ethanol yields (0.426 to 0.518 g ethanol g^-1 sugar) and final glycerol concentrations (3.79 to 5.42 g L^-1) both increased with increasing furfural concentrations (0.0 to 2.5 g L^-1). The kinetic model parameter values confirmed this observation. A higher inoculum-to-sugar ratio resulted in higher average rates of utilization of sugars: from 0.089 to 0.124 g xylulose L^-1 h^-1 (calculated for 120 h, as xylulose was not completely consumed during 120 h fermentation) and from 1.38 to 1.54 g glucose L^-1 h^-1 (calculated for the first 24 h, during which most of the glucose was consumed). The flux balance analysis (FBA) model simulation suggests an increase of ethanol of 1.047 mmol h^-1 g^-1 biomass due to the consumption of xylulose under the given conditions, and detoxifying furfural reduces the specific growth rate from 0.048 to 0.00043 g g^-1 biomass h^-1.