Designing simultaneous saccharification and fermentation for improved xylose conversion by a recombinant strain of Saccharomyces cerevisiae.

Wheat straw is an abundant agricultural residue which can be used as a raw material for bioethanol production. Due to the high xylan content in wheat straw, fermentation of both xylose and glucose is crucial to meet desired overall yields of ethanol. In the present work a recombinant xylose fermenting strain of Saccharomyces cerevisiae, TMB3400, cultivated aerobically on wheat straw hydrolysate, was used in simultaneous saccharification and fermentation (SSF) of steam pretreated wheat straw. The influence of fermentation strategy and temperature was studied in relation to xylose consumption, ethanol formation and by-product formation. In addition, model SSF experiments were made to further investigate the influence of temperature on xylose fermentation and by-product formation. In particular for SSF at the highest value of fibre content tested (9% water insoluble substance, WIS), it was found that a fed-batch strategy was clearly superior to the batch process in terms of ethanol yield, where the fed-batch gave 71% of the theoretical yield (based on all available sugars) in comparison to merely 59% for the batch. Higher ethanol yields, close to 80%, were obtained at a WIS-content of 7%. Xylose fermentation significantly contributed to the overall ethanol yields. The choice of temperature in the range 30-37 degrees C was found to be important, especially at higher contents of water insoluble solids (WIS). The optimum temperature was found to be 34 degrees C for the raw material and yeast strain studied. Model SSF experiments with defined medium showed strong temperature effects on the xylose uptake rate and xylitol yield.