Two of the most important parameters to reduce the production cost are the ethanol yield and concentration in the fermentation broth. High ethanol concentration requires high water insoluble solids in simultaneous saccharification and fermentation or in the enzymatic hydrolysis in separate hydrolysis and fermentation. This usually results in lower ethanol yield due to the increased inhibition and poorer mass transfer. However, the ethanol yield and concentration can also be improved by co-fermentation of glucose and xylose; especially from agricultural by-products, such as wheat straw, due to the high amount of hemicelluloses in these materials.
Saccharomyces cerevisiae TMB3400 has been improved and selected for better xylose fermentation, inhibitor- and thermotolerance. Glucose and xylose co-fermentation ability have been investigated using the original and two selected mutants in batch fermentation of steam pretreated wheat straw hydrolysate (non-diluted, 1.5 and 2 times diluted). Experiments with one of the selected strains showed improved xylose uptake, and resulted in increased ethanol yield, although xylitol formation also slightly increased. The study has now been continued with assessment of their performance in fed-batch fermentation. The results of these experiments will also be presented.