Alcoholic fermentation of glucose and xylose by Pichia stipitis, Candida shehatae, Saccharomyces cerevisiae and Zymomonas mobilis: oxygen requirement as a key factor

Summary

To investigate simultaneous alcoholic fermentation of glucose and xylose derived from lignocellulosic material by separate or co-culture processes, the effect of oxygen transfer rate (OTR) on the fermentation of 50 g/l xylose by Pichia stipitis NRRL Y 7124 and Candida shehatae ATCC 22984, and the fermentation of 50 g/l glucose by Saccharomyces cerevisiae CBS 1200 and Zymomonas mobilis ATCC 10988 was carried out in batch cultures. The kinetic parameters of the xylose-fermenting yeasts were greatly dependent on the OTR. The optimum OTR values were found to be 3.9 and 1.75 mmol·1⁻¹·h⁻¹ for C. shehatae and P. stipitis, respectively. By contrast the fermentative parameters of S. cerevisiae were poorly affected by the OTR range tested (0.0–3.5 mmol·l⁻¹·h⁻¹) Under these conditions the ethanol yields ranged from 0.41 g·g⁻¹ to 0.45 g·g⁻¹ and the specific ethanol productivity was around 0.70 g·g⁻¹·h⁻¹. Z. mobilis gave the highest fermentative performance under strictly anaerobic conditions (medium continually flushed with nitrogen): under these conditions, the ethanol yield was 0.43 g·g⁻¹ and the average specific ethanol productivity was 2.3 g·g⁻¹·h⁻¹. Process considerations in relation to the effect of OTR on the fermentative performance of the tested strains are discussed.