Oxygen utilization rate (OUR) alone does not define the optimal condition for ethanol production with Scheffersomyces stipitis

Lignocellulosic materials offer a potential source of carbon substrates for the production of ethanol by fermentation. Scheffersomyces stipitis is a native yeast strain best capable of utilizing xylose to ethanol. Since S. stipitis is a respiratory yeast strain, its fermentation performance depends significantly on the OUR. It has been reported that microaerobic condition promotes ethanol production by maintaining cell viability and NADH balance, while too high or too low OUR conditions would result in either biomass and/or byproduct accumulation. Many studies have been reported on the optimum oxygenation conditions for ethanol fermentation by S. stipitis, but the reported values are inconsistent. One reason for such inconsistency is that accurate control of either OTR or OUR was not implemented in many of these studies. In fact, most of them did not measure the OUR condition during the experiment. To quantitatively study and better understand the effect of OUR on the performance, we conducted experiments under chemostat operation that enables accurately controlled OUR conditions. In addition, our results suggest that OUR condition alone (i.e., without information on carbon utilization) does not fully define the condition for optimal ethanol production. It was found that under the same OUR conditions tested at different times under controlled chemostat, the ethanol yield can be quite different due to different metabolic states of S. stipitis. Different phenotypes could exist under a single OUR condition. To better define the optimal condition for xylose fermentation by S. stipitis, ratio between carbon and oxygen uptake is suggested.