(688b) Quantifying the Effects of Oxygen Utilization Rate on Ethanol Production By S. Stipitis Under Controlled Chemostat

Quantifying the effects of oxygen utilization rate on ethanol production by S. stipitis under controlled chemostat

Min Hea Kim¹, Q. Peter He² and Jin Wang¹

(1)Department of Chemical Engineering, Auburn University, Auburn, AL 36849, USA

(2)Department of Chemical Engineering, Tuskegee University, Tuskegee, AL 36088, USA

  Abstract

Lignocellulosic materials offer a potential source of carbon substrates (glucose and xylose) 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, the xylose fermentation performance depends significantly on the oxygenation level of the culture. High aeration rate results in fast cell growth and acetic acid production, while very low aeration (oxygen-limited) often results in xylitol production, both at the expense of reduced ethanol production. Only optimized microaerobic condition promotes ethanol production by maintaining cell viability and NADH balance. Hence, it is critical to determine the optimal oxygen utilization rate (OUR) for ethanol production by S. stipitis. In order to quantitatively study the effect of OUR on the fermentation performance, accurate control of OUR is essential. Several studies have been reported on the optimum oxygenation conditions for ethanol fermentation by S. stipitis [1-4]. Among these studies, most of the experiments were carried out using batch cultures grown in flasks where the Oxygen Transfer Rate (OTR) and/or Dissolved Oxygen (DO) were not effectively controlled. Different OTR levels have been tested simply by changing volume of media, airflow rate, and agitation speed. However, our research shows that the inaccurate control of OTR/OUR is problematic in these studies. In addition, our research indicates that the optimum OUR alone (i.e., without information on carbon utilization) does not correlate well with ethanol yield. Our research shows that with the same OUR condition tested under controlled chemostat, the ethanol yield changes due to the change of metabolic status or phenotypes of S. stipitis. Different phenotypes exist under a single OUR condition, which was verified by Principal Component Analysis (PCA) which enables the extraction of correlations between different cellular physiology with respect to  carbon uptake and OUR. This helps us understand and quantify the metabolic mechanism of OUR in continuous fermentation. References

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