Effect of controlled oxygen limitation on Candida shehatae physiology for ethanol production from xylose and glucose
Résumé
Carbon distribution and kinetics of Candida shehatae were studied in fed-batch fermentation with xylose or glucose (separately) as the carbon source in mineral medium. The fermentations were carried out in two phases, an aerobic phase dedicated to growth followed by an oxygen limitation phase dedicated to ethanol production. Oxygen limitation was quantified with an average specific oxygen uptake rate (OUR) varying between 0.30 and 2.48 mmolO2 g dry cell weight (DCW)−1 h−1, the maximum value before the aerobic shift. The relations among respiration, growth, ethanol production and polyol production were investigated. It appeared that ethanol was produced to provide energy, and polyols (arabitol, ribitol, glycerol and xylitol) were produced to reoxidize NADH from assimilatory reactions and from the co-factor imbalance of the two-first enzymatic steps of xylose uptake. Hence, to manage carbon flux to ethanol production, oxygen limitation was a major controlled parameter; an oxygen limitation corresponding to an average specific OUR of 1.19 mmolO2 g DCW−1 h−1 allowed maximization of the ethanol yield over xylose (0.327 g g−1), the average productivity (2.2 g l−1 h−1) and the ethanol final titer (48.81 g l−1). For glucose fermentation, the ethanol yield over glucose was the highest (0.411 g g−1) when the specific OUR was low, corresponding to an average specific OUR of 0.30 mmolO2 g DCW−1 h−1, whereas the average ethanol productivity and ethanol final titer reached the maximum values of 1.81 g l−1 h−1 and 54.19 g l−1 when the specific OUR was the highest.