Effect of high-cell-density fermentation of Candida utilis on kinetic parameters and the shift to respiro-fermentative metabolism
Candida utilis NRRLY-900 was grown in a high-cell-density continuous culture without oxygen limitation. Glucose or molasses was used as carbon source at 30 g l–1 or 100 g (reducing sugars) l–1. At 30 g glucose l–1, the dilution rate (D) immediately before the change in respiratory metabolism (D r) was approximately 0.40 h–1. At this value of D, the corresponding culture in molasses did not reach the D r value. When the reducing sugar concentration in the feed was 100 g l–1, the D r was 0.15 h–1 for glucose and 0.3 h–1 for molasses. When D>D r, accumulation of ethanol and organic acids occurred, due to physiological changes in C. utilis. The changes observed were a decrease in the biomass yield coefficient per gram of oxygen consumed (Y O2) and a sudden increase in the specific oxygen consumption rate (qO2) for each substrate. Therefore, at growth rates above D r, in a high-cell-density culture, C. utilis acquired a flexible catabolism directed towards alternative fermentation routes. The D at which metabolic changes took place seemed to depend on the nature and concentration of the carbon source. Biomass productivity was higher with molasses than with glucose when the fermenter was operated at high D values.
KeywordsBiomass Fermentation Carbon Source Biomass Yield Dilution Rate
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