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 cultur e 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 cu lture in molasses did not reach the D-r value. When the reducing sugar conc entration in the feed was 100 g l(-1), the Dr was 0.15 h(-1) for glucose an d 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 obse rved were a decrease in the biomass yield coefficient per gram of oxygen co nsumed (Y-O2) and a sudden increase in the specific oxygen consumption rate (qO(2)) for each substrate. Therefore, at growth rates above D-r, in a hig h-cell-density culture, C utilis acquired a flexible catabolism directed to wards alternative fermentation routes. The D at which metabolic changes too k place seemed to depend on the nature and concentration of the carbon sour ce. Biomass productivity was higher with molasses than with glucose when th e fermenter was operated at high D values.