A kinetic study on the oleaginous yeast, Candida curvata D
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Candida curvata D was grown in nitrogen-limited broth using both batch and single-stage continuous fermentations. In batch fermentations, the concentrations of cellular nitrogen, nonlipid biomass, and lipid increased more or less sequentially. The concentration of nonlipid biomass was proportional to the concentration of cells, and both concentrations seemed limited by the available nitrogen. A constant ratio of lipid to nonlipid biomass was observed in the batch fermentations during the initial period when most of the accumulation of nonlipid biomass occurred. The percentage of cellular nitrogen in the nonlipid biomass changed dramatically over the course of the batch fermentation and was greatly affected by the dilution rate in steady-state continuous fermentations. Lactose was used both for the accumulation of nonnitrogenous nonlipid biomass and lipid;The specific rates of change in various substrates and products were calculated with respect to the cellular nitrogen concentration. The results for batch fermentations indicated that there was a constant specific rate of lactose uptake. The specific rate of lipid accumulation which increased during the fermentation was accompanied by a parallel decrease in the specific rate of conversion of lactose to nonnitrogenous nonlipid biomass. During the later stage of the fermentation when most of the lipid accumulation occurred, a constant and maximum specific rate of lipid accumulation was observed. This observation was consistent with the hypothesis that a single enzyme step controls the rate of lipid accumulation;The batch and continuous fermentations were satisfactorily described by a kinetic model in which the specific rates of accumulation or uptake were expressed with respect to the cellular nitrogen concentration. The sets of parameters required to describe the batch fermentations and continuous fermentations were significantly different. Estimates from the model showed that a single-stage continuous fermentation required a shorter residence time for the utilization of lactose than a batch fermentation.