MACROAPPROACH KINETICS OF ETHANOL FERMENTATION BY SACCHAROMYCES-CEREVISIAE - EXPERIMENTAL STUDIES AND MATHEMATICAL-MODELING

The kinetics of ethanol fermentation by Saccharomyces cerevisiae was studied both experimentally and theoretically. Batch fermentations were performed anaerobically on synthetic media with an initial sucrose content of 130-210 g dm-3 under both controlled and uncontrolled pH conditions. A series of ten unstructured models involving microbial growth, substrate utilization and ethanol formation were applied to the data. The models were based on a formal macroapproach and either Pirt's or Herbert's concept of microbial energetics. The model giving the best fit to experimental data at constant pH satisfied Herbert's concept of endogenous metabolism and included an exponential term responsible for the effect of ethanol inhibition on the growth rate. A correction factor was introduced to account for the effect of pH on the biomass growth rate in fermentations with no pH control. A linear relationship between hydrogen ion consumption rate and growth rate was assumed and confirmed experimentally. Finally, it is shown that models identified exclusively by fitting the concentration data might result in a very poor estimate of the ethanol yield coefficient Y(P/S) at varying growth rates. In order to make the kinetic equations stoichiometrically consistent and hence to have an idea of what type of constraint should be used for the purpose of parameter estimation, a biochemically structured model based on the Roels approach was developed. Two parameter constraints, one resulting from fermentation stoichiometry and another from the yield function monotonicity, are proposed.