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

The kinetics of ethanol fermentation by Saccharomyces cerevisiae was s tudied both experimentally and theoretically. Batch fermentations were performed anaerobically on synthetic media with an initial sucrose co ntent of 130-210 g dm-3 under both controlled and uncontrolled pH cond itions. 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 fo r the effect of ethanol inhibition on the growth rate. A correction fa ctor was introduced to account for the effect of pH on the biomass gro wth rate in fermentations with no pH control. A linear relationship be tween hydrogen ion consumption rate and growth rate was assumed and co nfirmed experimentally. Finally, it is shown that models identified ex clusively by fitting the concentration data might result in a very poo r estimate of the ethanol yield coefficient Y(P/S) at varying growth r ates. In order to make the kinetic equations stoichiometrically consis tent and hence to have an idea of what type of constraint should be us ed for the purpose of parameter estimation, a biochemically structured model based on the Roels approach was developed. Two parameter constr aints, one resulting from fermentation stoichiometry and another from the yield function monotonicity, are proposed.