A modified ghose model for batch cultures of Saccharomyces cerevisiae at high ethanol concentrations

Abstract

Saccharomyces cerevisiae has been grown on a well-defined medium under carefully controlled environmental conditions at several ethanol concentrations. Similar to earlier workers, the specific growth rate was found to decline linearly with ethanol concentration but above 78.7 g l −1, further increases in ethanol concentration had a less significant effect on the specific growth rate. In spite of this apparent switch in a key metabolic process the yield of ethanol produced per gram of glucose consumed appeared to remain constant at 0.456. The yield of biomass produced per gram of substrate consumed was found to decline linearly from a value of 0.099 once the ethanol concentration in the medium reached 40.9 g l −1. At this concentration, the cells appear to divert some of their energy previously used for biomass production into overcoming the osmotic stress caused by the high ethanol concentration. This switch in a second metabolic process also did not affect the yield coefficient for ethanol. Utilizing least squares procedures, the best ‘universal’ values of a modified Ghose kinetic growth model have been computed using the experimental data. This included an investigation of the effect of data weighting on the numerical values of the best coefficients. In this manner: (1) the maximum specific growth rate was found to be 0.476 h −1; (2) the product inhibition constant was found to be 97.9 g l −1 if the ethanol concentration was below 78.7 g l −1, but equal to 119.8 g l −1 above that concentration; and (3) the Monod constant was found to be 2.0 g l −1. Using these ‘universal’ constants, the modified Ghose model is shown to fit the kinetic data generated here and elsewhere to a good approximation.