Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Zymocidal strains of Saccharomyces cerevisiae excrete protein toxins which attack and kill other sensitive contaminant yeast strains that might be present during fermentation. A systematic perusal of the literature revealed that very little or no research has been done on the kinetics of cell growth and zymocidal toxin production by S. cerevisiae, and its stability characteristics. This thesis represents the first attempt to study systematically, and under controlled conditions, the kinetic parameters of the zymocidal system.;In this study, batch and continuous steady-state culture techniques were used to study the affects of pH, temperature and initial glucose concentration on the production of zymocidal toxin and biomass. In addition, gel electrophoresis methods were used to identify the plasmids present in the zymocidal S. cerevisiae strain associated with the excretion of the zymocidal toxin.;The zymocidal toxin concentration was quantitatively using the Well-test method. Correlations of the zymocidal toxin concentration versus the clearing diameter were derived from the experimental data.;The batch and continuous culture results were analyzed and correlated to develop new kinetic models for cell growth, glucose uptake, ethanol and extracellular toxin production. A new kinetic model was proposed to explain the observed reduced rate of glucose uptake by zymocidal S. cerevisiae in the presence of toxins in the fermentation medium. The new model corrects the Monod equation to account for the effects of the zymocidal toxin and immunity molecules on the cell wall. It was hypothesized that the toxin or the immunity molecule interfers with the uptake of glucose by the cell by causing an increase in the mass transfer resistance at the cell membrane. Fitting of continuous culture data to the new model suggested that a specific zymocidal toxin concentration of 11.69-Units/g of cells would be sufficient to stop all glucose uptake by the cell.;Cell-free broth was used to study the stability of the toxin at different conditions of pH and temperature. An Arrhenius temperature deactivation model was developed for the zymocidal toxin and the appropriate kinetic parameters were evaluated and correlated with respect to temperature and pH. The optimum pH range for toxin activity was found to be 4.2 to 4.6, and the thermal deactivation was found to be first order with a deactivation rate of 14.5-kcal/mole at pH = 4.5.



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