Ethanol vapour pressure as a control factor during alcoholic fermentation

Aqueous solutions of glucose/fructose mixtures with varying concentrations of ethanol were used to study the effects on fermentation of ethanol vapour pressure and water activity. Water vapour pressure was found to increase significantly with temperature in the range 15 to 30 degrees C. The effects on glucose fermentation by Saccharomyces cerevisiae Bg7FL of the variables glucose concentration, Tween 80 concentration, temperature and ammonium and ethanol concentrations were examined using central composite design. A best fit equation describing the main, quadratic and interactive effects of the five variables on yeast growth rate was produced. Further model systems were analysed in which the effects of ethanol vapour pressure, water vapour pressure and ethanol concentration on maximal growth rate of the yeast strain were studied. Above 18 degrees C, neither ethanol concentration nor ethanol vapour pressure controlled the fermentation rate. Ethanol toxicity was shown to be associated with its vapour pressure rather than its concentration.