Ethanol tolerance of thermotolerant yeasts cultivated on mixtures of sucrose and ethanol

Abstract

The final levels of ethanol (levels of ethanol produced plus that added initially to the media) reached by the thermotolerant yeasts were highest (16.5–20.3%, v/v) at 8% initial ethanol. The thermotolerant yeasts were found to have the following characteristics: constant levels of ethanol formation (10.5–12.3%, v/v), for additions of external ethanol within the range 2–8% (v/v) of initial ethanol; constant values of product coefficients when initial ethanol was in the range of 2–6%, which increased or decreased, depending on the strain, when initial ethanol exceeded 6%; growth activity was inhibited at different levels of addition of external ethanol when final biomass and specific rate of growth were compared; significant differences among the yeast strains in the amount of external ethanol capable of reducing biomass formation by one half. In addition, the viability of the strains (early stationary phase) varied with the amount of external ethanol, the lowest viabilities occurring at concentrations of initial ethanol ranging from 4 to 7% and the highest in the range of 7 to 8% (v/v). The relative levels of trehalose (with/without 7% ethanol added initially) in the yeast strains (the stationary phase) ranged from 1.03 to 1.75, suggesting that the effect of produced ethanol on trehalose accumulation was stronger than that of external ethanol. The levels of final ethanol shown by the yeast strains were also correlated with the cellular levels of glycerol-3-phosphate dehydrogenase (increase in enzyme levels with decrease in final ethanol) for cells harvested at the stationary phase.