Ethanol tolerance is decreased by fructose in Saccharomyces and non-Saccharomyces yeasts

The maximal values of osmotic (fructose) stress and ethanol tolerances dependence on hexose type present in media were quantified for a collection of yeasts isolated from mezcal covering ten different genera, including Saccharomyces. The yeasts clustered in five groups where in the least tolerant group yeasts were not able to grow at a fructose concentration above 200 g/l, as compared to yeasts in the most tolerant group that were able to grew at concentration of fructose above 700 g/l. In ethanol agar plates without a carbon source, the maximum tolerance was of 9% v/v of ethanol for all of the yeasts. When ethanol was combined with glucose (20 g/l), a number of Saccharomyces cerevisiae strains were able to grow at up to 15% v/v ethanol, whereas the maximum was 10% v/v ethanol for the non-Saccharomyces yeasts. However, when fructose was used instead of glucose, none of the yeasts tested was able to grow on plates containing above 9% v/v ethanol, including S. cerevisiae. Hence fructose did not improve the tolerance to ethanol as observed for glucose, but rather fructose acted as an inhibitor or increasing the toxicity of ethanol.