The effects of osmotic pressure and ethanol on yeast viability and morphology.

The selection of a brewing yeast strain with the required fermentation and recycling characteristics is critical. The yeast strain will influence the rate and extent of fermentation, the flavour characteristics and the overall quality and stability of the finished beer, and consequently, the economic viability of the brewery. Since high gravity worts can have a deleterious effect on yeast fermentation performance, it is imperative that the strain selected be suitable for this environment, which includes a capacity to withstand high osmotic pressures and elevated ethanol levels. Under controlled in vitro osmotic and ethanol induced stresses, there was a decline in mean cell volume in both lager and ale yeast strains. Whilst significant reductions in viability were observed in the lager strains, the ale strains studied were not affected. Cell surface investigations revealed shrinkage of the yeast cells and crenation of the outside envelope under both stresses, although exposure to ethanol had a more marked effect on the yeast cell surface than sorbitol-induced elevated osmotic pressure.