Physical Responses of Yeast Cells to Osmotic Shock

Shock exposure of the osmophilic yeasts Saccharomyces rouxii, Torulopsis globosa and T. apicola to solutions of sucrose and polyethylene glycol of MW 200 (PEG 200) at water activity ( a_w ) values between 0.88 and 0.98 caused gradual changes in cell volumes in the test solutions, though cell volumes were consistently lower in PEG 200. Non-osmophilic yeasts, Sacch. cerevisiae , bakers'yeast and Schizosaccaromyces pombé showed responses similar to the osmophiles in sucrose solutions only, as cells in PEG 200 apparently equilibrated with external osmotic pressures by uptake of solute. Optical densities of cell suspensions supported the findings on cell volumes. Viability tests at the highest and lowest solute concentrations indicated that all test organisms withstood the osmotic shock over the test period. As the theoretical relationship between a_w and osmotic pressure breaks down in concentrated solutions, it is considered that the water relations of yeasts should not be considered exclusively in terms of a_w , but also in conjunction with osmotic pressure and solute uptake.