Short communicationInfluence of thermal and osmotic stresses on the viability of the yeast Saccharomyces cerevisiae
Introduction
The preservation of biological systems is currently achieved by decreasing the water content and hence the water activity of the medium. The classical dehydration processes are: freeze-drying, spray-drying and thermal drying. These processes change the physical state of water by the variation of temperature or pressure or by the combined action of these parameters. Therefore, cells dehydrated using such methods are subjected to a simultaneous variation of temperature and water activity.
We first carried out experiments where the effects of water activity and temperature variation rates on cell survival were separately studied. For this purpose we subjected yeast cells to isothermal osmotic dehydration, i.e. by solute addition, or to iso-osmotic heating. Finally, we studied the combined effects of temperature and water activity variation on cell viability.
Section snippets
Strains and growth conditions
Saccharomyces cerevisiae CBS 1171 was grown in 250-ml conical flasks containing 100 ml of a modified malt Wickerham medium (Gervais et al., 1992, Gervais and Martı́nez de Marañón, 1995). Cultures were held at 25°C on a rotary shaker and allowed to grow to early stationary phase.
Viability measurement
Cell viability was determined by vital staining (methylene blue stain) and microscopy as previously described (Martı́nez de Marañón et al., 1999). Briefly, the yeast suspension was mixed (v/v) with a methylene blue
Effect of isothermal osmotic dehydration
In a previous work (Marechal and Gervais, 1994), the rate of osmotic pressure increase was found to have a great effect on yeasts viability. S. cerevisiae can survive at very high levels of osmotic pressure (Π=100 MPa) when the extracellular osmotic pressure is increased slowly (1.6 MPa min−1) up to 100 MPa. On the other hand, cell viability drastically decreases when yeast cells are suddenly placed (hyperosmotic shock, i.e. fast kinetics of osmotic pressure increase) in a hyperosmotic solution
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