SUMMARY: Undercooling and ice nucleation in yeast cells exposed to increasing hypertonic polyvinyl-pyrrolidone (PVP) concentrations were measured. Ice nucleation rates were analysed in terms of classical nucleation theory. Contrary to earlier reports, nucleation was found to be of the catalysed type, by active catalytic sites within the cell. With increasing PVP concentration, the nucleation temperature tends to a limiting value (approximately 236 K), whereas the homogeneous nucleation temperature of the extracellular PVP solution continues to decrease with increasing PVP concentration. The calculated parameters in the nucleation equation indicate that the nucleation mechanism within the cell is unaffected by hypertonic stress. The experimentally determined freezing kinetics of undercooled water (in oil emulsions) were found to parallel closely previously reported death rates of yeast cells as a function of temperature. The observed kinetics are compatible with a slow crystallization or precipitation of emulsifying agent at the oil/water interface, to yield catalytic sites capable of promoting ice nucleation. Experiments with water- (or yeast)-in-oil dispersions containing no emulsifying agents led to long-term freezing resistance and high recoveries of viable cells.


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