Influence of cooling rate on Saccharomyces cerevisiae destruction during freezing:: unexpected viability at ultra-rapid cooling rates

The purpose of this work was to study cell viability as a function of cooling rate during freezing. Cooling rate strongly influences the viability of cells during cold thermal stress. One of the particularities of this study was to investigate a large range of cooling rates and particularly very rapid cooling rates (i.e., faster than 20 000 degreesC min(-1)). Four distinct ranges of cooling rates were identified. The first range (A') corresponds to very slow cooling rates (less than 5 degreesC min(-1)), and results in high cell mortality. The second range (A) corresponds to low cooling rates (5-100 degreesC min(-1)), at which cell water outflow occurs slowly and does not damage the cells. The third range (13) corresponds to rapid cooling rates (100-2000degreesC min(-1)), at which there is competition between heat flow and water flow. In this case, massive water outflow, which is related to the increase in extracellular osmotic pressure and the membrane-lipid phase transition, can cause cell death. The fourth range (C) corresponds to very high cooling rates (more than 5000 degreesC min(-1)), at which the heat flow is very rapid and partially prevents water exit, which seems to preserve cell viability. (C) 2002 Elsevier Science (USA). All rights reserved.