Starch gelatinization corresponds to a melting phase transition in aqueous medium. Such a transition involves important mass transfer of water. Using a high-pressure bomb including optical ports, the volume variation of starch granules in suspension was related to gelatinization during a high-pressure treatment up to 420 MPa. Microscopic observations of wheat and potato starch granules were compared with macroscopic measurements of compressibility up to 600 MPa and gelatinization intensity using differential scanning calorimetry on treated suspensions. Wheat starch gelatinization started below 300 MPa and was completely achieved at 600 MPa. Potato starch was not altered under 600 MPa. The behavior of the volume variation of starch granules under pressure compared with starch suspensions compressibility could be explained by simultaneous compression and hydration mechanisms. Keywords: Starch gelatinization; high hydrostatic pressure; image analysis

This work studied the viabilities of five types of cells (two yeast cells, Saccharomyces cerevisiae CBS 1171 and Candida utilis; two bacterial strains, Escherichia coli and Lactobacillus plantarum; and one human leukemia K562 cell) as a function of cooling rate during freezing. The range of investigated cooling rates extended from 5 to 30,000°C/min. Cell viability was classified into three ranges: (i) high viability for low cooling rates (5 to 180°C/min), which allow cell water outflow to occur completely and do not allow any intracellular crystallization; (ii) low viability for rapid cooling rates (180 to 5,000°C/min), which allow the heat flow to prevail over water outflow (in this case, cell water crystallization would occur as water was flowing out of the cell); (iii) high viability for very high cooling rates (>5,000°C/min), which allow the heat flow to be very rapid and induce intracellular crystallization and/or vitrification before any water outflow from the cell. Finally, an assumption relating cell death to the cell water crystallization as water is flowing out of the cell is made. In addition, this general cell behavior is different for each type of cell and seems to be moderated by the cell size, the water permeability properties, and the presence of a cell wall.The freeze-thawing process remains the principal method of cell preservation to date, and the high survival rates achieved by this method are of interest from both the biophysical and practical points of view. This is to ensure that the recovery of entire cell populations is free from the risk of possible subsequent alteration of its genetic composition.Cell cryopreservation, which is commonly used in the food and pharmaceutical industries, requires optimization for each type of microorganism. Moreover, each type of cell has its own protocol for freezing. Numerous researchers have attempted to develop methods that permit 100% preservation of freezethawing of diverse cellular specimens (3, 6), but some microorganisms cannot yet be preserved by freezing.For a better cell preservation some cryoprotectants such as glycerol or dimethyl sulfoxide can be used (8). These molecules improve the cell preservation by minimizing the cell water content (6) and/or supporting the vitrification occurrence (1) and finally by protecting the cell's constitutive macromolecules (2, 5).The freeze-thawing process constitutes a double stress for the cell, i.e., thermal and hyperosmotic stresses, which act simultaneously during cooling (15, 16). The scenario of cell evolution during slow freezing is well known (13). The water surrounding the cell freezes before the cell contents, because the cytoplasm is more concentrated than the growth medium, and because thermodynamically, the component with the largest volume will nucleate first (6, 16). This freezing increases the osmotic pressure of the medium, and the extracellular solutes become concentrated in the remaining liquid extracellular water. Consecutive osmosis will then dehydrate cells as water diffuses from the cy...

The effects of combined cold, acid and ethanol on the membrane physical state and on the survival of Oenococcus oeni were investigated. Membrane fluidity was monitored on intact whole O. oeni cells subjected to single and combined cold, acid and ethanol shocks by using fluorescence anisotropy with 1,6-diphenyl-1,3,5-hexatriene (DPH) as a probe. Results showed that cold shocks (14 and 8 degrees C) strongly rigidified plasma membrane but did not affect cell survival. In contrast, ethanol shocks (10-14% v/v) induced instantaneous membrane fluidisation followed by rigidification and resulted in low viability. Acid shocks (pH 4.0 and pH 3.0) exerted a rigidifying effect on membrane without affecting cell viability. Whatever the shock orders, combined cold (14 degrees C) and ethanol (14% v/v) shocks resulted in strong membrane rigidification. Interestingly, O. oeni survived combined cold and ethanol shocks more efficiently than single ethanol shock. Membrane rigidification was induced by ethanol-and-acid (10% v/v - pH 3.5) shock and correlated with total cell death. In contrast, O. oeni recovered its viability when subjected to cold (8 degrees C)-then-ethanol-and-acid shock which strongly rigidified the membrane. Our results suggested a positive short-term effect of combined cold, acid and ethanol shocks on membrane fluidity and viability of O. oeni.

Membrane fluidity in whole cells of Saccharomyces cerevisiae W303-1A was estimated from fluorescence polarization measurements using the membrane probe, 1,6-diphenyl-1,3,5-hexatriene, over a wide range of temperatures (6-35 degrees C) and at seven levels of osmotic pressure between 1.38 MPa and 133.1 MPa. An increase in phase transition temperatures was observed with increasing osmotic pressure. At 1.38 MPa, a phase transition temperature of 12 +/- 2 degrees C was observed, which increased to 17 +/- 4 degrees C at 43.7 MPa, 21+/- 7 degrees C at 61.8 MPa, and 24 +/- 9 degrees C at an osmotic pressure of 133.1 MPa. From these results we infer that, with increases in osmotic pressure, the change in phospholipid conformation occurs over a larger temperature range. These results allow the representation of membrane fluidity as a function of temperature and osmotic pressure. Osmotic shocks were applied at two levels of osmotic pressure and at nine temperatures, in order to relate membrane conformation to cell viability.

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I. POIRER, P.-A. MARÉ CH A L, S. R IC HA R D A ND P . G ER V AI S. 1999. The effects of rehydration kinetics and temperature on the viability of Saccharomyces cerevisiae dehydrated by drying were studied. During rehydration, a water activity range of 0·117-0·455 must be crossed slowly in order to maintain cell viability. If this range is crossed rapidly, cell viability can be preserved if rehydration takes place at 50°C. Several hypotheses have been proposed to explain previous results. One hypothesis, which relates cell mortality after rapid rehydration to water flow through the membrane in phase transition, is the more plausible and requires further investigation.

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