095 Hyperbaric theory of cryoinjury of microorganisms

Abstract

It has been shown that cells are captured by a growing ice crystal, forming inclusions, if the speed of crystallization front is higher than a critical speed, which usually does not exceed 10 μm/s. Further freezing of water leads to an increase of the hydrostatic pressure in the inclusions and to cell dehydration. Both of these processes depend on the rate of plastic relaxation of stress in the ice crystal near the inclusions. It is shown that the diffusion mechanism of plastic relaxation in ice is not sufficiently rapid to prevent the development of high (up to 108 Pa) pressures in inclusions at cooling rates greater than 3 °C/min. The average distance between dislocations in ice is much greater than the cell (inclusion) radius. So, there are no dislocations in the vicinity of inclusions, where the stress is large enough to create a plastic deformation. Hence, an expansion of the plastic relaxation region in an ice crystal containing small dilatating inclusions is possible only if the average tensile stress is more than the shear stress. The average tensile stress is equal to the product of twice the shear modulus, and the power and the dilatation center concentration. As shown by our theoretical analysis, the process of plastic relaxation of pressure in small dilatating inclusions in ice is percolating in character. Percolation begins when the volume concentration of cells is more than 0.1%. This unusual prediction agrees with the dependence of the survival rate of slowly frozen Streptococcus cremoris and Escherichia coli on the cell concentration. So, one can conclude that high hydrostatic pressure is a strong damaging factor for these microorganisms. The theory also suggests the possibility of cell protection from high pressure effects during freezing by adding neutral latex particles or by substantially decreasing the medium tonicity. This is also confirmed by experimental results. Source of funding: Universal Stabilization Technologies, Inc. Conflict of interest: None declared. victorb@ustsd.com