The Significance of the Aqueous-Ice Phase Transformation during Controlled Rate Cooling of Biological Specimens*

The controlled cooling of biological materials to subzero temperatures involves passage through several phases: the liquid aqueous phase, the aqueous to ice transformation or heat of fusion phase, anti the solid ice phase. Cooling rate in the liquid phase prior to entering the heat of fusion-transformation phase appears to be of least significance in most biological systems. The time in-volved for liquid to solid (aqueous to ice) phase transformation and concomitant latent heat of fusion removal appears to be of considerable biological significance. Decreasing biological viability is obtained as the duration of the specimen in the heat of fusion phase is prolonged, presumably due to prolonged exposure to dehydration effects of water-ice crystals and increased concentration of solutes. Rate of heat removal in the post-heat of fusion solid state is also important in maintaining viability of the biological specimen. Experimental biological evidence supports these contentions and provides a basis upon which to formulate and verify some basic concepts of freezing injury