212 EARL W. F L O S D O U F 



less rich in solute than the remaining; liquid phase. In the case of 

 cellular materials, disruption of cell walls and the like is usually 

 avoided. Protein molecules, which frequently are large and rela- 

 tively labile, are locked in place as a result of quick-freezing and de- 

 naturation does not occur. Viability of microorganisms is main- 

 tained. Enzymic changes are retarded if the product is frozen 

 quickly. In a word, quick-freezing is applied for the basic purpose of 

 avoiding change. 



The speed of freezing of biological materials depends upon several 

 factors in addition to the temperature of the refrigerant. These addi- 

 tional factors control the speed of freezing by influencing the rate of 

 transfer of the latent heat of fusion from the material being frozen 

 to the refrigerant. Of obvious importance is the extent of the surface 

 exposed to the source of low temperature. The existence of any in- 

 sulating barrier is a deterrent to rapid freezing, although many sub- 

 stances must be protected by a satisfactory type of container of 

 metal, glass, or other material. Ice itself forms an insulating layer 

 that retards further freezing. Heat transfer at a liquid-liquid inter- 

 face is more rapid than at a liquid-gas or solid-gas boundary. The 

 effect of the latter factor on quick-freezing has been discussed by 

 Goetz (1), who pointed out that heat transfer from a warm object to 

 liquid air can be slow in spite of the large temperature gradient, owing 

 to the formation of a gaseous layer between the liquid air and the ma- 

 terial being frozen. With liquids, agitation or stirring is beneficial. 



B. PRINCIPLE OF DEHYDRATING FROZEN MATERIALS 



The objectives and principles involved in the dehydration of 

 frozen biological materials have been discussed in detail in a recent 

 book by the author (2). These may be summarized as follows. 



First, the temperature is below that at which many labile sub- 

 stances undergo chemical change. This applies to labile components 

 in blood, to viruses and most forms of microorganisms, and to other 

 biologicals and pharmaceuticals. 



Second, because of the low temperature, the loss of volatile con- 

 stituents is minimal. This is particularly important in application to 

 many foods like orange juice and pineapple juice. 



Third, since the product is frozen, there is no bubbling or foaming. 

 Thus, changes due to surface action such as the surface denaturation 



