432 SCIENCE PROGRESS 



Law of Cooling, that the rate of cooling is proportional to the 

 difference in temperature between the cooling body and the 

 external medium. Stirring, by removing warmer gas or 

 liquid from the surface of the cooling body, also increases the 

 rate of freezing. As regards conductivity for heat different 

 substances vary greatly, but in practice naturally the choice 

 of a freezing medium is limited to those which can be used 

 without damaging the cooled material. The only materials 

 which have so far been employed at all on a commercial scale 

 are air, water, and brine. The great advantage of air as a 

 freezing medium obviously lies in the simplicity of the pro- 

 cedure involved in its use. It is no wonder that the vast 

 majority of food materials kept in the frozen condition are 

 frozen in cold air. But there is a great difference between air 

 on the one hand and water or brine on the other in regard to 

 heat conductivity. The conductivity of water and brine is 

 about twenty-eight times as great as that of air. Consequently 

 their capacity for removing heat from the surface of the cooled 

 body is very much greater, and so freezing will be accomplished 

 much quicker in a bath of brine than in air, other conditions 

 being the same. Generally brine has to be used, as it can be 

 obtained in the liquid condition at a temperature of — 21 ° C, 

 whereas water freezes into a solid mass at o° C. Nevertheless 

 for some food produce freezing in ice-block has been success- 

 fully employed. 



It is thus possible to vary the rapidity with which a par- 

 ticular food material is frozen, and so we are in a position to 

 examine how far differences in the freezing velocity affect the 

 loss of liquid or " drip " which occurs on thawing the frozen 

 tissue. For this purpose some experiments made with gela- 

 tine and agar-agar are very instructive. If a block of gelatine 

 jelly is frozen slowly in air at a temperature only a few degrees 

 below o° C, after thawing the water can be squeezed out of it 

 as the water can be squeezed out from a wet sponge. If, 

 however, a thin piece of gelatine gel is frozen rapidly, as, for 

 example, by freezing in mercury (which has a particularly 

 high conductivity) at say — 20 C, then, after thawing, the 

 water loss is no greater than that which takes place from 

 similar material unfrozen. The same phenomenon can be 

 observed with agar-agar gels if these are concentrated enough 

 and the freezing is carried out sufficiently rapidly. 



These experiences lead us to expect that the application 

 of more rapid freezing to the preservation of food materials 

 will overcome the difficulties resulting from changes in the 

 water relations of the tissues brought about by slower freezing 

 methods. Accounts of work in which a more rapid method of 

 freezing is employed are now available and fully bear out this' 



