REFRIGERATION OF FISH 



597 



tion. Lower temperatures are possible with solutions of other sub- 

 stances. With calcium-chloride brine a temperature of 67° below 

 zero is possible, and 28.5° below zero may be attained with magnesium 

 chloride. These brines and other liquids of low freezing point can be 

 used provided the fish are protected from direct contact with them by 

 being inclosed in some impervious container, preferably a good con- 

 ductor of heat. When fish are so frozen, the surface is free from 

 any trace of foreign salt and may be glazed as easily as air-frozen 

 fish. Numerous methods based on these facts have been proposed or 

 practiced from time to time, and some of the more promising ones 

 are in successful practical use. 



As will be seen in the discussion, the difficulties in freezing in cans 

 or molds arise from (a) less perfect contact of the fish with the brine 

 than is obtained in direct brine freezing, and (b) lack of flexibility 

 because of difficulty of making cans or molds that conform to the 

 shapes of many varieties of fish. 



Usually several fish are packed close together in the mold. The 

 surfaces in contact with one another are, of course, not effective for 

 heat transfer, only the outer surfaces of the mass as a whole serving 

 this purpose. This outer surface, not being uniformly flat, generally 

 has incomplete contact with the walls of the mold. The rate of freez- 

 ing in molds, therefore, is considerably lower for the same brine tem- 

 perature than it is where the fish are in direct contact with the brine. 

 To overcome this difficulty, recourse is had to the lower temperatures 

 that are possible with calcium chloride brine. An objection to this 

 procedure lies in the diminished efficiency of refrigeration machinery 

 as lower temperatures are reached, expressed in terms of tons of 

 refrigeration per horsepower, as it is generally recognized that 

 refrigeration machinery becomes less efficient as the temperature 

 lowers. 



In ideal refrigerating machines the amount of work done is pro- 

 portional to the difference between the temperature at which heat 

 is absorbed and that at which it is rejected. We may say that the 

 steeper the grade up which the heat must be pushed the more work 

 is required to push it. The efficiency is expressed mathematically 

 by the ratio T 1 -^-(T 2 —T 1 ) where T x is the cold side and T 2 the 

 warm side, expressed in absolute temperature units. Table 24 shows 

 these ratios for several temperatures commonly dealt with in 

 refrigeration. 



Table 24. — Ideal efficiency of refrigerating machines 



