598 U. S. BUREAU OF FISHERIES 



To take an example : If brine is to be maintained at 25° F. below 

 zero, the temperature of the evaporating ammonia may be 30° below 

 zero. This, expressed in absolute units (238.59°), is 7\. If the 

 condensed water is 75° F., we must maintain a temperature of, say, 

 80° in the compressed ammonia gas. This, also expressed in absolute 

 units (299.64°), is T 2 . The efficiency, T 1 -^-(T 2 —T 1 ), is 3.9; that is, 

 3.9 times as much energy will be handled in the form of heat as is 

 expended in the form of work. If a temperature of 5° F. below zero 

 is to be maintained, we may evaporate the ammonia at 10° below 

 zero. If the condenser water is still 75° and the compressed am- 

 monia 80°, the efficiency by ratio will be 5.3. At 25° below zero brine 

 temperature the ideal efficiency is only about 74 per cent of what it 

 would be at 5° below zero brine. This neglects actual inefficiency in 

 the machine itself, friction, losses, and smaller amount of gas handled 

 per piston stroke. 



Apart from the diminished efficiency of the machinery and the 

 economic factors involved, it has been pointed out by Plank 6G that 

 these lower temperatures are objectionable in their effect on the tis- 

 sues of the fish. The juices of fish contain mineral salts and colloid 

 substances. Because of their presence a very low temperature 67 

 (around 75° C.) is necessary to cause all the water to freeze solid. At 

 a temperature warmer than this more and more of the water is left 

 in the tissue substance. If a fish originally containing 75 per cent 

 of water is frozen at 5° F., there remains about 13 per cent water of 

 the 75 per cent still in the tissues unfrozen; while if frozen at 31° F. 

 below zero only 2 per cent would remain — that is, the tissues would 

 be almost completely dehydrated and would return, if at all, with 

 much more difficulty to their original condition on defrosting. For 

 these reasons Professor Plank recommends that speed in freezing 

 be secured by direct contact and brisk circulation rather than by 

 very low temperatures with less perfect contact. 



Opposed to these objections are the advantages of compactness, 

 convenience, and cheaper handling of cakes or blocks instead of indi- 

 vidual fish, and the ease and perfection of glazing. Freezing in 

 molds is particularly well suited to the freezing of fillets or steaks 

 of fish, which readily conform to the shape of a mold and are more 

 seriously affected by penetration of salt than round fish. 



EARLIER METHODS 



Among the earliest methods of freezing fish were those that in- 

 volved the packing of fish in pans with lids, 68 which were embedded 

 in cracked ice and salt. Hesketh and Marcet, who were also pioneers 

 in brine freezing, covered in their patent™ the freezing of perish- 

 ables by placing them in compartments, boxes, or cells surrounded 



m R. Plank, " Thjories' concerning the changes taking place in the celL membranes of 

 animal flesh during the process of refrigeration." Ice and Cold Storage, October, 1925. 



67 H. W. Foote and Blair Saxton ("The effect of freezing on certain inorganic hydro- 

 gels." Journal. American Chemical Society, Vol. XXXIX, pp. 1103-1125. Easton, 1917. 

 See also ibid., Vol. XXXVIII, p. 588, 1916.) have shown that the jellylike consistency 

 also has an effect of lowering the freezing temperature, the contained water being in 

 the capillary condition. 



68 D. W. and S. H. Davis, IT. S. Patent 101596. Apr. 6, 1875. Referred to, also, in 

 D. W. Davis, U. S. Patent 709751. Sept. 23, 1902. 



<*> British Patent 6117, Apr. 9. 1889. 



