REFRIGERATION AND FREEZING OF FISH 335 



that condition for the entire period of storage. Freezing and storage, if done by 

 the best known methods make possible the holding of fish in good edible condi- 

 tion for a year or more. 



In the freezing, storing, and defrosting of fish, which are intended to prevent 

 autolysis and putrefaction, certain other undesirable changes may supervene 

 which must be clearly recognized and avoided as far as possible. These are, 

 chiefly: 



Internal Crystallization of Water. A fish contains from 65 to 80 per cent water, 

 varying with the amount of fat present. If a small piece of fish is frozen with 

 extreme rapidity, as by immersion in cold mercury or liquid carbon dioxide, and 

 examined microscopically, no physical change is observed. But if frozen slowly, as 

 in an ordinary fish freezer, it will be found that much of the contained water has 

 escaped from the cells and muscle fibres and frozen as ice crystals. This phe- 

 nomenon is accompanied by a rupturing of cell structures, rapid autolysis, and 

 loss of juices on defrosting; the practical result is that fish frozen slowly are not 

 capable of being held long on being defrosted, but must be consumed without 

 delay. 



Desiccation. During the periods of freezing and cold storage the rooms or 

 chambers in which the fish are kept are refrigerated by means of a refrigerant, 

 ammonia or brine, circulating in pipes. These pipes are usually the coldest objects 

 in the rooms, and consequently their surfaces have the lowest water-vapor 

 pressure. Since the fish in the freezer or storage are usually warmer than the pipes, 

 they are of higher vapor pressure. The fish thus lose water by subhmation and 

 the pipes accumulate water as snowy crystals. The fish, unless carefully protected, 

 may in this way rapidly lose 50 or 60 per cent of their weight in a few months. 

 As the fish dries, the skin shrinks and loses its lustre, the tissues become like cork, 

 and the fish is not acceptable as food. 



Loss of Flavor. Fish that are quickly frozen, heavily glazed, and stored under 

 the best conditions keep for months with first-rate edible qualities. Yet when 

 storage is unduly prolonged, they become insipid, though still wholesome. The 

 chemical reason for the loss of flavor is not known, though it is generally sup- 

 posed to be caused by the evaporation of volatile constituents. In addition to loss 

 of flavor there is a development, in fat fish, of a rancid flavor due to changes in 

 the fats. 



Rusting. Unless precautions are taken in storage, the fatter fish become "rusty." 

 By this term is meant an exuding of fat to the stirface, followed by oxidation and 

 possibly polymerization and other changes. When this condition occurs, the fish 

 has a yellowish or rusty appearance, especiaUy around the vent and belly and 

 on cut surfaces; it is rancid in flavor and probably indigestible. Fat fish, like 

 salmon, mackerel, and lake trout, rust within 2 or 3 months unless proper pre- 

 ventive measures are taken. The exact nature of the changes which constitute rust- 

 ing is not understood. Some writers think that shrinkage in the fish during freezing 

 forces the fat out in droplets, and the freezing temperatures should be lower 

 than the freezing point of the oil. 



Incidental Changes. Other undesirable things may happen more or less in- 

 cidentally to frozen fish. Snouts, fins, and tails may become broken or scarred; 

 eyes may become glassy; the red corpuscles may be hemolyzed with a diffusion 

 of hemoglobin into the surrounding tissue; and the gall bladder may be broken. 



