REFRIGERATION OF FISH 523 



into the protein. Virtually, if not absolutely, the changes that occur 

 in freezing of this degree of rapidity are reversible on thawing. In 

 a yet slower rate of freezing, such as occurs in the inner portions of 

 brine-frozen fish, one single large column of ice forms in the muscle 

 fiber; but here, also, little practical damage is done. 



In very slow freezing large ice crystals are formed — so large that 

 they do not remain in the muscle fibers or cell. Cell walls are 

 ruptured and the crystals form in the interstitial spaces. When 

 such fish are defrosted, the juice runs out of the fish, carrying much 

 of the valuable fish substance with it. Unfortunately, this is the 

 kind of freezing that commonly occurs in " sharp " freezers and 

 accounts for the many efforts that have been made in recent years 

 to achieve more rapid freezing. Especially in large fish, such as 

 halibut, large crystals, half an inch or more long, may be extracted 

 easily from the tissues; and the tissues when defrosted are char- 

 acterized by a honeycomb structure. Compared with fresh fish, the 

 flesh is dry and the taste flat. 



Furthermore, it was shown that autolysis is promoted by bruising. 

 The formation of large internal crystals is equivalent to a severe 

 bruising or physical damage to all the tissues of the fish. Such fish 

 on defrosting autolyze very rapidly. 



CHANGES DURING COLD STORAGE 



If frozen fish are held for a protracted time in cold storage, other 

 changes may occur. 



BLOOD PIGMENTS 



The red coloring matter of blood — hemoglobin — is sensitive to the 

 gases of the air. On long standing in the presence of air, hemo- 

 globin is converted to methemoglobin, a brownish-gray substance 

 that gives the color to corned beef. The blood of fish after pro- 

 longed storage undergoes this change. The writer has prevented it 

 experimentally by the use of substances like carbon monoxide and 

 nitric oxide, which form stable red compounds with hemoglobin. 



DESICCATION 



It is universally known that water will evaporate when exposed to 

 dry air. That ice also will evaporate is not so well recognized, yet 

 it is so. An understanding of the laws of vapors is necessary to a 

 clear conception of the movements of moisture in a cold-storage 

 room. 



If a pan of water is placed in a closed chamber, and if the air in 

 the chamber is not already saturated with water vapor, water will 

 evaporate until the vapor (or air) in the chamber is saturated — 

 that is, until it contains all the water that it will hold. How much 

 water will it hold? This depends on the temperature of the cham- 

 ber. The warmer the chamber the more water is required to saturate 

 it. If the temperature in a saturated chamber is raised, more water 

 will evaporate; if it is lowered, some will condense as dew if the 

 temperature of the condensing surface is above 32° F., or as ice 

 or snow crystals if below this point. 



