REFRIGERATION OF FISH 525 



When decomposed, they are readily attacked by oxygen and form 

 various unpleasant substances; that is, they become rancid. This 

 decomposition and oxidation occurs in the cold-storage room unless 

 measures are taken to prevent it and is manifested by a yellowish, 

 rusty, gummy accumulation on the surface of the fish, especially 

 around the bases of the fins and, in the case of gutted fish, in the 

 belly cavity. Cut surfaces are prone to rust. 



These changes are chemical and, in common with chemical changes 

 generally, are accelerated by heat and retarded by cold. For exam- 

 ple, ethyl acetate, a fatty substance, at 72° decomposes one-half as 

 fast as it does at 104° ; at 32° it decomposes about one-seventh as 

 fast, and at 18° only one-twentieth as fast as it does at 104°. The 

 rate of decomposition would be even lower at the temperatures com- 

 monly found in fish cold storages, namely, 10 to 0°. It follows that 

 low temperatures will prevent rusting, or at least greatly reduce it. 

 In practice it has been found that 8° F. is about the highest tempera- 

 ture at which fat fish can be kept for ordinary storage periods with- 

 out serious rusting. Lower temperatures are recommended, and 

 many fish freezers regularly maintain temperatures of 5 or even 0°. 



The oxidation of fat generates heat. The rising temperature in 

 turn accelerates oxidation. This vicious cycle may ruin fish like 

 smelt, which are particularly liable to rust. The writer has seen 

 smelt completely ruined and covered with mold, though they had 

 been kept at 5° F. The mold could not have grown at that tempera- 

 ture. Rapid oxidation had raised the temperature to a point where 

 the mold could grow. The fat had run out of the boxes copiously. 

 This is, of course, an extreme condition — small fish, exposing much 

 surface without glaze, and very rich in highly oxidizable fat. Glaz- 

 ing is a great help in preventing oxidation of fat but is not com- 

 pletely efficacious without a low temperature. 



LOSS OF SAVORY SUBSTANCES 



Fish that have been frozen and stored for a long time frequently 

 are observed to have lost much of their flavor. We do not know 

 definitely just what substances in fish are responsible for flavor. 

 Most probably the flavor is the blend of flavors and odors from 

 many substances that are present in small quantities. The loss of 

 flavor or odor possibly may be caused by (1) escape of volatile 

 substances by evaporation, (2) reaction of the atmospheric gases 

 (oxygen and carbon dioxide) with the constituents of the fish, or (3) 

 reactions between or among the various constituents of the fish them- 

 selves. The last-mentioned of these would seem more likely to in- 

 crease than decrease the flavor. If loss is caused by evaporation of 

 savory substances or by reaction with atmospheric gases, the remedy 

 lies clearly in a protecting glaze that seals in the natural constituents 

 of the fish and excludes the air. In practice it is common experi- 

 ence that fish held at low temperature and fully protected by a glaze 

 lose nothing of taste or flavor in months or even years. It seems 

 much more likely that the loss of flavor often observed occurs at 

 the time of defrosting fish that have been frozen slowly. The loss 

 of juice made possible by internal crystallization easily accounts 

 for the loss of soluble principles that give taste and flavor as well 

 as nutriment. 



