REFRIGERATION OF FISH 519 



PUTREFACTION 



Putrefaction is caused by bacteria or microscopic living things 

 entirely too small to be seen without a microscope. They exist almost 

 everywhere. Sea water and other natural waters contain millions of 

 them. As long as fish are alive and healthy bacteria do them no 

 harm, though they occur in the body slime, on the gills, and in the 

 intestines and probably, also, to some extent in the blood and flesh. 

 When the fish dies there is no longer any resistance to the growth of 

 bacteria; they begin at once to multiply rapidly in the body slime 

 and in the intestines, soon invading the flesh and gills, break down 

 and disintegrate the intestinal walls, and ultimately spoil the fish. 



Bacteria do not obtain their nourishment by biting or digging in. 

 They are plants by nature and live, as other plants do, by absorbing 

 food from their surroundings. They secrete enzymes from their 

 bodies; these enzymes liquefy or digest the surrounding flesh, and 

 this digested flesh is then absorbed by the bacteria. The products 

 of bacterial action, unlike those from autolysis, are usually offensive 

 and unwholesome. The process of decomposition brought about by 

 bacteria is known as putrefaction. 



To prevent putrefaction, the bacteria must be killed or their 

 activities prevented. Bacteria are killed by cooking and also by 

 certain chemicals, both of which agencies are impracticable for 

 fresh fish. Their activities can be retarded by low temperature, 

 which greatly slows down the multiplication of bacteria. Ice tem- 

 perature serves well to delay putrefaction for several days. 



Bacteriological investigation 17 has shown that development of 

 bacteria in fish is arrested by freezing, and that the bacterial content 

 of fish after a term of storage was essentially the same as it was 

 when the fish went into storage. 



As the source of infection of fish is usually the surface slime, gills, 

 and intestine, it follows that fish should be washed before freezing, 

 unless they are perfectly fresh at the time, and that preferably they 

 should be gutted and the gills removed. Gutting is, however, not 

 always desirable because of trade requirements. 18 



CHANGES DURING THE FREEZING PROCESS 



Some reference already has been made to the freezing of water in 

 the tissues of fish. When fish are exposed to a low temperature they 

 freeze — that is, they become solid — apparently simple enough, but 

 in reality a very complex phenomenon. Exactly what happens de- 

 pends largely on just how the freezing is done. 



GROSS EFFECTS OF SLOW AND RAPID FREEZING 



The most important single factor that affects the internal condi- 

 tion of the fish is the speed of freezing. If a fish is placed in a 

 sharp freezer or cold room the temperature of the entire fish is gradu- 



17 See, for example, H. D. Pease, " Effect of prolonged periods of cold storage on the 

 bacteria in the tissues of fish." Proceedings, Third International Congress of Refrigera- 

 tion, Chicago, 1913, Vol. I, Sec. Ill, pp. 560-573, 



18 See footnote 25 on p. 537. 



