REFRIGERATION OF FISH 517 



taken together with the desirability of rapid freezing of fish, ex- 

 plains why such cold temperatures are necessary in sharp freezers. 

 Ice is about four times as good a conductor of heat as water. When 

 a fish is freezing the outer frozen part (containing about 80 per cent 

 ice) conducts heat outward about four times as fast as it did before 

 it was frozen; but when a frozen fish is thawing the outer thawed 

 layer conducts heat only about one-fourth as rapidly as it did while 

 it was frozen. A fish will therefore freeze more rapidly than it will 

 thaw if there is the same difference between the temperature of the 

 air and the fish in both cases. 



CHANGES THAT TAKE PLACE IN FISH AND THEIR 

 PREVENTION BY COLD 



Two classes of changes that take place in fish will be considered 

 in this section, (a) those that occur in fresh fish and (b) those that 

 occur in frozen fish. 



CHANGES IN FRESH FISH" 



When a live fish is taken from the water its parts are all intact 

 stomach usually full of food, intestines with digested food and 

 residue, and its body and gills covered with a heavy mucus. Its 

 muscle tissue is able to contract, and the entire fish is chemically nor- 

 mal and wholesome and all internal tissues sterile or nearly so. 

 When the fish dies there begins a series of many kinds of changes 

 that affect the wholesomeness and appearance of the fish. To pre- 

 vent these changes from taking place is to preserve the fish. In 

 order to do this intelligently an understanding of the nature of these 

 changes is valuable. 



COLOR OF THE SKIN 



The first noticeable change in the fish after death is in the coloring 

 of the skin. This color is due largely to variousty colored pigment 

 cells, which are contractile and controlled by nerves. Upon release of 

 nervous control at death these pigment cells contract, the blend of 

 colors becomes dull, and the appearance is generally less attractive 

 than that of the living fish. 



RIGOR MORTIS 



One of the earlier changes in the fish after it dies is a development 

 of lactic acid in the muscles, causing them to contract, the fish in 

 consequence becoming rigid. This condition is called rigor mortis. 

 The warmer the fish the sooner rigor mortis begins and the shorter 

 time it lasts. The colder the fish (not frozen) the slower rigor is 

 to appear and the longer it lasts. Rigor is a sign of freshness and 



u See A. G. Anderson, " On the decomposition of fish." Fishery Board for Scotland, 

 26th Annual Report, 1907, Part III, pp. 13-39. Edinburgh. Clough, R. W., "A biochemi- 

 cal study of Pacific coast salmon, with particular reference to the formation of indol and 

 skatol during decomposition." University of Washington, Department of Chemistry. 27 

 pp., bibliography. The University Press, Seattle. L. Gross, "An investigation into the 

 rate of putrefaction in the common food fish caught in and around Passamaquoddy Bay, 

 New Brunswick." Department of the Naval Service, Biological Board for Canada, Con- 

 tributions to Canadian Biology, 1918-1920, Vol. IX, pp. 99-102. Ottawa. Hunter, A. C, 

 " Bacterial growths in decomposing salmon." Journal of Bacteriology, Vol. V, 1920, pp. 

 543-552. Also, " The sources and characteristics of bacteria in decomposing salmon." 

 Ibid., Vol. VII, 1922, pp. 85-109. 



