524 



U. S. BUREAU OF FISHERIES 



If moisture is present in a sealed room, and if the temperature of 

 the room is constant and uniform throughout, the vapor will soon 

 come to saturation and remain saturated as long as the conditions 

 are not changed. 



Table 6 shows the number of grains of water vapor per cubic foot 

 of saturated vapor at cold-storage temperatures. 



These conditions of constancy and uniformity of temperature are 

 not realized in a fish cold-storage room. The temperature fluctuates 

 from hour to hour or day to day. If a room is at 0°, each cubic foot 

 of saturated air contains 0.479 grain of water vapor. If the tem- 

 perature rises next day to 10°, 0.780 grain, or nearly twice as much 

 water, is required to saturate the air, and this must evaporate from 

 the fish. (The quantities are actually somewhat smaller because the 

 juice of fish is not pure water.) The temperature in the room also is 

 not uniform. Heat is coming into the room through the walls, etc., 

 and is being absorbed by the cold pipes. The fish, being near the 

 walls, on the floors, and surrounded by air that is warmed from the 

 same sources, are warmer than the pipes, which are absorbing the 

 heat. The saturation point is lower at the pipes and higher at the 

 fish. Under these conditions the moisture will evaporate steadily 

 from the fish but can not saturate the air because the cold pipes con- 

 dense the moisture. There is thus a continuous travel of moisture 

 from fish to pipes, which will dry the fish completely unless remedial 

 measures are taken. 



The commonest remedial measure is an ice glaze on the fish. The 

 glaze, being of pure water, has a slightly greater tendency to evap- 

 orate than the juice of the fish, and, being exposed, evaporates first. 

 Other practical measures will be considered in more detail later. 



RUSTING 



The fats in fish are a mixture of fatty substances, some of which 

 are unsaturated. That is to say, they are capable of combining 

 with either oxygen or hydrogen under proper conditions. They com- 

 bine directly with oxygen on exposure to air, or they may become 

 "hydroxylated " by combining with both oxygen and hydrogen, in 

 which case they are rancid. When they take oxygen from the air 

 they become viscous or rubbery, as linseed oil becomes on drying 

 when applied as paint. 



The tissues of fish contain enzymes that are capable of decompos- 

 ing fats, and as long as the fats are in the presence of their mother 

 tissues (as they are in stored fish) they are subject to decomposition. 



