REFRIGERATION AND FREEZING OF FISH 345 



for putting in the fish straight. If large fish are thus loaded in, they must not 

 overlap so much as to freeze together. 



In large sharp freezers and especially where brine is circulated through the 

 pipes there is a comparatively large reserve of refrigeration in the brine, pipes, 

 and air. Consequently, a comparatively large quantity of fish can be placed in the 

 rooms without excessive rise in temperature. Since the rate of extraction of heat 

 from a body is proportional to the difference of temperature between it and 

 its surroundings, it is evident that fish will freeze more rapidly if placed in a 

 cold room than they will if placed in a room which is warm at the start. The 

 temperatures often available in such large rooms (i.e., rooms of 60,000-pound 

 capacity) range from to — 12° F (— 17.8 to — 24.4° C), which not only makes 

 possible a relatively well-preserved fish, but also shortens the time of freezing. 



The small individual freezer is, however, more popular, especially where direct 

 expansion is employed. There are usually several rooms, side by side, each of 

 a capacity of from 12,500 to 30,000 pounds. While such a room is being filled 

 with a large quantity of fish, the temperature rises rapidly to 15 to 35° F (— 9.4 

 to 1.7° C), though lower temperatures often prevail on completion of loading. 

 When the room is filled, the door is closed and the expansion valve opened. The 

 temperature gradually falls as the heat is extracted. The lot is judged to be frozen 

 when the room has reached a temperature of about — 10° F (— 23.3° C). This 

 figure not only varies from freezer to freezer, but in any particular freezer is 

 subject to change with the exigencies of the day's rush. When there is no hurry, 

 the room may be allowed to reach - 15 or - 20° F (- 26.1 or - 28.9° C), but if 

 the freezer is needed for a fresh lot, the frozen lot may be taken out at 0° F 

 (-17.8°C). 



The time required to freeze the different species of fish is usually not so im- 

 portant as might otherwise appear. The filled pans are almost always left in the 

 freezer overnight (i.e., 24 hours or less). If the fish freeze in a shorter time, no 

 advantage can be taken of the saving because the time for change would fall in 

 the night. Large fish, such as halibut and salmon, are left in the sharp freezer 

 over 2 nights (i.e., 40 to 48 hours). Very large fish may remain in the freezer 3 

 or 4 days. 



The rate of freezing (i.e., the extraction of heat) is not uniform. When the 

 charge is first put in the room, the temperature falls rather rapidly until freezing 

 begins. A long period of little change in temperature ensues while the fish are 

 freezing. Then there is a more noticeable lowering of temperature. In plants 

 where ice tanks are cut oflF during the day, but are making ice at night, the in- 

 fluence of the heavier night load is also observable in the sharp freezers. 



Defrosting of the pipes may be done by warming the room and flooding the 

 pipes with warm liquid ammonia or brine, thus thawing out the ice which adheres 

 to the pipes. Another method is to scrape the pipes with irons having a semi- 

 circular recess to fit the pipes. The former wastes refrigeration and is said to be 

 very bad for the fish frozen immediately afterward because of the excessive 

 dehydrating effect on the fish. Although the latter method is rather laborious, the 

 fish are much less likely to dehydrate. In some plants the pipes are defrosted 

 after each freezing, in others only once every 1 or 2 years. 



Brine Freezing. When fish freeze slowly, the water contained in the muscle 

 plasma and blood separates out and freezes as crystals of pure water ice. The 



