Superchilling means reducing the temperature of fish uniformly to a point slightly below that obtained in melting ice, thereby extending the storage life of the fish.
When fish is kept in melting ice, the temperature of the fish falls to about 31°F; this is because salt, blood and other substances in the mixture of fish and ice depress the temperature a little below the natural melting point of freshwater ice, 32°F. White fish consists of about 80 per cent water, and all of the water remains unfrozen at 31°F.
When the mass of fish and ice is further refrigerated, some of the water in the fish begins to freeze and the temperature falls. In present practice superchilling means reducing the fish temperature to about 28°F, at which point half the water is frozen, and keeping it there. Bacterial action, and hence spoilage, is slowed a little at this lower temperature, so that the fish remain edible longer.