To obtain rapid and thorough curing, therefore, it is necessary—especially in warm weather—to use salt which contains as little calcium and magnesium salts as possible.
Pure salt, used dry, produces a soft, yellow-meated fish which is flexible in the hand. Salt containing calcium chloride or magnesium chloride produces a harder and stiffer fish with a markedly whiter colour.
Salted fish can only be stored satisfactorily in a dry place. Fish which has been cured with impure salt is hygroscopic and will run wet in the store.
This hygroscopic moisture weakens the preserving action of the salt. Fish that has been cured with a pure salt will keep much drier under ordinary storage conditions.
(2) _Its Grain._ The crystals of Fishery Salt should be coarse and hard. Coarse crystals dissolve slowly, and so produce a more gradual cure than fine-grained salt does. Fine-grained salt extracts the water so rapidly from the surface tissues that it coagulates them. This retards the further penetration of the salt into the fish, so that the fish has the appearance of being slack salted.
=Round versus Cleaned Fish.= The thoroughness with which a cut fish is cleaned and washed influences the temperature at which the fish can be salted successfully, and materially affects the quality and taste of the product. Tressler[1] has shown that the chief cause of fish spoiling when salted in hot weather is the decomposition of the blood which remains in the flesh. Even in cold weather, it is found that the extra washing and cleaning greatly improve the quality of the fish. As the presence of blood in the fish also leads to discolouration during the salting process, a thoroughly cleaned and washed fish is, after salting, much whiter in appearance and has a finer taste.
Many fish are skinned before they are salted. It has been observed that a skinned fish will cure almost twice as quickly as an unskinned fish. This is because salt penetrates the meat of the fish at approximately twice the rate at which it penetrates the skin. It is desirable, therefore, particularly in hot climates, to skin the fish before salting. This, of course, is only commercially practicable with certain large kinds of fish such as cod.