Page:Principles preservation fish by salt.djvu/13

Rh, there are strong reasons for believing the softer and yellowish fish produced in pure salt to be superior. There is reason for believing that the whitening of the fish in impure salt is explained by the fact that the calcium coagulates the protein, just as heat by coagulating egg white causes it to be white and firm. But where there is no calcium in the salt the protein retains its natural translucency and yellowish color. The calcium in impure salt is retained by the fish, a matter that will be discussed later under the subdivision on flavor of salted fish.

While no investigations appear to have been made on the influence of temperature on the permeability of fish flesh, investigations have been made on a great variety of other living things, so that it is probably safe to generalize cautiously regarding such influences on fish. Osmotic pressure varies, approximately, as absolute temperature. That is, if we double absolute temperature osmotic pressure is doubled, other factors being held constant. The range from 32 to 100° F. within which fish salting is usually done is, on the absolute scale, rather narrow (491.4 to 559.4° A.), so the maximum variation due to this cause would be about 14 per cent. It is, however, a common experience in pickling fish that the warmer the temperature the more rapid the striking through, a difference too great to be accounted for by temperature variations of osmotic pressure. The cell membrane itself must change. Whether any more free permeability caused by warm temperature is permanent after the fish is chilled again is not known, but the question would be well worth investigating. Cold, when in the neighborhood of freezing, also promotes permeability, as has been proved by various experiments. It is quite possible that fish chilled to a point near freezing (as in the mild curing of salmon) would strike through much more quickly than fish at the customary warmer temperatures. This matter also should be investigated.

Stale fish–that is, fish whose cell membranes have “died”–are more permeable than fresh fish. Some fish were held in the laboratory all day at a temperature of about 75° F. and toward night were salted in pure salt and put in an incubator at 100° F. By the next day they were struck through. The combination of stale fish, high temperature, and pure salt brought about extraordinarily rapid penetration.

At this point mention should be made of another effect of salt upon the protein constituents of fish. Strong solutions of salt precipitate certain protein substances, different substances falling out successively from a mixture of dissolved proteins as the concentration of salt is increased. The nature of the proteins is not altered by this precipitation, for upon replacement of the salt solution with fresh water the proteins redissolve and appear to be restored to their original condition. Salt thus causes a temporary precipitation or fixation of proteins in fish, to ascertain extent hardening the tissues and reducing the likelihood of changing. Not only does quite pure salt penetrate the fish more rapidly, but when the time comes to cook the fish it is found to soak out more rapidly also. Practical experiments in the experimental kitchen of the Bureau of 77635°–22—2