PRESERVATION OF FISH BY SALT, 7 



merce, there are strong reasons for believing the softer and yellowish 

 fish produced in pure salt to be superior. There is reason for be- 

 lieving that the whitening of the fish in impure salt is explained by 

 the fact that the calcium coagulates the protein, just as heat by coagu- 

 lating egg white causes it to be white and firm. But where there is 

 no calcium in the salt the protein retains its natural translucenc}' 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 prob- 

 ably 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 com- 

 mon exj3erience 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 mem- 

 brane 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 investi- 

 gating. 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 cur- 

 ing 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 labora- 

 tory 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 pre- 

 cipitate certain protein substances, different substances falling out 

 successively from a mixture of dissolved proteins as the concentra- 

 tion 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 precipita- 

 tion or fixation of proteins in fish, to a certain 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 



* Absolute temperature is Ixised on absolute zero, the point of no lioat, or absolute 

 cold, which is —273° C. or —459.4° F. If we use degrees the same size as Fahrenheit's 

 degrees, then 0° F. is 4.^9.4 ab'^oUito ; '>0° F. is 459.4 + 50^500.4 absohite, etc. 



77635°— 22 2 



