Protein, the colloid, can not pass through an osmotic membrane, but proteins can be decomposed into simpler substances which readily dissolve and pass through. The agency which breaks down protein into these simpler substances is called an enzyme, and protein must always be so liquefied or digested by enzymes before it can be absorbed through membranes; hence the necessity of digestion in the stomach of animals preparatory to absorption of food through the intestines. Now, animals, including fish, require a certain amount of new protein to support the body activities, which, failing, the animal would immediately perish. But the hazards in the existence of any animal often make it obligatory to do without food for a shorter or longer period. If the stomach became empty because of temporary shortage of food or an injured mouth, the animal would die unless special provision were made to supply protein from some other source. But nature has provided a means whereby the proteins in the less important parts of the body can be used for the time being to support the activities of the absolutely necessary vital parts. The stored protein is within cells and could not possibly be carried by the blood stream to the point of need unless it could get out. So there is in each cell stored along with the protein some enzyme ready in case of threatened starvation to break the protein down into simpler substances which penetrate outward into the blood for transportation to the point of need. Fish may thus live for a time at the expense of their own bodies.
These enzymes, present in every part of the fish, while almost an absolute necessity to the living fish, become the greatest enemy of the dead fish, for they soften and liquefy the cell contents, cause unpleasant tastes and odors, and permit the contents to escape from, the cell into brine. The proteins could not escape as long as they were proteins, but when they are broken down by autolysis into simpler substances the latter rapidly diffuse into the brine and are lost. This at least is the hypothesis, supported by some facts.
What factors promote autolysis and what factors oppose it? Warm temperatures promote it directly. A temperature sufficiently high to destroy the enzyme stops it. Low temperatures retard it directly.
If cells are ruptured, as they often are by rough handling of the fish, autolysis rapidly decomposes the protein, and for this reason every bruise received by the fish during capture and subsequent handling results in the loss of so much protein during salting. A bruise on a fish has about the same effect as does a bruise on an apple, promoting rapid decomposition. Perhaps if the bruised fish turned brown, as the bruised apple does, the fisherman and packer would be more careful in the handling of their fish.
Factors that increase permeability of membranes seem to promote autolysis. Low temperatures seem to increase the permeability of the cells, so that fish that have been chilled decompose more rapidly on being warmed than fish that have never been chilled, though as long as the fish remain on ice the low temperature may prevent the enzymes from doing their work. It is as if increased permeability increases the escape of the enzymes, and that once escaped they play havoc if temperature conditions are allowed to become favorable. The optimum temperature for autolytic activity is about human body
