12 U. S. BUREAU OF FISHERIES. 



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 ab- 

 sorbed 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 im- 

 portant parts of the body can be used for the time being to support 

 the activities of the absolutely necessary vital parts. The stored pro- 

 tein 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 sub- 

 stances 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 sim- 

 pler 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 b}^ rough handling of the 

 fish, autolysis rapidly decomposes the protein, and for this reason 

 every bruise received by the fish during capture and subsequent han- 

 dling 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, pro- 

 moting 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 temjDeratures seem to increase the permeabilitj'^ 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 

 enzjnnes 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 temi^erature for autolytic activity is about human body 



