PEESERVATION OF FISH BY SALT. 17 



as herring, salmon, and alewives, contain fat well distributed through- 

 out the body tissues. In others, such as cod and haddock, the fat 

 is localized in some particular part of the body, as in the species 

 mentioned the oil is contained in the lever, the flesh being almost 

 entirely destitute of oil. For reasons that will be set forth later fat 

 fish must not be exposed to the air because of untoward changes that 

 air causes in the fat ; but no harm is done to the protein constituents. 

 Therefore fish which do not contain fat may be dried in air after 

 they are salted. 



In practice these differences are well recognized. In the case of 

 cod and haddock, in which the muscle tissue is free from fat, the 

 greater part of free water is extracted in the usual way by salt, 

 later assisted by the pressure of piles or kenches, in which the lower 

 layers are pressed by the weight of the upper layers in the kench, 

 and finally by drying out of doors or in artificial drying tunnels. 

 Fish prepared by this method are packed and shipped in the dry 

 state, with advantages in saving of freight and simpler handling in 

 general. In the case of mackerel and herring and such other fishes 

 as have fat tissues the fish must at all times be carefully excluded 

 from contact with air. If the fish are directly exposed to air for a 

 time, the fish " rust " — that is, the fat becomes reddened and rancid — 

 and the value of the fish for food is very greatly impaired. This 

 rusting, especially of salt mackerel, is of immediate and pressing 

 practical importance, for there is a regular waste of a large per- 

 centage of mackerel on our northeastern coast for no other cause than 

 rustiness and rancidity. This aspect of the subject has not been in- 

 vestigated to any great extent, but there is just as much reason to ex- 

 pect valuable results to accrue from work on this problem as have 

 accrued from the work already described. 



Fats consist of a combination of glycerin with fatty acids. In 

 the absolutely pure state, which is scarcely attainable, in fact, they 

 would presumably be colorless, odorless, and tasteless. They usu- 

 ally contain a greater or smaller quantity of coloring matter dis- 

 solved, and under certain conditions the combination, glycerin-fatty 

 acid, may be broken down, free glycerin and free fatty acid resulting. 

 Free fatty acid has both taste and odor ; in fact, our choicest fishes, 

 such as salmon, shad, and mackerel, owe much of their peculiarly pala- 

 table flavor to the small amount of free fatty acid present. But 

 many of the free fatty acids of fish oils readily oxidize on exposure 

 to air and light, developing during the process a darker color and 

 an unpleasant odor and taste which we call rancidity. Once fats 

 have become rancid they can never be restored to their original 

 sweetness. 



What conditions promote rancidity? First, the fat must be de- 

 composed or " split " into glycerin and free fatty acid. Next it must 

 oxidize. Just as fish contain autolytic enzymes that decompose pro- 

 tein, so they also contain fat-splitting enzymes. These enzymes re- 

 quire moisture and warmth for their activities. Fat that has been 

 removed from the tissue that produced it may be kept under proper 

 condition for a long time, because only a small amount of fat-split- 

 ting enzyme goes with the oil, but when the fat is not removed from 

 the original source all the enzyme is present and available to produce 



