as herring, salmon, and alewives, contain fat well distributed throughout 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 percentage of mackerel on our northeastern coast for no other cause than rustiness and rancidity. This aspect of the subject has not been investigated to any great extent, but there is just as much reason to expect 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 usually contain a greater or smaller quantity of coloring matter dissolved, 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 palatable 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 decomposed or "split" into glycerin and free fatty acid. Next it must oxidize. Just as fish contain autolytic enzymes that decompose protein, so they also contain fat-splitting enzymes. These enzymes require 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-splitting 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
