In gadoid fishes, e.g. cod, as well as in skates and rays, the oil is almost entirely confined to the liver. During the summer the liver grows larger and richer in oil, until sometimes the oil amounts to more than half the total weight of the liver. (When cod are caught the livers are removed and kept apart, to be treated subsequently for their oil.) The percentage of oil in the flesh of the cod varies from 0·1 per cent to 1·0 per cent. Unlike that of the herring, therefore, the food value of the flesh of the cod does not fluctuate markedly according to the season.
When fish are dry-salted a certain proportion of the proteins and mineral salts in the flesh is extracted by the brine pickle that is formed. In Russia and Poland, where the greater proportion of salted herrings are consumed, the peasants eat them without further cooking, and also consume the pickle.
A great gain in food value per pound results from the removal of so much water from the flesh of the fish. Freshly caught cod flesh contains about 80 per cent water and 17 per cent protein; after being dry-salted for export it contains about 25 per cent of water and 55 per cent protein.
Thus, 1 lb. of dry cod is equal in food value to about 3 lbs. of fresh cod. The increased food value of salted fish will be seen from the following analyses—
| The Effect of Curing and Drying upon the Food Value of DifferentFishes | |||||||
|---|---|---|---|---|---|---|---|
| Food. | Protein. | Fat. | Carbohydrate. | Ash. | Water. | Food Value. | |
| Cal. per lb. | |||||||
| Haddock | (fresh) | 12·0 | 0·2 | — | 0·9 | 51·6 | 232 |
| „ | (smoked) | 14·9 | 0·2 | — | 3·4 | 57·4 | 286 |
| Herring | (fresh) | 14·0 | 10·4 | — | 1·5 | 45·7 | 699 |
| „ | (salted) | 21·2 | 15·4 | — | 7·7 | 30·9 | 944 |
| „ | (bloater) | 15·7 | 9·6 | — | 1·5 | 52·0 | 697 |
| „ | (kipper) | 14·1 | 11·1 | — | 3·4 | 46·9 | 730 |
| Sprats | (fresh) | 12·6 | 10·7 | — | 1·3 | 49·4 | 686 |
| „ | (smoked) | 21·2 | 14·9 | — | 3·2 | 39·4 | 1,023 |
Thus, the food value of salted sprats or herrings per pound is 50 per cent more than that of the same fish when fresh.
The original food value of a fish is generally diminished by the cooking process. The fish may be boiled or broiled for direct consumption, or it may be steam cooked in cans and sealed up for future consumption, as in the canning industry. When oily fishes, such as herrings, are cooked, the oil globules burst and some of the oil is lost, and the food value of the fish becomes correspondingly less. When salted fish is soaked in fresh water before being cooked, some of the gelatin and other coagulable proteins are extracted from the flesh. This loss of protein can be checked either by broiling the fish, when the protein near the surface becomes coagulated and so prevents the loss of protein from the interior of the fish, or by placing the fish that is to be boiled direct into boiling water, and not into the cold water before the heating has begun.
In addition to this diminution of the food content of the fish, the process of cooking, contrary to general expectation, also diminishes slightly its digestibility.
In the canning process the fish to be canned are cleaned (gutted) and boned, and packed into tins, together with the necessary sauce or seasoning. The tins are then closed, a small hole being left temporarily in the lid. The tins are placed on steam-heated racks, and the contents thoroughly cooked. In this way the contents are sterilized as well as cooked, and the air originally present in the tin is all driven out by the steam through the small hole in the lid. This hole is sealed with a spot of solder while the contents of the tin are still at boiling point. The tin and its contents are allowed to cool down, and are dispatched to the store-room. During storage the contents of the sealed tin gradually “mature.” This maturing process may last from six months to ten years. During this period the bones soften, the flesh becomes soft and pasty, and the taste becomes richer. The precise nature of the changes that take place during this maturing process is not fully understood; probably maturing is partly due to the action of certain enzymes in the flesh of the fish, and partly to the slow but continuous chemical action of the various juices present in the tin. Attempts to pickle herrings from the Zuyder Zee have been unsuccessful owing to a lack of the enzyme action that makes other herrings tender when pickled. The enzyme, although present, is apparently rendered inactive by the presence of an anti-enzyme.