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.
The last, but by no means the least, important factor to be considered in estimating the food value of any particular fish is its retail price. The price of the different kinds of fishes is by no means proportional to their individual food values. It is determined primarily by the abundance or otherwise of the available supply of each individual species. Thus, the various pelagic fish—mackerel, herring, sprat—that are easily caught in enormous quantities at certain seasons of the year are by far the most valuable. Of trawl-caught fish, cod and whiting are more plentiful and are, therefore, cheaper than hake, although, again, the cheaper fish has the greater food value.