The preservation of fishes for use as food long after they have been caught is a matter of constantly increasing importance to the prosperity of the fishing industry. In most other food supplying industries the produce can be kept fresh for the market comparatively easily. Dry grain will keep indefinitely; vegetables and fruits with proper care will generally remain “fresh” long enough to reach distant markets. Oxen, sheep and pigs may be transported to the market alive, and then slaughtered as required. But a fish as soon as it is taken from the water dies and speedily begins to decay.

Fish, like other foodstuffs, whether animal or vegetable, decays as a result of the growth in it and on it of certain micro-organisms (bacteria, moulds). These micro-organisms swarm in the air and on exposed surfaces all the world over. Generally speaking, they flourish best at ordinary temperatures and in a moist environment.

Foodstuffs can be preserved from decay only by preventing the growth and development of these decay organisms. They can be killed outright by any of the ordinary sterilizing processes such as exposure to sufficient extremes of heat or cold, or by treatment with disinfectant substances (germicides) such as carbolic acid or hypochlorites. Clearly, however, foodstuffs cannot be preserved indefinitely by the simple process of killing all the organisms that are resident on the foodstuff at the time of treatment, for, as soon as the foodstuff is exposed to the air, it will become infected afresh.

They can be preserved—

(1) By boiling, and packing immediately afterwards in air-free containers.

This process is, of course, the basis of the great meat packing industry. The meat is packed in a tin, the tin and its contents are heated in steam or boiling water until the meat is cooked and all the decay organisms are destroyed. The tin is then sealed, air-free and air-tight.

(2) By freezing.

Cold storage is a widely used method of preserving foodstuffs. The low temperature prevents the growth and development of decay organisms and, as long as the foodstuff is kept sufficiently cold, arrests decay.

Prehistoric animals long extinct are sometimes found firmly embedded in the Polar ice, as fresh as they were on the day they were drowned.

It is found that the stability and subsequent quality of frozen meat or fish depend directly upon the manner in which it has been frozen. It may be frozen in air, or when immersed in brine. Of these two methods the latter is much quicker, because brine is over twenty-five times as good a conductor of heat as air is. During the slower air-freezing process the quality of the flesh is impaired by the separation of the contained water into comparatively large crystals of ice. This leads to the displacement of the membrane and tissues of the meat, so that in thawing again the meat drips and becomes tough. When immersed in brine freezing occurs too rapidly for this separation of water to occur to any marked extent.

The keeping qualities of brine-frozen fish also are greater than those of air-frozen fish, owing to the protecting coating of ice which effectively prevents contact with bacteria or mould spores.

(3) By drying.

Primitive man preserved his meat by drying it in the sun, or in the smoke of a fire. To-day the preparation of fish, dried fruits, desiccated vegetables, etc., is a world-wide industry.

Generally speaking, decay organisms can only develop in a moist environment. All fresh foodstuffs contain a large proportion of water. The removal of this water effectively checks decay. Drying alone, however, does not always produce a permanent “cure,” as the foodstuff is always liable to get moist again. For that reason it is customary to combine the drying process with treatment with an antiseptic substance such as salt. Smoke drying is better than sun drying, for although the ultra-violet rays of the direct sunlight effectively kill bacteria and mould spores wood smoke contains antiseptic substances with which the meat becomes impregnated, so that even the chance of any subsequent infection is greatly reduced.

(4) By treating with an antiseptic substance such as salt.

Antiseptic substances differ from disinfectant substances in that they do not kill micro-organisms, but only arrest their development.

As a rule, they are effective preserving agents, and do not make the food poisonous or unpalatable.

All these methods can be, and are, used for preserving fish, the method most commonly used being treatment with salt. Fish, however, are often kept in ice on board during a fishing trip and are then either packed in ice for transit under special storage conditions (if required fresh) or they are salted down.

Methods of Salting. Different methods of salting are used, according to the character of the fish and the locality. The fish are either cleaned (split and gutted) or salted “round” (whole). In general, the method used is one of the following—

(1) Dry-salting. The fish are cleaned, rolled in dry salt, and packed in layers in open casks. Each layer of fish is covered with a layer of salt.

(2) Brine Pickling. The fish are immersed in saturated brine, salt being added from day to day to restore the strength of the brine as it becomes weakened by the water which it extracts from the fish.

(3) Kenching. The fish, either split or round, are piled in layers in the hold of the ship, or on the floor of the warehouse, each layer being covered in turn with a layer of salt. The brine, as it forms, is allowed to drain away.

Of all these three methods, the first is undoubtedly more effective, more economical, and requires less attention than the second. The third method is often used on board ship and sometimes on shore as a temporary expedient when the catch is too large for the number of containers available.

In the dry-salt method, the fish are packed tightly in the casks, and are not afterwards disturbed. When cured they possess a characteristic dry, shrunken appearance.

Fish pickled in brine need attention every day. The brine has to be closely watched so that it shall not become too weak. Fresh salt has to be added daily, and the fish stirred up with wooden paddles to ensure uniform pickling.

Fish cured in this way are softer and more plump than those cured by the dry-salting method.

When a fish is packed in salt the salt rapidly extracts water from the flesh and a strong brine results.

The salt dissolves in the remaining flesh juices of the fish, and rapidly diffuses throughout the fish, thoroughly permeating it. By this process, therefore, the fish is partially dried and becomes thoroughly impregnated with salt.

The gradual change in the composition of the flesh is reflected in the following analysis—

The efficiency of the cure and the appearance of the finished product will be influenced by the following factors—

(a) The temperature—whether summer or winter;
(b) The freshness of the fish;
(c) The quality of the salt—its purity and grain;
(d) The quantity of salt used;
(e) The duration of the process.

(a) The Temperature. As soon as a fish is dead, it commences to decay.

In hot weather, decay proceeds more rapidly and the interior portion of the meat may become soured before the salt reaches it. Clearly, if the rate at which the salt penetrates the fish is retarded by the salt being impure, or of too fine a grain, or by the brine being too weak, the probability of the fish being spoilt is very much increased.

The dry salt method leads to a much quicker penetration of the fish than the brine method, and should always be used in warm weather.

(b) The Freshness of the Fish. The decay processes gather impetus day by day. It is clear, therefore, that in order to avoid the possibility of “souring,” the fish should be salted with the least possible delay.

(c) The Quality of the Salt. (1) Its Purity. The impurities commonly present in Fishery Salt are the sulphates and chlorides of calcium and magnesium.

The following analysis show the composition of typical samples of Fishery Salt.

The Spanish and Italian salts are solar salts, obtained by evaporating sea-water by the heat of the sun. Solar salt nearly always contains more magnesium salts than brine salt does. This constitutes a serious disadvantage to the fish curer.

Of the calcium salts which occur as impurities in Fishery Salt, the sulphate is practically insoluble in brine, and is probably without action upon the salting process.

Calcium chloride, on the other hand, resembles magnesium chloride and is an undesirable constituent of Fishery Salt, for calcium chloride, and to a lesser extent magnesium chloride and magnesium sulphate, diminish the rate at which the salt penetrates the fish. Curing will, therefore, be delayed, and in warm weather (above 70°F.) this may result in the souring of the fish.

To obtain rapid and thorough curing, therefore, it is necessary—especially in warm weather—to use salt which contains as little calcium and magnesium salts as possible.

Pure salt, used dry, produces a soft, yellow-meated fish which is flexible in the hand. Salt containing calcium chloride or magnesium chloride produces a harder and stiffer fish with a markedly whiter colour.

Salted fish can only be stored satisfactorily in a dry place. Fish which has been cured with impure salt is hygroscopic and will run wet in the store.

This hygroscopic moisture weakens the preserving action of the salt. Fish that has been cured with a pure salt will keep much drier under ordinary storage conditions.

(2) Its Grain. The crystals of Fishery Salt should be coarse and hard. Coarse crystals dissolve slowly, and so produce a more gradual cure than fine-grained salt does. Fine-grained salt extracts the water so rapidly from the surface tissues that it coagulates them. This retards the further penetration of the salt into the fish, so that the fish has the appearance of being slack salted.

Round versus Cleaned Fish. The thoroughness with which a cut fish is cleaned and washed influences the temperature at which the fish can be salted successfully, and materially affects the quality and taste of the product. Tressler1 has shown that the chief cause of fish spoiling when salted in hot weather is the decomposition of the blood which remains in the flesh. Even in cold weather, it is found that the extra washing and cleaning greatly improve the quality of the fish. As the presence of blood in the fish also leads to discolouration during the salting process, a thoroughly cleaned and washed fish is, after salting, much whiter in appearance and has a finer taste.

Many fish are skinned before they are salted. It has been observed that a skinned fish will cure almost twice as quickly as an unskinned fish. This is because salt penetrates the meat of the fish at approximately twice the rate at which it penetrates the skin. It is desirable, therefore, particularly in hot climates, to skin the fish before salting. This, of course, is only commercially practicable with certain large kinds of fish such as cod.

The Reddening of Salted Fish. Salted fish sometimes undergo a change, either during the salting process if improperly carried out, or more generally in the store, which is characterized by the development on the surface of the fish of irregular red and brown patches. This reddening occurs not only on the fish, but also on the floors and walls of the curing factories, on the sides and decks of fishing boats, and even on the salt itself. It occurs most readily in warm weather.

The reddening has been shown to be due to the growth of a micro-organism (a micro-coccus). With this micro-coccus are generally associated a bacillus and a micro-fungus which produce the brown mould on the fish.

Fish become infected with these micro-organisms by contact with boats or docks or warehouses.

Every precaution should be taken to keep such places clean and properly disinfected.

The “rusting” of fatty fish, e.g. herring, is due to the oxidation of certain free, fatty acids split off from the fats by enzyme action.