THE CURING AND PRESERVATION OF FISH
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—
---------------------------------+--------+--------+-------
Sample. | % | % | %
| Water. | NaCl. | Fat.
---------------------------------+--------+--------+-------
Fresh herring, ungutted | 67·33 | 0·63 | 13·78
| | |
Herring lightly salted, before | | |
gutting | 66·33 | 1·27 | 12·11
| | |
Herring from rousing tub, gutted | | |
and salted, ready to pack into | | |
barrel | 61·09 | 1·41 | 16·14
| | |
Herring, after 7 days salted in | | |
barrel | 52·67 | 7·43 | 17·10
| | |
Herring, after 8 days salted in | | |
barrel | 46·90 | 11·49 | 22·50
---------------------------------+--------+--------+-------
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.
-----------------------+--------+--------+--------+--------+--------
| German | | | |
Composition. | Rock |Italian.|Spanish.|French. |English.
| Salt. | | | |
-----------------------+--------+--------+--------+--------+--------
| % | % | % | % | %
Salt (Sodium chloride) | 97·28 | 96·59 | 96·63 | 95·86 | 98·9
| | | | |
Calcium chloride | -- | 0·32 | -- | 0·16 | --
| | | | |
Magnesium chloride | 0·25 | 1·19 | 0·96 | 0·35 | 0·08
| | | | |
Magnesium sulphate | -- | 1·75 | 0·73 | -- | --
| | | | |
Sodium sulphate | 0·44 | -- | -- | -- | 0·04
| | | | |
Sodium bicarbonate | 0·01 | -- | -- | -- | --
| | | | |
Insoluble (Calcium | | | | |
sulphate, sand, etc.)| 2·02 | 0·15 | 1·68 | 3·63 | 0·98
-----------------------+--------+--------+--------+--------+--------
| 100·00 | 100·00 | 100·00 | 100·00 | 100·00
+--------+--------+--------+--------+--------
Moisture | 0·20 | 6·54 | 4·47 | 1·39 | 3·25
-----------------------+--------+--------+--------+--------+--------
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. Tressler[1] 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.
[Footnote 1: U.S. Bureau of Fisheries, Document No. 884 (1920).]