follows:—The canister, which has been made either by the use
of solder or by folding machinery only, is packed with the material
to be preserved, and a little water having been added to fill the
interstices the lid is secured by soldering or folding, generally
the former. Sterilization is effected by placing the tins in
pressure chambers, which are heated by steam to 120° C. or more.
The tins are exposed to that temperature for such time as
experience has shown to be necessary to heat the contents
throughout to at least 100° C. The temperature is then allowed
to fall slowly to below the boiling-point of water, when the
tins can be taken out of the pressure chamber, or they are placed
in pans filled with water or a solution of calcium chloride and
are therein heated till thoroughly cooked. Sometimes a small
aperture is pierced through the lid, to allow of the escape of the
expanding air, such holes before cooling closed by means
of a drop of solder. This process, which was originally introduced
by François Appert early in the 19th century, is employed
on an enormous scale, especially in America. The use of
lacquered tins, having the inner surface of the tin covered with
a heat-resisting varnish, is gradually extending. Imperfect
sterilization shows itself in many cases by gas development
within the tin, which causes the ends to become convex and
drummy. More frequently than not the contents of the larger
tins, containing meat or other animal products, are not absolutely
sterile, but the conditions are mostly such that the organisms
which have survived the cooking process cannot develop. When
they can develop without formation of gas dangerous products
of decomposition may be produced without showing themselves
to taste or smell. Numerous cases of so-called ptomaine poisoning
have thus occurred; these are more frequently associated
with preserved fish and lobster than with meats, although no
class of preserved animal food is free from liability of ptomaine
formation. The formation of poisonous substances has never
been traced to preserved fruit or other material poor in nitrogen.
The mode of preserving food in china or glass is quite similar,
but the losses by breakage are not inconsiderable. Food which
has been preserved in tins is sometimes transferred to glass and
re-sterilized, the feeling against “tinned” food caused by the
“Chicago scandals” not having entirely subsided. Were it not
for the facts that sterilization is rarely quite perfect, and that the
food attacks the tin, the contents of tin canisters ought to keep
for an indefinite length of time. Under existing circumstances,
however, there is a distinct limit to the age of soundness of
canned food.
Preservation by Chemicals.—Salt is the oldest chemical preservative and, either alone or in conjunction with saltpetre and with wood-smoke, has been used for many centuries, mainly as a meat preservative. It is used either dry in layers strewn on the surface of the meat or fish to be preserved, or in the form of brine in which the meat is submerged or which is injected into the carcasses. The preserving power of salt is but moderate. It has the great advantage that in ordinary doses it is non-injurious, that an excess at once betrays itself in the taste, and that it can be readily removed by soaking in water. When aided by wood-smoke, which depends for its preservative power upon traces of creosote and formaldehyde, it is, however, quite efficient. The addition of saltpetre is principally for the purpose of giving to the meat a bright pink tint. The strongly saline taste of pickled meat or salted butter appears gradually to have become repugnant to a large part of mankind, and other preservatives have come into use, possessing greater bactericidal power and less taste. The serious objection attaching to them is discussed in the article Adulteration. At the present time the use of borax or boracic acid is almost universal in England. Meat which has been exposed to the vapours of formaldehyde, and has thus been superficially sterilized, is also coming into commerce in increasing quantities. Formaldehyde in itself is distinctly poisonous, and has the property of combining with albuminoids and rendering them completely insoluble in the digestive secretions. Salicylic and benzoic acids are not infrequently used to stop fermentation of saccharine beverages or deterioration of so-called “potted meats,” which are supposed to last fresh and sweet on the consumer’s table for a considerable length of time. Sulphurous acid and sulphites are chiefly used in the preservation of thin ales, wine and fruit, and sodium fluoride has been found in butter. The whole of these substances possess decided and injurious physiological properties. Alcohol now rarely forms a preservative of food material, its employment being confined to small fruit. The use of sugar as a preservative depends upon the fact that, although in a dilute solution it is highly prone to fermentation and other decomposition, it possesses bactericidal properties when in the form of a concentrated syrup. A sugar solution containing 30% of water or less does not undergo any biological change; in the presence of organic acids, like those contained in fruit, growth of organisms is inhibited when the percentage of water is somewhat greater. Upon this fact depends the use of sugar in the manufacture of jams, marmalades and jellies. Moulds may grow on the surface of such saccharine preparations, but the interior remains unaffected and unaltered.
Preservation by Drying.—Food materials in which the percentage of moisture is small (not exceeding about 8%) are but little liable to bacterial growths, at most to the attacks of innocent Penicillium. Nature preserves the germs in seeds and nuts, which are laden with otherwise decomposable food material, by the simple expedient of water removal. The life of cereal grains and many seeds appears to be unlimited. By the removal of water the most perishable materials, like meat or eggs, can be rendered unchangeable, except so far as the inevitable oxidation of the fatty substances contained in them is concerned and which is independent of life-action. The drying of meat, upon which a generation ago inventors bestowed a great deal of attention, has become almost obsolete, excepting for comparatively small articles or animals, like ox tongues or tails and fish. It has been superseded even among less civilized communities by the spread of canned food. Fruit, however, is very largely preserved in the dried state. Grapes are sun-dried and thus form currants, raisins and sultanas, the last variety being often bleached by the addition of sulphites. Plums, apples and pears are artificially dried in ovens on wooden battens or on wire sieves; from the latter they are apt to become contaminated with notable quantities of zinc. Excellent preparations of dried vegetables, including potatoes, carrots, onions, French beans and cabbages, are also manufactured.
The utilization of meat in the form of meat extract belongs to some extent to this class of preserved foods. Its origin is due to J. von Liebig and Max von Pettenkofer, and dates from the middle of the 19th century. The soluble material is extracted mainly from beef, in Australia to some extent from mutton, by means of warm water; the albumen is coagulated by heat and removed, and the broths thus obtained are evaporated in vacuo until the extract contains no more than about 20% of water. One pound of extract is obtained from about 25 ℔ of lean beef.
Preservation by Refrigeration.—At or below the freezing-point of water fungoid organisms are incapable of growth and multiplication. Although it has been asserted that many of them perish when kept for some time in the frozen condition, it is certain that the vast majority of bacteria and their germs remain merely dormant. Even so highly organized structures as cereal seeds do not suffer in vitality on being kept for a considerable length of time at the far lower temperature of liquid air. Biological change is, therefore, arrested at freezing-point, and as long as that temperature is maintained food material remains unaltered, except for physical changes depending upon the evaporation of water and of volatile flavouring matters, or chemical alterations due to oxidation.
Refrigeration, therefore, affords the means of keeping for a reasonably long time, and without the addition of any preservative substance, food in a raw condition. It is the only process of preservation which from a sanitary point of view is entirely unobjectionable as ordinarily and properly employed. Its introduction on a commercial scale has more powerfully affected the economic conditions of England and, to a less degree,
