Many of the most important food-staples require nothing more than favourable natural conditions for their preservation, till they are ordinarily required for consumption. Such is the case with the cereal grains, which are sufficiently ripened and dried in the harvest field, and with all hard farinaceous and oleaginous seeds, nuts, and fruits. Most soft succulent fruits and vegetables, on the other hand, and all varieties of animal food require artificial preservation, and it is to these that the various processes in use are applied. These processes resolve themselves into four groups,—(1) drying, (2) use of antiseptics, (3) exclusion of air, and (4) refrigeration. Several hundreds of patents have been obtained in the United Kingdom alone for preservative processes coming under one or more of these heads; but in reality the methods of preservation in practical operation are not many.

1. Drying is the most ancient and primitive of all processes for preserving food, and, although it answers but imperfectly for most animal substances, yet in dry hot countries it is very extensively practised. In the River Plate regions of South America a large quantity of beef is annually prepared for export to Brazil and the West Indies under the name of “tasajo” or “charqui dulce,” principally by drying. The meat is simply cut into pieces, freed from fat, bone, and tendon, powdered with maize meal, and dried hard by exposure to the sun, care being taken to keep it protected from rain. The dried product has about one-fourth the weight of fresh meat, and is of a dark colour. It requires to be soaked in water and cooked for a long time, yielding at best a tough indigestible meat; but it makes a well-flavoured nutritive soup. The greater part of the charqui or jerked beef of South America is, however, slightly salted as well as sun-dried; and among many races where drying is practised the use of salt and smoking are also appreciated. Many attempts have been made to introduce dried meat in the form of powder or meal. For this purpose fresh meat, deprived of fat, is cut into thin slices and slowly dried at a low heat in an oven or heated chamber till the meat is hard, crisp, and dry. When powdered, such a preparation keeps well if it is not exposed to damp; but it cannot be said to offer any advantages for general use, although it might be of value to an army during a campaign. Nevertheless a company, under the name of the Carne Pura Company, has been established in Berlin within the last few years for the manufacture of such meat-powder. Of an analogous nature are the concentrated soup tablets or cakes, prepared, principally in Russia, by the rapid evaporation of rich soups, with which dried vegetables and flour are sometimes incorporated in proportion sufficient to yield a good soup on dilution with boiling water. These soups are generally deficient in aroma and have frequently an unpleasant gluey consistency and taste. Concentrated meat biscuits, in which flour and extract of beef are prepared in a thoroughly dry condition, and which were largely used in the American Civil War, the German pea sausage (“Erbsen wurst ”), made famous during the Franco-Prussian War, and pemmican are examples of food in which dried meat may be well preserved in conjunction with farinaceous substances. Preservation by simple drying is extensively practised among the Chinese for their gelatinous foods, such as trepang, dried tendons, skins, mussels and other molluscs, and fish. Milk also may be preserved in the form of a dry powder, but the result is not sufficiently attractive to command a market.

Succulent fruits and vegetables are satisfactorily preserved by simple drying. The principal dried saccharine fruits of commerce are raisins, currants, figs, dates, and prunes. These differ in their nutritive properties considerably from the natural fruits they represent, as do also the farinaceous fruits and vegetables preserved by drying, such as the banana, bread-fruit, mandioc, <fcc. A process of drying and compressing ordinary pot-vegetables and potatoes, invented by Μ. Masson about 1845, is now carried out on a large scale by Messrs Chollet & Co. of Paris. The vegetables to be treated are carefully picked, plunged into boiling water to coagulate the vegetable albumen, shred, and promptly dried in a current of heated air. They are then submitted to powerful hydraulic pressure, condensing them into thin dense cakes, ιvhich retain from 9 to 15 per cent, of the weight of the original green substances, or 20 per cent, of the weight in the case of potatoes, but all in greatly reduced compass. The saving of space is, equally with the preservation, of the utmost importance for use on board ship or by soldiers in the field. Within the space of a cubic metre 25,000 rations of Chollet’s compressed vegetables can be packed, each ration weighing 25 grammes and representing about 200 grammes of green vegetables. As anti-scorbutics such preserved vegetables are inferior; but they are nevertheless exceedingly useful, and when well cooked almost equal in taste to the fresh vegetables.

2. Use of Antiseptics.— The variety of antiseptic substances which have been experimented with for the curing of food is numberless. Bodies solid, liquid, and gaseous have been proposed, and these have been variously recommended for superficial application, for injection, and for forming an artificial atmosphere around the substance to be preserved; and further, it has been suggested that the creature whose flesh is to be preserved should, before killing, be impregnated with the antiseptic by inhalation or otherwise. In practice the antiseptics used are very few in number, since many of them have a physiological effect on the digestive and other internal organs into which they are introduced with the food, and so must injure the health. Besides, many proposed antiseptics are either in themselves unpleasant in smell or taste, or alter the appearance, colour, taste, or consistency of the food preserved. The least objectionable are substances which enter into human food themselves, such as certain salts, sugar, vinegar, and alcohol. The most ancient, most commonly used, and throughout most effective is common salt. Salt acts on meat by withdrawing the animal juices, the place of which it takes, and by hardening the muscular tissue. Consequently it seriously lessens the nutritive value of animal food, and renders it much less digestible than fresh meat. It appears to be least injurious in the case of pork, the fat of which it renders more digestible, and, as a consequence, no animal food is more largely preserved by the process of salting. A certain proportion of other saline bodies, notably saltpetre (nitrate of potash), and of sugar is frequently combined with salt in curing, and so also are other antiseptic and preservative agencies. Bacon, for example, is both salted and smoked, while tongues and fish are not only salted and smoked but also dried. Smoking alone is very effective in preserving and flavouring fish intended for consumption within a limited time after curing. The quantity of fish prepared for human food by salting, smoking, and drying, together or separately, is incalculably great. Of other antiseptics which have been suggested, and which may be used effectively for the preservation of food, few possess any advantage whatever over common salt, which is certain in its action, abundant, cheap, and, within limits, harmless. Among the substances which have of recent years come into prominent notice are bisulphite of lime and various preparations of boracic acid, notably that known as “glacialin” salt and the boro-glycerin introduced by Professor Barff. Boracic acid is a powerful, inodorous, and tasteless preservative; but in repeated small doses it exercises a specific influence on the excretory organs which must be detrimental to health. Salicylic acid has also been extensively tried as a foodpreserver, more especially for milk, but, in addition to the unpleasant taste it communicates to the substances, there are physiological objections to its use.

The use of non-saline preservative agents is exemplified on a large scale in the pickling in vinegar of succulent fruits and vegetables (see Pickles, p. 80 above). Sugar plays a similar part in the preparation of jams, jellies, candied fruits, <fcc., ajιd alcohol is also occasionally employed as a medium for the preservation of fruits. Oil acts as a preservative more by its power of excluding atmospheric air than from any antiseptic influence it possesses, and therefore comes under the next category.

3. Exclusion of Air.— The principal method of food preservation dependent on the exclusion of air is the invention of François Appert and dates from 1809. It consists essentially in securing cooked food in hermetically sealed vessels from which the atmospheric air is as far as possible driven off before sealing, and in killing by heat or otherwise such germs or ferments as may remain within the vessel either before or after it is sealed up. The process does not depend for its success on the perfect exclusion of air,—indeed, originally there was no attempt to drive it off, but air sealed up with the food was immediately submitted to a temperature sufficiently high to kill all germs introduced with it and existing in the food itself. Quite recently experiments have been conducted by Mr J. J. Coleman, the inventor of the cold-air process described below, with the view of preserving food in hermetically sealed vessels, which, instead of being exposed to heat, are subjected to an intense cold, supposed to be sufficient to kill all minute putrefactive organisms; but he has found that a cold of 130° Fahr, below freezing-point is insufficient to destroy all organic germs. At present the innumerable varieties of tinned foods, both animal and vegetable, are entirely the result of the application of Appert’s principle. In practice there are several processes of “tinning” food, but the general method adopted is everywhere uniform in principle.

The tins used are manufactured with the greatest care, and most ingenious machinery has been devised for their thorough and expeditious preparation. The proper quantity of meat, generally, though not necessarily, free from bone, tendon, and undue proportion of fat, is weighed out and placed raw in the tin, over which the cover is soldered. In the cover a small “pin-hole” is left, and the tins are placed in a bath or boiler of solution of chloride of calcium, which boils at a temperature of from ‘260° to 270° Fahr. Each tin is immersed to within an inch or two of the top, and as the heat is gradually raised steam issues from the pin-hole, carrying off the atmospheric air from within the tin. When all the air has been expelled the pin-hole is promptly closed with a drop of solder, and the tin, hermetically sealed, is entirely immersed for some time in the superheated solution. When withdrawn and cooled, the tins are placed in a heated testing-house, in which after a few days those that have been imperfectly treated manifest their defects by a bulging of the sides, due to the generation of gases from the putrefying mass they contain. Those which have been successfully preserved generally show both ends collapsed or depressed by the pressure of the air outside; and usually on a well-preserved tin being pierced the air is audibly sucked in.

The process is applicable to all classes of food, vegetable as well as animal, which may without destruction be submitted to a temperature sufficient to render putrefactive organisms inert, and in experience the amount of heat to which different substances must be exposed varies very considerably. The variety of substances preserved by tinning is now very great, and the total weight of human food so stored is enormous. Numerous modifications of the airexclusion principle, effectual within certain limits, are in use. The preservation of sardines is due partly to cooking them in oil and surrounding them with .it and partly to sealing them in tins, and potted meats, the “conserves fines” of the French, are partly preserved by the use of fat. The most effective means of preserving eggs consists in coating the shells, as soon as they are laid, with butter or some other fat, gum, or varnish. Such coating prevents the transfusion of water from the egg which ordinarily goes on, the place of the water being taken by atmospheric air, rendering the egg specifically lighter and promoting its putrefactive change. Processes for the exclusion of air by the substitution of an atmosphere of some inert gas have not proved successful, neither has the method proposed and patented by Dr Redwood, which consisted in coating meat with a layer of paraffin.

4. Refrigeration.— That cold checks putrefaction has long been known from ordinary experience. Bodies of the prehistoric mammoth have been found in the ice of the Siberian tundra so well preserved that the flesh was eaten by dogs. Ice is much used by fish merchants and other provision dealers for the temporary preservation of their perishable stores; but the cost and inconvenience of the process in temperate, and still more in hot, climates render it applicable only for brief spaces of time and to the more costly of food-products. But about the year 1875 ice began to be used on a large scale for the preservation of fresh meat during its transit from America to the European markets. This, the first practically successful method of preserving fresh meat for such a period as enabled it to be sold in remote markets, consisted in cooling a large meat chamber hung full of carcases by continually blowing into it air which had previously been cooled to near the freezing point by being made to pass through reservoirs of ice. The process was not all that could be desired, but it successfully solved a question which had previously been attempted many times and ways. It continued to be the method by which large quantities of fresh meat were brought in good condition to the European market, till in 1879 Mr J. J. Coleman inaugurated a new era by the introduction, in conjunction with Mr H. Bell and Mr J. Bell, of his Bell-Coleman dryair refrigerator.

In the Bell-Coleman machine atmospheric air is compressed to one-fourth or one-third of its normal bulk in an air-pump by means of a steam cylinder. The air so condensed becomes hot, and is cooled by injecting water into the air-compressor, after which it is still further reduced in temperature and freed from moisture by passing it through a range of pipes in the cold air of the chamber that is being refrigerated. Being then conveyed to the expanding cylinder, the work or energy it contains in virtue of its compression is expended in moving a piston which forms part of the machinery. From the piston the air, now cooled as much as 50° to 100°, or even 200° Fahr., according to the degree of compression to which it has been subjected, is distributed through the cold chamber by suitable pipes. Mr A. Seale Haslam has since brought out a refrigerator in which the temperature of the air is lowered by passing it through pipes cooled externally, instead of by injecting water into the tubes containing it.

Numerous other machines have also been produced, the principle and action of which are illustrated in the article Ice, vol. xii. p. 612. By means first of the Bell-Coleman and subsequently of the Haslam method, fresh meat has been regularly imported into Europe from America since March 1879, when the Anchor liner “Circassia” delivered the first cargo. In February 1880 the first shipment from Australia, consisting of 34 tons of beef and mutton, was delivered in London by the “Strathleven,” and in June 1882 the sailing ship “Dunedin” brought from New Zealand to London, after a passage of ninety-eight days, 4909 carcases of sheep and twenty-two pigs, all in perfect condition, notwithstanding the prolonged voyage and the excessive heat encountered during the passage. The dry-air refrigerators have also been largely adopted in passenger and emigrant vessels for preserving fresh provisions for daily use throughout their voyages, and preserving chambers and freezing chambers have been erected on land at the ports of lading and delivery. The machinery at present in use is capable of freezing upwards of 300,000 tons of meat per annum, and it is rapidly being added to; and it may be said that these machines have accomplished a perfect solution of the great problem of fresh-meat preservation and distribution.

Commerce.— It is impossible to tabulate any reliable figures relating to the trade in products which are properly classed as preserved food. Within the first five years in which the Bell-Coleman machine was in use there were brought from America by its agency alone 563,568 quarters of beef and 113,633 carcases of mutton. The following figures illustrate the development of the frozen-mutton trade from the great sheep-growing localities, giving the number of carcases imported. [table]

As these imports bear only an insignificant relation to the supplies which might be drawn from the several countries at the present moment, it is obvious that a most important factor has been introduced into the meat trade which will exercise a powerful influence on the markets. (j. pa. )