PRESERVED FOOD Immen, shred, and promptly dried in a current of heated air. They are then submitted to powerful hydraulic pres sure, condensing them into thin dense cakes, which 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 neverthe less exceedingly useful, and when well cooked almost equal in taste to the fresh vegetables. 2. Use of Antiseptics. The variety of antiseptic sub stances 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 recom mended 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 com monly 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 pre served 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 pre servation 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 food- preserver, 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, etc., and 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 Francois Appert and dates from 1809. It consists essentially in securing cooked food in hermetic ally 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 im mediately 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 her metically 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 ex peditious preparation. The proper quantity of meat, generally, though not necessarily, free from bone, tendon, and undue propor tion 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 sub mitted 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 con siderably. 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 air- exclusion 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