Exclusion of air from contact with food is accomplished not only in the process of canning, but by such means as varnishing or covering it with substances which are comparatively impermeable, as in the case of varnishing eggs, covering fish with oil, or pate de foie gras with lard, etc.

Eggs undergo decomposition from the entrance of the atmospheric air and germs through their shells, and this process may be prevented, sometimes for several years, by covering fresh eggs with almost any substance which is more impermeable than their shells, such as gum, fat, butter, oil, beeswax, or fresh milk of lime. The sawdust or salt in which eggs are commonly packed serves the double purpose of insuring safety in transportation and excluding to some degree the air. Similarly meat may be preserved by coating It with paraffin, gelatin, collodion, or layers of powdered charcoal or of lard after the manner of potted meats. Beef has been sent in good condition from Australia to England by merely dipping it into hogsheads of melted fat, in which it was allowed to remain after the fat cooled and solidified. The preservation of meats in air-tight skins, like sausages, has long been practised.

When food is preserved by any of these methods care must be exercised to have it perfectly fresh at the start and to drive off by heat or otherwise any air which may be present in the food itself or in its containing vessel.

Canning

The original idea of the preservation of foods by canning was that the exclusion of air was the sole object necessary of accomplishment. It is now known, however, that many putrefactive bacteria are anaerobic, and that the food must be thoroughly sterilised before the can is closed. This should be done by heat, but since it can also be accomplished by the addition of antiseptics, the introduction of the latter is practised by some unscrupulous manufacturers to the detriment of the public health.

The process of canning meat or vegetables is conducted as follows: The food is placed in clean new tin cans, filling them as completely as possible. Lids are then tightly soldered on the cans, leaving a minute pinhole opening only for the escape of air and steam. The cans are then immersed in a bath of boiling fluid, such as zinc-chloride solution, having a higher boiling point than the water within the cans. The latter boils, expels at first air, then steam, and thoroughly cooks the food, making it aseptic by killing all germs. Before the cans cool, their minute openings are soldered, and they are then ready for storage.

The long boiling of meat in this manner toughens its fibres by hardening the syntonin. Such meat is apparently tender, but in reality it is not very digestible (Williams).

To avoid the necessity of cooking food at a high temperature in order to exclude the air, various modifications in the process of canning are employed. One of these - McCall's - is based on the disinfection of the air by sodium sulphite. In another process sulphurous acid and nitrogen are used to replace the air.

H. W. Wiley, who has made an exhaustive study of canned foods, says, in an instructive report on Foods and Food Adulterants, made by him for the United States Department of Agriculture (Bull. No. 13, Part VIII):

"All manner of food is canned, and that at prices which place it within the reach of the humblest pockets. Preserved food has been a great democratic factor, and has nearly obliterated one of the old lines of demarcation between the poor and the wealthy. Vegetables out of season are no longer a luxury of the rich.... In the American grocery pineapples from Singapore, salmon from British Columbia, fruit from California, peas from France, okra from Louisiana, sweet corn from New York, string beans from Scotland, mutton from Australia, sardines from Italy, stand side by side on the shelves".

Much light is thrown by Wiley upon the economic value of the substances under consideration in the following important statements from the report above quoted:

"The quantity of dry food material in canned goods varies within wide limits. It is very low in such vegetables as string beans, asparagus, etc., and quite high in such materials as canned corn, succotash, and other bodies of that description. The lowest percentage of dry matter in string beans of American origin was 4.17. In other words, in buying one hundred pounds of such material the consumer purchases 95.83 pounds of water.

"The price of the packages of string beans [bought in open market] varied within wide limits, depending both upon the size of the packages and the labels they bore. The highest price paid was thirty-five cents, and the weight of the contents of the package was a little over three pounds. The lowest price paid was ten cents, and this was paid in many instances. The highest price paid, according to the percentage of dry matter, was in sample 10,928, costing thirty cents and containing only two hundred and fifty-four grammes of string beans, 31.1 grammes of dry matter, and 94.37 per cent of water. The price of the dry matter in this package was nearly one cent per gramme, which would be almost five dollars per pound. The enormous cost of food in canned goods is illustrated to the fullest extent by this sample, showing in a striking way that such food materials must be regarded in the light of luxuries or condiments rather than as nutrients to support a healthy organism. An expenditure of ten or fifteen cents for a good article of flour or meal will procure as much nutriment for a family as the investment of three or four dollars in canned goods would.

"A general view of the digestive experiments must lead to the conviction that the process of canning, especially when preservatives are employed, such as salicylic acid and sulphites, tends to diminish the digestibility of the albuminoid and other bodies. The low percentage of digestible albuminoids will be remarked with some degree of astonishment in all the analytical tables".

Of the dangers of poisoning from canned foods Wiley says: "Vegetables are usually canned in the fresh state, and if they are in any degree spoiled at the time the fact is usually conspicuously evident to the taste, so that the canner cannot afford to use them. Bacterial action seldom occurs in the can without bursting it or rendering it unsalable. Ptomaines may, however, develop where the canned food is allowed to stand for some time after opening, though even then this is unlikely in the case of preserved vegetables.

"It may be said, therefore, that the principal risks to health which may arise from the use of canned foods are those due to the use of preservatives, or to the presence of the heavy metals - copper, tin, lead, and zinc.... In this country there is no restriction whatever in regard to the character of the tin employed, and as a result of this the tin of some of the cans has been found to contain as high as 12 per cent of lead.... The analyses of numerous samples of solder employed show that it contains fully 50 per cent of lead. In addition to this there is no care taken to prevent the solder from coming in contact with the contents of the can. It is a rare thing to carefully examine the contents of a can without finding pellets of solder somewhere therein.

" Another great source of danger from lead has been disclosed by the analytical work, viz., in the use of glass vessels closed with lead tops or with rubber pads, in which sulphate of lead is found to exist".

The frequency of poisoning by eating canned lobster, crabs, or shellfish is due mainly to the rapidity with which they decompose and develop ptomaines after the can has been opened. The contents of such a can partially used should not be kept until the next day.

Canned beef is boiled and steamed under pressure at 2500 F. It loses most of its extractives and has the tastelessness, therefore, of meat from which soup has been made. Vegetables may be added to improve the taste, either in the canning process or subsequently during cooking.

Canned beef should be eaten promptly after opening, for it is liable to spoil within a few hours, especially in tropical climates. Severe gastro-enteric disorders may arise from eating such spoiled meat, and epidemics of such poisoning have been observed in a French garrison at Tours (1898), among the British troops in South Africa (1900), and among our own troops in Cuba (1899). In the latter case much public scandal arose in connection with this subject, and from the further fact that manufacturers do not always resist the temptation to can meats of inferior quality, or which may be already partially decomposed, especially when hurried demands for canned goods are made to meet the exigencies of warfare. At the commencement of the Spanish-American War, in 1898, 7,000,000 pounds of canned roast beef were purchased by the commissary department for use by the United States soldiers.