The different methods of food preservation have of late years received much attention, for it is owing to them very largely that it is possible to maintain large armies and navies in action and to permit of the aggregation of men in communities away from all immediate sources of food supply.

It was originally believed that contact with air was per se the cause of the decomposition of food, but it is now known that the great number of germs contained in even comparatively "pure " atmospheric air are the agents of putrefaction rather than the air itself, but the exclusion of the latter implies the exclusion of the former.

The different methods of preserving foods are therefore all dependent upon the principle of preventing fermentative changes which are liable to occur when germs, derived either from the atmospheric air or contamination with unclean substances, come in contact with foods under favourable conditions of warmth and moisture. The conditions which are inimical to the development of bacteria are those which may be successfully adopted for the preservation of food.

Ferments and putrefactive germs require for their activity a fair degree of moisture, a moderately warm temperature, which for many putrefactive germs ranges between 6o° and 100° F., while certain germs must, in addition, have free oxygen derived from the atmospheric air.

The principal means employed for preserving food are included under the following headings:

I. Drying. II. Smoking. III. Salting. IV. Freezing. V. Refrigeration. VI. Sterilisation. VII. Exclusion of Air - Canning. VIII. Addition of Antiseptic and Preservative Substances.

I. Drying

Drying in the sun or before a fire is probably the oldest of methods of preserving food. At present it is used mainly for fruits and vegetables, although in some excessively dry and clear atmospheres, comparatively free from putrefactive bacteria, meat also may be preserved in this way. " Jerked " meat is kept by cutting it into thin slices and drying in the sun for several days. Familiar instances of preservation by drying are found in raisins, figs, dates, prunes, dried apples, peaches, desiccated cocoanut, etc. Some vegetables are also preserved in this manner, such as Lima beans, okra, corn, etc. Others are cut into slices and then dried, but the latter are apt to become tough and tasteless. Fish, such as the cod, is desiccated and preserved by drying, but with the addition of salt. The desiccated meats and vegetables which are used for making soups have the advantages of portability and permanency.

They are nutritious, and may be added to strong beef tea to increase its flavour and make it more palatable.

This subject is further discussed under the heading Food Concentration (p. 275).

II. Smoking

Smoking is the preservation of meat or fish by means of volatilised creosote and other substances developed from wood or peat smoke, which have an antiseptic action. It is chiefly applied to beef, tongue, ham, bacon, and fish. The meat or fish is hung in a confined chamber and saturated with wood smoke for a long time, so that it absorbs a small percentage of antiseptic materials, the fat is prevented from becoming rancid and the albumin from putrefying. The smoking is commonly employed after salting and in connection with drying. Painting the surface of meat with a solution of wood creosote in vinegar has the same preservative action.

The outer surface of meat, such as ham or bacon, preserved by smoking becomes considerably drier and tougher than the interior, but the latter is not made especially tough by the smoking if it was originally tender. Well-smoked bacon cut thin and thoroughly cooked is a digestible form of fatty food for tubercular patients, and smoked beef may sometimes be eaten for the sake of variety by patients who are placed upon a meat diet. The digestibility of hams is enhanced by the smoking process to which they are subjected.

The process of smoking is applied to fish upon a very large scale, and their digestibility and flavour cannot be said to be destroyed by it in many cases; in fact, in some instances, as in those of smoked mackerel, herring, and salmon, while the flavour is very different from that of the fresh fish, it is agreeable to many persons, and these articles afford an important and appetising variety of food.