If foods are to be kept successfully from one season to another, it is necessary to have as nearly as possible exact knowledge of the conditions that interfere with their preservation. This statement refers not only to fruits and vegetables that are pickled, preserved with sugar, or put up in cans, but to other foods, such as apples, winter vegetables, and eggs - foods that do not need to be kept indefinitely but that should be given a longer season of usefulness.

There are two main causes for the spoiling of foods: first, the presence of small living organisms that feed on the foods and change them so that they may cease to be desirable and may even become harmful; second, the normal occurrence in such foods as fruits, vegetables, eggs, meat, and seeds of all plants, of certain substances which, although not alive, are the products of living things and have the power of causing fruit to ripen, seeds to start growing, meat to soften, and all finally to decay.

Any food in which the life processes are still going on is subject to either of the types of changes just described. Yeast plants or bacteria may live on foods and cause them to spoil. Bacteria, yeasts, and molds grow everywhere - on the ground, in water, and in air. All micro-organisms need warmth, moisture, food, and oxygen for growth. Warmth, moisture, light, and oxygen also favor the maturing, ripening, and decay changes occurring in eggs, meat, fresh fruits, and vegetables.

If food is to be kept for any length of time, it is necessary to retard or prevent natural ripening or developing processes and to protect the food from invasion by invisible as well as visible enemies. Methods of food preservation may be grouped as follows: (1) By means of low temperature; (2) the removal of moisture; (3) preservatives; (4) high temperature.

A temperature even as low as the freezing point of water may not kill micro-organisms nor destroy in such foods as fruits and vegetables the substances that ripen and mature them. Such a temperature does, however, retard or check all life processes, and therefore its use is a highly efficient method of keeping foods during limited periods of time. Refrigeration and cold storage on a commercial scale are not discussed here, but storage at moderately low temperatures that is practicable for the average home is described.

When foods are dried until their water-content is reduced below 20 per cent, micro-organisms are not likely to develop in them. When foods are dried, the micro-organisms originally present are not destroyed, but their growth and multiplication are checked. If moisture is supplied, active life quickly begins again.

The word preservative covers a wide range of substances that are used in food preservation to retard or prevent the growth of micro-organisms. Foods containing any substance sufficiently active to destroy or check the growth of micro-organisms should be at least thoroughly questioned before being used. Preservative substances may be divided into three classes: (1) those known to be harmless, such as sugar, salt, vinegar, and spices; (2) those about which there is doubt, such as saltpeter, smoke, and liquid smoke; (3) those known to be harmful, such as boric acid and the borates, salicylic acid and the salicylates, benzoic acid and the benzoates, sulfurous acid and the sulfites, and formaldehyde.

The United States Department of Agriculture has issued the following statement concerning the use of canning powders. The possibility of harmful effects from these substances is still not fully recognized by many housekeepers.

"The attention of the Department of Agriculture has recently been called to the widespread use, especially in rural communities, of salicylic acid in putting up preserves. The Department is aware that this practice is not confined to salicylic acid under its own name alone, but that large quantities of this acid, and of boric acid as well, are sold under fanciful names as preserving powders or canning compounds at prices which are much in excess of their real value. In the directions for use, the housewife is told to fill the jar with the fruit or vegetables, cover with water, and add a teaspoonful of the powder. It is true that these powders may prevent the decay of the fruit or vegetable, but they also encourage uncleanly or careless work, and their excessive use may be attended with very serious effects on the health. Salicylic acid is a medicine of the greatest value in acute articular rheumatism and certain other diseases. It is well known as a poisonous substance, and one of the evils which may accompany its use is derangement of the digestion. It is therefore plain that its extensive use in food may lead to disturbance of digestion and of health. It is entirely practicable to put up both fruits and vegetables in such a manner that they will keep indefinitely by sterilizing the products by means of heat, and there is no excuse for running any risk by the use of preserving powders."

A temperature as high as that of boiling water is destructive to the vitality of both micro-organisms and the foods on which they may occur. This fact is the corner stone of canning.