PRESERVED foods of all kinds now enter so largely into the daily dietaries of nearly all classes of the community that some reference must be made to the different methods of preservation of foods, and to their effects on the nutritive value of the food.

1. Drying, Smoking, And Salting Food

(A) Dried Foods

Milk, vegetables, and fruits are preserved in this way. The watery constituents of the food may be partially removed, as in the case of condensed milk, or completely removed, as in the desiccated form, where the dried powder is used to increase the nutritive properties of other foods. Tea and coffee are examples of foodstuffs prepared in this way, so also are peas, beans, grapes (raisins), apples, prunes, and figs. Sugar or salt are frequently employed to aid the drying and to assist in preventing the decomposition of the food. Pemmican is a good example of a concentrated food of a mixed kind. It consists of slices of meat cut and dried in the sun, to which is added fat, sugar, and dried fruit.

(B) Smoked Food

The process of smoking is applied to beef, fish, tongue, ham, and bacon. It consists in hanging the meat in a chamber, in which the air is saturated with wood smoke, the antiseptic substances derived from the combustion of the wood impregnating the outer layers of the meat and so preserving the whole. This process is usually employed after salting. Meat so prepared is not so easily digested as fresh meat.

(C) Salting

The addition of salt to meat or fish absorbs the moisture, and dries and preserves the meat. In the process some of the extractives and salts of the meat may be removed. Food so prepared is rather less nutritious and digestible than fresh meats. Salted meat requires prolonged cooking.

2. Freezing And Cold Storage

(a) The process of freezing consists in subjecting meat to a temperature of about 260 F. below zero, the meat when hard being transferred to a refrigerating chamber. Meat so prepared can be kept almost indefinitely. Frozen meat should be cooked when thawed, as decomposition sets in more readily than in non-frozen meat. Frozen meat loses some of its nutritive value in cooking, this loss being greater than in the case of non-frozen meat; it also loses a little delicacy of flavour. These disadvantages, however, are more than outweighed by the diminished cost.

(B) Treatment By Refrigeration

Here the meat is not actually frozen, but is kept in chambers maintained at a temperature a few degrees above freezing point. The temperature is maintained by a draught of cold air. This process involves less alteration in the meat than freezing. In addition to meat, fish, eggs, fruits, and vegetables can be so treated.

3. Exclusion Of Air - Canned Foods

The decomposition of all foods is prevented or retarded by excluding air from the food. This is done by the process of canning, or by varnishing the food, or by covering it with a substance of an impermeable nature, such as melted fat.

Canned foods are very extensively used at the present time. The food is first made sterile by heat, or by the use of chemicals - the latter method being often abused in the hands of unscrupulous people. After the fresh food is placed in the new tin can, the lid is soldered on, leaving a minute hole on the top for the escape of air and steam. The can is then immersed in a bath of boiling fluid which has a higher boiling point than the water in the food. The latter boils, expels at first air, then steam, and so cooks the food, rendering it aseptic. The lid is then soldered, so as to render the can airtight. It should be noted, however, that the presence of anaerobic bacteria in the food may induce decomposition of the food so treated. Canned foods are on the whole less digestible and also less tasty than fresh foods. It has also to be borne in mind that some people may have an idiosyncrasy to the preservative used. The possibility of the occurrence of copper, tin, lead, or zinc poisoning from the prolonged use of canned meats must be borne in mind. The ingredients in many foods have a solvent action on the cans, as a result of which toxic symptoms are set up.

4. Antiseptic And Preservative Substances

A large number of substances are used for this purpose. Many of these are innocuous, e.g., sugar, salt, oil, and vinegar; others are harmless in minute doses, but injurious in the amounts that are occasionally present, such as salicylic acid, formalin, borax, sulphuric acid.

The process of food adulteration has called for legislative action, but the condition is not always easily detected by any analytical method. It has been shown experimentally that the addition of boric acid to the food in the proportion of 1/2 per cent, is not injurious to health; this would be equivalent to the consumption of about 7 1/2 grains of boric acid daily. In conditions where the excretory functions of the kidneys or bowels are defective, smaller amounts of salicylic acid, borax, formalin, and the like in the food would produce injurious results more readily than under normal conditions. The injurious results that may be induced are gastro-intestinal derangement, albuminuria, and an impairment in the general health.

General Effects Of Preservatives On The Food

Generally speaking, the effects of preservatives are to render the food a little less nutritious, probably by interfering with the normal action of the digestive ferments. In some cases the food is rendered less tasty, and its nutritive value thereby impaired.