Preservatives are old in use and are used to keep food as natural as possible. Originally this was practiced for out-of-season use of seasonally produced food, where the supply of food was limited. The substances used were those also used for condiments, as vinegar, alcohol, spices, salt. Smoking too was practiced; it preserves, because smoke contains creosote that is a germicide and that is so used as a drug.

Why preservatives did preserve food was long unknown. But with the discovery of bacteria came a knowledge of the cause of decomposition that is generally recognized as putrefaction or decaying of food. To overcome such changes they were studied. It has been found that only a few kinds of bacteria cause these changes. Extremes of temperature (see Sterilization, p. 152) are unfavorable to the growth of such bacteria, as are also many chemicals.

Modern chemical preservatives, refrigeration, sterilization, are used mainly for their effect upon these putrefactive bacteria, in order to prevent unpleasant tastes and odors. Some of the chemicals used are borax or boric acid, benzoate of soda, formaldehyde, sulphites, hydrogen peroxid. None of these are foods. Some that have been found not to injure healthy adults have affected young animals seriously, and are not advised even when not forbidden by law. For children, invalids, and the aged they may be perilous; for any one they may cause kidney-deterioration, so later disease. Sulphites, used to make meat red, cause hemorrhages of different organs. Hydrogen peroxid is not considered unsafe; when added to food it breaks up quickly into simply water and oxygen. But oxygen as it is being thus freed from chemical combination is particularly destructive to bacterial life; also to tissues, therefore is claimed by some to affect unfavorably the food-quality.