As has been pointed out, it is not the mere presence of micro-organisms that causes the spoiling of food, but their constant growth. Therefore, to keep milk from souring, meat from spoiling, bread from molding, canned fruit from fermenting, and so on, it is necessary to know what will prevent the growth of these minute organisms. Different foods require different treatment. Some foods must be kept very cold, some must be heated or cooked, others must be dried, and to others must be added preservatives. An unwarrantable prejudice has been raised in the minds of many persons against the use of preservatives, but this is due to the fact that the term is not properly understood. In this use, it means anything that helps to preserve or keep safe the food to which it is added. Sugar, salt, spices, and vinegar are all preservatives, and are added to food as much for the purpose of preserving it as for seasoning it.

CANNING AND DRYING OF FOODS

Among the common methods of caring for foods that are to be used at a future time are canning and drying. CANNING, which is discussed fully in another Section, consists in preserving sterile foods in sealed cans or jars. The aim in canning is to prevent the growth of micro-organisms, and to do this the process known as sterilizing--that is, the destroying of bacteria and other micro-organisms by means of heat--is resorted to. Canning theories are different now from what they were in former times. For example, housewives formerly made heavy, rich preserves of available fruits because it was thought that sugar must be used in large quantities in order to keep or prevent them from spoiling. While it is true that the sugar assisted, science has since proved that sterilizing is what must be done, so that now only the sugar desired for sweetening need be used.

The other method of keeping food, namely, DRYING, depends for its success on the fact that such micro-organisms as bacteria cannot grow unless they have a considerable quantity of moisture or water. Molds grow on cheese, bread, damp cloth or paper, or articles that contain only a small amount of moisture, but bacteria need from 20 to 30 per cent. of water in food in order to grow and multiply. This explains why in high altitudes and dry climates foods keep for a long time without artificial means of preservation. It also explains why the old-fashioned housekeeper dried fruits and why the preservation of certain meats is accomplished by the combined methods of smoking and drying, the creosote of the smoke given off from the wood used in this process acting as a preservative. All the grains, which are very dry, keep for long periods of time, even centuries, if they are protected from the moisture of the air. Peas, beans, and lentils, as well as dried biscuits and crackers, are all examples of how well food will keep when little or no moisture is present.