Although, as has just been stated, food may be considered as anything that the human engine can make over into tissue or use in living and working, not all foods are equally desirable any more than all materials are equally good in the construction of a steam engine and in the production of its working power. Those food substances which are the most wholesome and healthful are the ones to be chosen, but proper choice cannot be made unless the buyer knows of what the particular food consists and what it is expected to do. To aid in the selection of food, therefore, it is extremely necessary to become familiar with the five substances, constituents, or principles of which foods are made up; namely, water, mineral matter, or ash, protein, fat, and carbohydrate. A knowledge of these will help also in determining the cooking methods to adopt, for this depends on the effect that heat has on the various substances present in a food. Of course, so far as flavor is concerned, it is possible for the experienced cook to prepare many dishes successfully without knowing the effect of heat on the different food constituents; but to cook intelligently, with that success which makes for actual economy and digestibility, certain facts must be known concerning the food principles and the effect of dry and moist heat on foods.

Water.--Of the various constituents that are found in the human body, water occurs in the largest quantity. As a food substance, it is an extremely important feature of a person's diet. Its chief purpose is to replenish the liquids of the body and to assist in the digestion of food. Although nature provides considerable amounts of water in most foods, large quantities must be taken in the diet as a beverage. In fact, it is the need of the body for water that has led to the development of numerous beverages. Besides being necessary in building up the body and keeping it in a healthy condition, water has a special function to perform in cooking, as is explained later. Although this food substance is extremely essential to life, it is seldom considered in the selection of food, because, as has just been mentioned, nearly all foods contain water.

Mineral Matter.--Ranking next to water in the quantity contained in the human body is mineral matter. This constituent, which is also called ash or mineral salts, forms the main part of the body's framework, or skeleton. In the building and maintaining of the body, mineral salts serve three purposes--to give rigidity and permanence to the skeleton, to form an essential element of active tissue, and to provide the required alkalinity or acidity for the digestive juices and other secretions.

The origin and distribution of these mineral substances are of interest. Plants in their growth seize from the earth the salts of minerals and combine them with other substances that make up their living tissue. Then human beings, as well as other living creatures, get their supply of these needed salts from the plants that they take as food, this being the only form in which the salts can be thoroughly assimilated. These salts are not affected by cooking unless some process is used that removes such of them as are readily soluble in water. When this occurs, the result is usually waste, as, for instance, where no use is made of the water in which some vegetables are boiled. As is true of water, mineral matter, even though it is found in large quantities in the body, is usually disregarded when food is purchased. This is due to the fact that this important nutritive material appears in some form in nearly all foods and therefore does not necessitate the housewife's stopping to question its presence.