STARCH, one of the chief forms of carbohydrates, is found in only the vegetable kingdom. It is present in large quantities in the grains and in potatoes; in fact, nearly all vegetables contain large or small amounts of it. It is stored in the plant in the form of granules that lie within the plant cells.

Cooking applied to starch changes it into a form that is digestible. Moist heat cooks the granules until they expand and burst and thus thicken the mass. Dry heat changes starch first into a soluble form and finally into what is called dextrine, this being the intermediate step in the changing of starch into sugar.

SUGAR, another important form of carbohydrate, is mainly of vegetable origin, except that which is found in milk and called lactose. This, together with the fat found in milk, supplies the child with energy before it is able to digest a variety of foods. The sap of various plants contains such large quantities of sugar that it can be crystallized out and secured in dry form. The liquid that remains is valuable as food, for, by boiling it down, it forms molasses. Sugar is also present in considerable amounts in all fruits, and much of it is in a form that can be assimilated, or taken up by the body, quickly. A sugar very similar to this natural fruit sugar is made from the starch of corn and is called glucose. Much of the carbohydrate found in vegetables, especially young, tender vegetables, is in the form of sugar, which, as the vegetables grow older, changes to starch.

Sugar melts upon the application of heat or, if it is in a melted condition, as syrup or molasses, it boils down and gives off water. When all the water has boiled away, the sugar begins to caramelize or become brown, and develops a characteristic flavor. If the cooking is continued too long, a dark-brown color and a bitter taste are developed. Because the sugar in fruits and vegetables is in solution, some of it is lost when they are boiled, unless, of course, the water in which they are cooked is utilized.

CELLULOSE is a form of carbohydrate closely related to starch. It helps to form the structure of plants and vegetables. Very little cellulose is digested, but it should not be ignored, because it gives the necessary bulk to the food in which it occurs and because strict attention must be paid to the cooking of it. As cellulose usually surrounds nutritive material of vegetable origin, it must be softened and loosened sufficiently by cooking to permit the nutritive material to be dissolved by the digestive juices. Then, too, in old vegetables, there is more starch and the cellulose is harder and tougher, just as an old tree is much harder than a sapling. This, then, accounts for the fact that rapid cooking is needed for some vegetables and slow cooking for others, the method and the time of cooking depending on the presence and the consistency of the cellulose that occurs in the food.

IMPORTANCE OF A VARIETY OF FOODS.--Every one of the five food substances just considered must be included in a person's diet; yet, with the exception of milk, no single food yields the right amounts of material necessary for tissue building and repair and for heat and energy. Even milk is in the right proportion, as far as its food substances are concerned, only for babies and very young children. It will thus be seen that to provide the body with the right foods, the diet must be such as to include all the food substances. In food selection, therefore, the characteristics of the various food substances must be considered well. Fats yield the most heat, but are the most slowly digested. Proteins and carbohydrates are more quickly digested than fats, but, in equal amounts, have less than half as much food value. Water and mineral salts do not yield heat, but are required to build tissue and to keep the body in a healthy condition. In addition, it is well to note that a well-balanced diet is one that contains all of the five food substances in just the right proportion in which the individual needs them to build up the body, repair it, and supply it with energy. What this proportion should be, however, cannot be stated offhand, because the quantity and kind of food substances necessarily vary with the size, age, and activity of each person.