Every living cell has a little life history of its own, and constantly demands a certain amount of new material to replace old which it has worn out. Besides this, old cells die, and new ones have to be made to replace them. Hence even a full-grown person needs building material, and much more is required in proportion when the person is growing and perhaps adding several ounces a week to his weight. The foodstuffs which have especial value as building material are protein and mineral matter.
To help in the digestion of food, to keep the blood in proper condition, the muscles supple, and all the processes of the body at their best, ash constituents and water in the diet are necessary. A tabular summary of the functions of the foodstuffs and an outline of the changes which take place in digestion will be found in the appendix.
We are now able to give a more complete answer to our question, "What is Food?"
Food has been said to be that which taken into the body builds tissue or yields energy, or both. The food as a whole must contain all the chemical elements needed by the body, these elements being supplied in substances known as foodstuffs, viz., protein, fat, carbohydrates, mineral matter or ash, water. To be a food, a substance must contain one or all of the foodstuffs.
It must be noted here that our food materials as bought, contain inedible matter, as in the shells of eggs, the bone of meat, the skins and pods of vegetables. Moreover, the fiber that we eat in vegetable foods is not digested under ordinary circumstances, but seems rather to serve a useful purpose in giving bulk to our foods.
In preparing foods for the table, we have the habit of adding substances to develop or give flavor. With the exception of sugar, which we use largely for its agreeable taste, these substances have no nutritive value. They are not hurtful unless used in excess, although pepper and other spices sometimes disturb digestion. Pepper, too, irritates a delicate throat.
Only a few flavors are really detected by the sense of taste. These are salt, sugar, acids; and something in the spices that gives a sensation hard to describe, but is unmistakable in an overdose of mustard or horse-radish. "Pun-gent "is the descriptive word for such a flavor.
The other flavors are really odors, and are detected by the sense of smell. Have you not at some time seemed to lose the sense of taste when suffering from a severe cold in the head? Yet even then you could taste sugar, salt, vinegar, and feel the pungency of pepper. These other flavors or odors are due to a volatile oil in the flavoring material, that is, an oil that readily evaporates, especially when heated, as distinguished from the non-volatile oils and fats like olive oil and butter. This is a practical bit of knowledge in our cookery, for whatever passes off as fragrance during the cooking process, is lost as flavor. For instance, to cook vanilla essence in a soft custard is equivalent to throwing most of it away.