Protein.--The food substance known as protein is a very important factor in the growth and repair of the body; in fact, these processes cannot be carried on unless protein is present in the diet. However, while a certain quantity of protein is essential, the amount is not very large and more than is required is likely to be harmful, or, since the body can make no use of it, to be at least waste material. The principal sources of protein are lean meat, eggs, milk, certain grains, nuts, and the legumes, which include such foods as beans and peas. Because of the ease with which they are digested, meat, fish, eggs, and milk are more valuable sources of protein than bread, beans, and nuts. However, as the foods that are most valuable for proteins cost more than others, a mixed diet is necessary if only a limited amount of money with which to purchase foods is available.

So much is involved in the cooking of foods containing protein that the effect of heat on such foods should be thoroughly understood. The cooking of any food, as is generally understood, tends to break up the food and prepare it for digestion. However, foods have certain characteristics, such as their structure and texture, that influence their digestibility, and the method of cooking used or the degree to which the cooking is carried so affects these characteristics as to increase or decrease the digestibility of the food. In the case of foods containing protein, unless the cooking is properly done, the application of heat is liable to make the protein indigestible, for the heat first coagulates this substance--that is, causes it to become thick--and then, as the heat increases, shrinks and hardens it. This fact is clearly demonstrated in the cooking of an egg, the white of which is the type of protein called albumin. In a raw egg, the albumin is nearly liquid, but as heat is applied, it gradually coagulates until it becomes solid. If the egg is cooked too fast or too long, it toughens and shrinks and becomes less palatable, less attractive, and less digestible. However, if the egg is properly cooked after the heat has coagulated the albumin, the white will remain tender and the yolk will be fine and mealy in texture, thus rendering it digestible.

Similar results, although not so evident to the sight, are brought about through the right or wrong way of cooking practically all other foods that contain much protein. Milk, whose principal ingredient is a protein known as casein, familiar as the curd of cheese, illustrates this fact very plainly. When it is used to make cottage cheese, heating it too long or to too high a degree will toughen the curd and actually spoil the texture of the product, which will be grainy and hard, instead of smooth and tender.

FATS.--The food substances just discussed--water, mineral matter, and protein--yield the materials required for building and repairing the tissues of the body, but, as has been explained, the body also requires foods that produce energy, or working power. By far the greater part of the total solids of food taken into the body serve this purpose, and of these fats form a large percentage. Although fats make up such a large proportion of the daily food supply, they enter into the body composition to a less extent than do the food substances that have been explained. The fats commonly used for food are of both animal and vegetable origin, such as lard, suet, butter, cream, olive oil, nut oil, and cottonseed oil. The ordinary cooking temperatures have comparatively little effect on fat, except to melt it if it is solid. The higher temperatures decompose at least some of it, and thus liberate substances that may be irritating to the digestive tract.

CARBOHYDRATES.--Like fats, the food substances included in the term carbohydrates supply the body with energy. However, fats and carbohydrates differ in the forms in which they supply energy, the former producing it in the most concentrated form and the latter in the most economical form.

So that the term carbohydrate may be clearly understood and firmly fixed in the mind, it is deemed advisable to discuss briefly the composition of the body and the food that enters it. Of course, in a lesson on COOKING, not so much attention need be given to this matter as in a lesson on dietetics, which is a branch of hygiene that treats of diet; nevertheless, it is important that every person who prepares food for the table be familiar with the fact that the body, as well as food, is made up of a certain number of chemical elements, of which nitrogen, carbon, hydrogen, and oxygen form a large part. Protein owes its importance to the fact that of the various food substances it alone contains the element nitrogen, which is absolutely essential to the formation of any plant or animal tissue. The other three elements, carbon, hydrogen, and oxygen, go to make up the carbohydrates; in fact, it is from the names of these three elements that the term carbohydrate is derived. The carbohydrates include the starches and sugars that are used and eaten in so many forms, and these contain the three elements mentioned, the hydrogen and oxygen contained in them being in the proportion that produces water. Thus, as will readily be seen, by separating the name into its parts--carbo (carbon) and hydrate (hydrogen and oxygen in the proportion of two parts of hydrogen and one of oxygen, that is, in the form of water)--carbohydrate is simply carbon united with water. While the facts just brought out have much to do with food economy, they are of interest here chiefly because they help to make clear the term carbohydrate, which, as will be admitted, is the only correct name for the food substance it represents.