FIG. 20.-Composition of common vegetable foods. (Drawn from Charts of U. S

Dept. of Agriculture by Mabelle Baker.)

E

kind of food substances that we found in plants; and this is what we might expect, since the cow is wholly dependent upon grass and other vegetable foods. Indeed, when we analyze any other animal foods that we eat, we find that all consist of one or more of the food substances which closely resemble those that we have been studying in plant biology. In Figures 19 and 20 are represented not only the various nutrients found in some of our most common foods, but also their relative proportions in percentages.

52. Composition of various foods. - (Home study.)

1. Name the foods represented in Figures 19 and 20 that are derived from animals; name those obtained from plants.

2. Which of the two classes of foods, named in 1 above, has on the average the larger percentage of protein?

3. In which of these two classes do you find the larger amount of fats?

4. Which class has the larger percentage of carbohydrates? 5. What, then, are the principal differences in the composition of animal and vegetable foods?

6. Name the various food substances found in one animal food and in one vegetable food, giving in each case the percentage of each nutrient.

IV. USES OF THE FOOD SUBSTANCES

53. Uses of the food substances to plants and animals. In our study of green plants we learned that these living organisms can manufacture the food substances they need from the simple compounds (water, carbon dioxid, and mineral matters) found in earth, air, and water, and that these food substances are used for the making of protoplasm and the liberation of energy. Animals and human beings, on the other hand, since they cannot make their foods, are

either directly or indirectly dependent on plants for their food supply. They use these food materials, however, for the same purposes as do plants. The use of the individual food substances will now be considered.

54. Uses of proteins.

Protein is an essential constituent of plant protoplasm (P. B., 43). This class of nutrients is also essential for the growth and repair of the living substance in muscle, nerve, and all other tissues of the human body. Proteins may also be oxidized in the body and give heat and muscular energy.

55. Uses of fats and carbohydrates. The chief fuel ingredients of food, however, are the fats and carbohydrates. Most of the fat in our foods is probably oxidized soon after it reaches the cells to furnish heat and power, and this class of nutrients possesses two and a half times the fuel value of any other kind of food substance. This is the reason why the inhabitants of arctic countries eat such large quantities of fatty foods.

The starches and sugars of bread, potato, fruits, and milk are also used as fuel. The fat which we stated (44) is stored in various parts of the body, is derived partly from the carbohydrates and partly from the fats in our food, and this acts as a reserve fuel. That portion of the fat which is stored in the deeper layers of the skin helps to keep our bodies warm by preventing the escape of heat.

It is evident

56. Comparison of uses of the nutrients. that the three nutrients thus far studied may be used to supply the body with energy. If our diet is deficient in any one of the three, the others supply the need, and are burned instead. For growth and repair, however, proteins are absolutely essential; neither carbohydrates nor fat can be transformed

into this essential ingredient of protoplasm. Hence, an animal soon dies if it is not supplied with proteins.

If a machine is to do a large amount of work, it must be large enough and strong enough, and must have plenty of fuel. This is true of the body machine. A man who does hard work, and a good deal of it, needs plenty of proteins in his food to build up his tissues and keep them in repair, and plenty of fats and carbohydrates for fuel.

57. Uses of mineral matters and water. The mineral matters like phosphate and carbonate of calcium and magnesium are necessary for making bones and teeth, and for the making of protoplasm (P. B., 43). Salt is used in large quantities by all civilized nations; it makes food more palatable and it is important in the making of digestive fluids.

Water is an essential constituent of protoplasm, and hence the body needs it constantly. Water also aids in dissolving foods. A considerable amount is supplied by the water contained in some of our solid foods, and we get the rest from the water and other beverages that we drink.

V. COOKING OF FOODS

58. Importance of proper cooking. Some of our foods, like milk, nuts, and fruits, are eaten without being cooked. The great majority, however, before they are taken into our bodies are changed considerably. It is important for us to learn the essential principles of good cooking, since food, as often prepared, loses much of its flavor, becomes more or less indigestible, and is deprived of a considerable percentage of its nutrition.

59. Reasons for cooking animal foods. — In civilized communities meats and other animal foods are usually cooked by broiling, roasting, boiling, or frying. The reasons for cooking the flesh

of animals are these: (1) proper cooking loosens and softens the fibers, thus preparing the meat for mastication and for the action of the digestive juices; (2) the heat kills the harmful bacteria and other parasites (e.g. tapeworms) that are sometimes found in foods. of animal origin; (3) cooking makes the meat more attractive in appearance and often improves its flavor; and (4) cooked meat is more completely digested. It is probably true, however, that raw or partly cooked meats are more easily digested.

60. Frying. If meats are fried, the skillet should be very hot, so that the surface of the meat may be coagulated at once, thus preventing the escape of nutrients and the entrance of fats. Frying usually involves the use of additional fats, and since frying tends to make foods indigestible, this is doubtless the poorest method of cooking meat.

61. Soups. If we wish to obtain nutritious soups, the meat should be cut into rather small pieces and first put into cold water to which a little salt has been added. A small proportion of the proteins, and large amounts of so-called "extractives," or flavoring matters, are drawn out by the water and salt, and since the meat is in small pieces, a considerable proportion of the mineral matter is thus dissolved. When we warm the mixture, we cause the fats to melt, and when it is boiled, much of the tough connective tissue is made more or less soluble by being turned into gelatin. The soups thus obtained are made more palatable by the addition of condiments.

The meat which is left after the soup has been prepared is more or less tasteless. Only small percentages, however, of the nutrients have been withdrawn; hence the soup meat should not be thrown away, but should be used as described in 71.

62. Stewing. It is unfortunate that stews are not more highly regarded in American families, for by this method of preparing animal foods all the nutritive ingredients are utilized. To make a good stew the meat should be cut into rather small pieces and placed in cold water. Some of the flavoring matters and soluble proteins