5. The composition of all cereals is similar, yet each one has its distinguishing feature. While all the five food substances--water, mineral matter, protein, fat, and carbohydrate--are to be found in cereals, they occur in different quantities in the various kinds. Some contain large quantities of protein and others practically none, and while certain ones have considerable fat others possess comparatively small quantities. A characteristic of all cereals, however, is that they contain a large amount of carbohydrate and a small amount of water. It is well to remember, though, that while the food substances of cereals are found in sufficient quantities to sustain life, they will not permit a person to live for long periods of time exclusively on this form of food. Likewise, it will be well to observe that the foods made from a certain grain will be quite similar in composition to the grain itself; that is, any change in the composition of the foods must be brought about by the addition of other substances.
6. All grains are similar in general structure, too. The largest proportion of carbohydrate lies in the center, this substance growing less toward the outside of the grain. The protein lies near the outside, and grows less toward the center. Fat is found in small amounts scattered through the entire grain, but most of it is found in the germ, which is a tiny portion of the grain from which the new plant sprouts. The mineral matter of cereals is found chiefly just inside the bran, or outer covering, so that when this covering is removed, as in the process of preparation for food, a certain amount of mineral matter is generally lost.
7. PROTEIN IN CEREALS.--The cereals are essentially a carbohydrate food, but some also yield a large proportion of protein. In this respect they differ from the animal foods that produce the principal supply of protein for the diet, for these, with the exception of milk, do not yield carbohydrates. The grain that contains the most protein is wheat, and in the form in which protein occurs in this cereal it is called gluten, a substance that is responsible for the hardness of wheat. The gluten, when the wheat is mixed with water or some other liquid, becomes gummy and elastic, a fact that accounts for the rubbery consistency of bread dough. Cereals that contain no gluten do not make bread successfully. Next to wheat, rye contains protein in the greatest amount, and rice contains the least. Although protein is the most expensive of the food substances, the kind of protein found in cereals is one of the cheaper varieties.
8. FAT IN CEREALS.--The fat of cereals helps to contribute to their heat-and energy-producing qualities, and, besides, it is one of the cheaper sources of this food substance. Of the eight grains, or cereals, used as food, oats and corn contain the most fat, or heat-producing material. The oil of corn, because of its lack of flavor, is frequently used in the manufacture of salad oil, cooking oil, and pastry fat. The fat that occurs in cereals becomes rancid if they are not carefully stored. In the making of white flour, the germ of the wheat is removed, and since most of the fat is taken out with the germ, white flour keeps much better than graham flour, from which the germ is not abstracted in the milling process.
9. CARBOHYDRATE IN CEREALS.--The food substance found in the greatest proportion in cereals is carbohydrate in the form of starch. Cereals contain many times more starch than any of the other food substances, rice, which is fully three-fourths starch, containing the most, and oats, which are less than one-half starch, the least. Starch is distributed throughout the grain in tiny granules visible only under the microscope, each being surrounded by a covering of material that is almost indigestible. In the various grains, these tiny granules differ from one another in appearance, but not to any great extent in general structure, nutritive value, or digestibility, provided they are cooked thoroughly. The large amount of carbohydrate, or starch, in cereals explains why they are not hard to digest, for, as is well known, starch is more easily digested than either protein or fat. This and the fact that some grains contain also a large amount of fat account for the high energy-producing quality of cereals. While it is safe to say that cereals are chiefly valuable for their starch, the tissue-building material in some grains, although in small proportion, is in sufficient quantity to place them with the protein foods.