Wheat is the only grain which contains gluten in the proper proportion and of the desired quality essential to the making of light, spongy bread. It contains all the elements necessary for the growth of the body; but, to meet all the requirements of nutrition, the whole of the grain, with the exception of the outer husk, should be used. Wheat has several layers of bran coats, the outer one of which is almost wholly pure silica and is perfectly indigestible. Underneath this husk lie the inner bran coats, containing gluten, a dark substance which is the nitrogenous or flesh-forming element, the phosphates and other mineral matters which help to make up the bony parts of the body, and the oil which gives the characteristic odor to wheat grains. The centre, or heart, of the grain consists of cells filled with starch, a fine, white, mealy powder, which has little value as food except as a heat producer. There is also a small amount of gluten diffused among the starch cells. For convenience, these different parts of the wheat will be designated as bran, or the outer husk; gluten, or the inner bran coats; and starch, or the heart of the wheat. The proportion and quality of the gluten and starch in different kinds of wheat vary according to the climate and soil in which they are grown. They are also affected by the method of grinding the grain. Wheat grown in Southern or warm climates, and in the intense, though short, summer of our own Northwest, contains more nitrogen than that grown in cold, damp climates. It loses more water by evaporation, and consequently the seed is smaller and harder. In some varieties of wheat the outer husk is thin and smooth, and peels off readily under the stones. In others, it is thick and rough, and adheres closely to the kernel. In some, it is light-colored or brittle; in others, dark-colored or tough. The husky portion of wheat is about fourteen or sixteen per cent of the whole weight.
Fig. 1 Grain of wheat, showing outer coat of silex and woody fibre.
The gluten of wheat is a gray, tough, elastic substance, consisting chiefly of vegetable fibrine. It can be examined easily by making a dough of flour and water, and working it on a sieve under a stream of water. The water will carry the starch, sugar, gum, and mineral matters into the pan below, leaving a lump of gluten on the sieve. It closely resembles a piece of animal skin, and, when dried, has a glue-like appearance; hence its name, gluten. The proportion of gluten varies from eleven to fifteen per cent. This tough, elastic quality of the gluten determines the quality of the flour. The more gluten and the tougher or stronger it is, the better the flour. The gluten of good flour will swell to four or five times its original bulk; while that of poor flour does not swell, but becomes watery and sticky, and sometimes gives off a disagreeable odor, owing to the deterioration of the fatty or oily element.
Fig. 2. Grain of wheat with bran coat removed.
Fig. 3. Grain of wheat magnified. A is the bran; B the gluten; C the starch.