This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
It has been well said that the quality of the bread used by the inhabitants of any country is a fair measure of their civilisation. Flour is prepared from various grains by crushing and grinding processes. The grains consist of (1) an outer layer, the husk or skin, which is woody, fibrous, and indigestible, and which in the milling process is separated into "bran"; (2) the kernel within the husk, which is composed of gluten, fats, and salts; (3) the starch.
To appreciate the important details of bread-making, it will be necessary to first review the structure and composition of the grain from which the bread is derived.
The wheat kernel is subdivided into four layers. The first or outermost layer (Fig. 1, H) consists of two or three strata of elongated cells the long diameters of which correspond with the long axis of the grain. From these cells slender filaments or tapering, hair-like processes project outward. The cell margins are irregular in outline, and appear somewhat beaded.
Immediately beneath the outer hairy layer lies the second layer (Fig. 1, F), consisting of more or less quadrangular cells, with rounded angles, which are more uniform in size than the others, and grow at right angles to them. The third layer (Fig. i, K) consists of a delicate, transparent membrane-like structure. The fourth or internal layer (Fig. i, S) is composed of large, almost rectangular cells arranged in one or two strata, and which contain a dark granular material which may be easily separated from the cell walls.
The grains of other cereals conform in a general way to the structure of the wheat grain, although they differ in the thickness of the several layers, the number of their strata, and the size of the individual cells.
Fig. 1. - Section through wheat kernel (from Rupp). H, hairlike processes; F, K, second and third layers; S, fourth internal rectangular cell layer; a, b, c, d, successive layers represented as partially stripped off.
Fig. 2. - Microscopic characters of wheat (+ 200) (from Landois and Stirling), a, cells of the bran; b, cells of thin cuticle; c, gluten cells; d, starch cells.
Bran (Fig. 2, a) contains carbohydrate material which is but little if at all digested in the human alimentary canal, although the lower animals derive abundant nutrition from it. The nutritive salts of wheat are chiefly contained in the bran; and for this reason, if bread constitutes the principal food for a time, it is best to eat that which contains some bran. But if too much is consumed it hastens peristalsis, and nutrition suffers because the food is hurried out of the alimentary canal before absorption is complete. When bread is eaten with other food containing: nitrogen and salts, white bread is preferable.