Page:Popular Science Monthly Volume 23.djvu/706

688 The starch of every plant differs from its neighbors both in size and shape, and this has a considerable influence on the character of the vegetable organ in which it is stored up; the hardness of rice, for instance, being due to the fact that rice-granules are extremely minute, with angular corners which fit closely and firmly together; whereas potato-starch is large and round, with considerable interspaces filled with water, and so forms a comparatively soft mass. But, notwithstanding their outward points of difference, in chemical composition the starches are all identical, consisting of carbon, hydrogen, and oxygen—exactly the same materials as sugar is composed of, and better known as the component elements of coal and water. Leaving the many varieties of starch in the mean while, let us consider one species, namely, that of wheat, because it is the most important in this country, forming the basis of our daily bread.

An ordinary grain of wheat, if sliced through the middle and examined as to its structure, will be found to consist of several layers, the outer a hard coating, which contains mineral salts, lime, sand, etc. Beneath this is a zone of matter very rich in gluten, the flesh-forming constituent of the wheat; while the central portion of the grain is occupied by a white, powdery mass, which is nearly pure starch. In manufacturing flour, the two outer layers, which together form the bran, are usually removed, leaving the white starchy flour of the central portion.

Let us now briefly consider the chief points in the chemistry of bread-making. If flour be worked up with water, it forms a sodden, insipid, indigestible mass; but, if heated to the temperature of boiling water, the starch-granules burst; and it is thereby rendered a little more digestible, although still forming a close, stiff, and not very palatable cake. Such is the character of unleavened bread, and of sea biscuits, a slightly different form of the same thing. To be fit for digestion, starch must be dissolved or softened by boiling or baking; hence the reason why raw nuts are so indigestible as compared with the favorite roasted chestnuts; and hence one reason for cooking food, which mankind has been taught by experience, ages before chemistry could give a scientific explanation of the reason why. Cooking is, in fact, a partial digestion; and the same is the case with baking, both being preliminary aids to the changes which take place in the mouth and stomach before the food is in a fit state for the preparation of the blood. Accordingly, we bake our bread; and we bake it in the way we do because a soft, spongy loaf is more readily moistened and acted on by the saliva and the juices of the stomach.

There is a good deal in the chemistry of bread-making; and our bread might be much improved if bakers had a more intelligent understanding of the science involved in their business; for, although several improvements have been introduced of late years, the most of our bread is still prepared in the old fashion. The necessary quantity of flour is