Bread properly made with yeast undergoes certain chemical changes which render it lighter, more porous, more pleasant to the taste, and more healthful, because more easily digested, and more convenient for general use. It is generally recommended by scientific and medical men as the best form of bread.

Fig. 4. Yeast Plant.

Fig. 4. Yeast Plant.

Wheat contains a larger percentage of starch than of anything else. We learn, in the chapter on Digestion, that starch as such is not absorbed into the human system. It must first be transformed into sugar. All starch that is not changed into sugar by the process of cooking or before our food is eaten, is so changed by the ptyalin, or ferment of the saliva, and the ferment of the pancreatic fluid. Any process which produces this change for us makes our food more digestible. "Powdered alum will dissolve in water sooner than a crystal of alum." Any fluid will penetrate more easily through a sponge than through putty, and the salivary and gastric fluids are no exception to this rule. Wheat starch in its natural state is close and compact; and bread made simply with flour and water, and baked at once, will be close, dry, and difficult to masticate and digest. Good bread should be sufficiently soft to be easily crushed in the mouth, and of such a light, spongy texture that all the starch cells may be ruptured, and the greatest possible amount of surface be presented to the action of the digestive fluids. To obtain these qualities in bread, we try to expand the dough as much as possible without destroying its natural sweetness. Owing to the peculiar elasticity and tenacity of the wheat gluten, this is very easily accomplished by alcoholic fermentation. The flour is moistened with some warm liquid, yeast and salt are added, and it is then exposed for some hours to a temperature of about 70°. The yeast changes some of the starch of the flour into sugar, and the sugar into alcohol and carbonic acid gas. This gas, being lighter than the dough, rises, and, in its efforts to escape, expands the elastic, glutinous dough into a mass two or three times its original bulk. The toughness or elasticity of the gluten prevents the gas from escaping; and when this expansion has reached the desired limit, - that is, before the alcoholic fermentation has changed to the acetic and soured the dough, or the tough, glutinous walls of the air cells are broken, - we check the formation of gas, and kill the ferment by baking the dough in a hot oven. The alcohol escapes into the oven; some of the starch is changed into gum, and forms the crust; and the rapid decomposition, produced by the intense heat, causes the crust to assume a brown color.