When bread is made by yeast or leaven the process, which may be divided into three stages, is as follows:

(1) The wheat or other flour, finely ground, is mixed into a thick paste with water, which may be either warm or cold, and to which the ferment, together with a little salt, is added; the mass is thickened with flour to form a dough, and the dough is well worked by hand or kneaded so that its several ingredients may become most thoroughly incorporated. If this is not done properly the bread is lumpy or of uneven porosity. The entire mass may be prepared at once, or a small part of the flour is first allowed to ferment for a short time, and then is kneaded into the remainder.

(2) The dough is next set aside for some hours in a warm place to rise. This process consists of a fermentation which is produced in the flour by the action of the yeast, resulting in the freeing of carbon dioxide and water. The accumulating carbonic-acid gas endeavours to escape in bubbles, which become entangled in the more or less tenacious gluten of the flour. Upon the size and number of these bubbles depends the porosity of the bread, and this in turn is modified somewhat by the kind of flour used, the quantity of the ferment, and the rapidity with which the development of carbonic acid is allowed to proceed.

(3) The final stage consists in the baking of the dough after it has risen. The heat of the oven, by expanding the carbonic-acid gas, makes the bread still more porous and "sets" the walls of the little cavities which have formed so that the loaf maintains its shape. The gas is finally driven off, together with a large quantity, but not all, of the water. The baked bread is therefore considerably lighter in weight than the dough, much drier, and porous.

The water added to make the dough escapes in part through evaporation, and the external portion of the bread becomes drier and browner than the interior or crumb, and constitutes the crust. The thickness of the crust will depend upon the character of the flour used, the temperature of the oven, and the duration of the process of baking. Bauer claims that the crust contains less nitrogenous material than the crumb, but this is contradicted by Dujardin-Beau-metz, and there is no definite reason why it should.

With the exception of these changes, bread has practically the same composition as its original flour.

Mixing meal or flour with fat tends to prevent the evaporation of water from the bread.

The chief art in baking bread consists in arresting the yeast fermentation of the dough by the heat of the oven (3000 to 400° F.) at exactly the right period. If fermentation has not proceeded far enough the bread is tough, or sodden, or lumpy, whereas if it has gone too far it acquires a sour taste by the development from the carbohydrates or organic acids, such as acetic, butyric, and lactic, which are both unpalatable and unwholesome. Fermentation produced by the use of leaven instead of yeast is much more difficult to control, and these acids, therefore, are more likely to be formed. The bread made with fresh brewers' yeast is by many esteemed to have the best flavour. The process of bread-baking also causes the starch granules to burst, if this has not already occurred from absorption of water in the dough, and results in the conversion of some of the starch into dextrin, with the further formation of alcohol and sugar (glucose). About 6 to 8 per cent of the starch is thus made soluble.

There are several minor chemical changes, and the entire action of yeast and heat upon the dough is thus summarised by H. Snyder and L. A. Voorhees (U. S. Department of Agriculture Bulletin No. 67, 1899):

"(1) The fermentation of the carbohydrates and the production of carbon dioxide and alcohol; (2) the production of soluble carbohydrates, as dextrin, from insoluble forms, as starch; (3) the production of lactic and other acids; (4) the formation of other volatile carbon compounds; (5) a change in the solubility of the proteid compounds; (6) the formation of amid and ammonium compounds from soluble proteids; and (7) the partial oxidation of the fat".

About 2 per cent of the weight of the flour used is lost in volatilising various organic products. With prolonged fermentation this loss may amount to 8 per cent.

The soluble dextrin has the physical properties of a gum. It is obtained by heating starch to 3000 to 4000. In steam-cooked cereals also, the starch is partially dextrinised.