The sour-dough principle is the ancient method of aerating the dough and bread, still used to this day in the making of rye bread The sour dough may be obtained in the first place by letting dough prepared in the usual way with an addition of yeast get over-ripe or sour; then using this sour dough in place of yeast, retaining a small part of the sour dough to mix with unyeasted dough in the proportion desired to obtain new leaven or sour dough within the desired period. Two fermentations proceed in the development of sour dough; yeast fermentation producing mainly carbonic acid gas and some lactic acid gas; and lactic fermen tation producing lactic acid, but very little car bonic acid gas. It is the lactic acid that ripens the dough, the gas that aerates it.
The yeast generally employed in the form of pressed yeast cakes or bricks or mixed with corn meal and dried is composed of minute vegetable organisms, consisting of single cells, which are plainly visible as such under the microscope, but are invisible to the unaided eye. A speck of yeast just about visible contains as much as a hundred individual yeast organ isms. The yeast, being a living organism, re requires food for its sustenance, growth and reproduction. In utilizing this food it produces out of the sugar in the dough the gas which aerates the bread, an dthis fermentation is the aerating principle. Bakers' yeast is now specially prepared by yeast factories, where it is gained by the fermentation of malt and corn mashes under the influence of intense aeration. The yeast is allowed to settle, is then centri fuged and pressed and employed in the bakery within a few days. Dried yeast will keep serv iceable for a long time.
Sugar and malt extract are added to the dough, not so much for sweeteningpurposes as for the necessity of these bodies for yeast fermentation. The sugar is decomposed in dough fermentation by the yeast into alcohol and carbonic acid gas; the latter is held in place by the tenacity of the above mentioned protein particles, which permits of thorough and uni form aeration; the malt extract contains sub stances besides sugar which, like the amino bodies and phosphates, are utilized by the yeast as foods, which causes a more vigorous fer mentation. Saccharose (cane or beet sugar), maltose, glucose, solid or in syrup form, are employed.
In the use of baking powders the aeration is accomplished on quite a different principle. Baking powders are composed of a weak acid, generally tartaric acid or cream of tartar, and bicarbonate of soda. Through the interaction
of these two ingredients, in the presence of i water, the carbonic acid contained in the bicar bonate of soda is liberated, and the aeration ensues. Baking powders are usually employed in the preparation of quick-rising products, like homemade cakes or biscuits, griddle cakes, etc.
Baking Process, Wheat Bread.—Mixing.— In making wheat bread on a large scale the various ingredients, flour, water, salt, sugar and malt extract, lard or oil and yeast, are mixed in a mixing machine, the yeast being previously dissolved by stirring or whipping it in water. After mixing thoroughly to a homogeneous mass, the dough is placed in a trough, con structed of wood or sheet iron, the dimensions of the trough varying from 2 to 3 feet in width; depth about the same; length from 5 to 10 feet, in which dough fermentation gradually sets in and progresses for about 2 to hours to the extent of causing the dough to rise in the trough to treble and more its original volume. Now the dough is punched in the trough or kneaded and worked until it has fallen back to nearly its original volume; it is again left to rise. The second dough fermentation being finished in about hours, when it receives its second punch, that is, it is again kneaded and worked until it has fallen down. It is again allowed to ferment, this time for about 45 minutes, when it receives its third punch, and is then ready, after about 10 minutes more, to go to the bench, that is, in hand-operated bakeries, the dough is placed on a large flat, oaken table, is kneaded, cut into pieces, weigh ing 10, 12, 16, 21 ounces, or other denominations. These pieces are properly molded in loaf shape and are placed, flattened out, into the bread pans. These are placed on racks that are con veyed on rollers to the steam, or proofing cabinet, usually constructed of sheet iron, filled with an atmosphere of steam, where the pans are left for about 50 minutes. The dough here achieves its last rise, filling the pans quite to the level of the rim. Then they are taken out and placed in the oven, where they remain for about 35 minutes, which is usually the time of a bake. Here the bread increases its volume once more, mainly through expansion of the gases caused by the high temperature of the oven. This is the spring, the proper volume of which is very important to the quality of the bread, and should be one-fourth of the con tents of the pan, increasing the volume of the bread by one-fifth of the whole loaf.