Aerated bread is made light by a mechanical process. The liquid with which the flour is mixed into dough is charged with carbon dioxide. The manipulations are so performed as to prevent the gas escaping until the dough is made, when the imprisoned gas expands and renders the dough light at once. The flavor of such bread is not considered so fine as that of the best yeast-risen bread, and people usually tire of it in time.
Baking powders render doughs light by means of carbon dioxide gas. The gas is produced in the dough usually by the action of some acid on sodium bicarbonate. This action is, in some powders, brought about by the presence of moisture alone, while in others heat is necessary to its action. In using the powders which act by the presence of moisture alone, one must work rapidly, that the dough may be placed in the oven and become set by the heat before the gas escapes, in order that the air cells may remain distended.
Baking powders may be classed under three general heads, - tartrate powders, phosphate powders, and alum powders. Some baking powders contain two or more of the above acids in their composition. The tartrate powders, as the name indicates, have tartaric acid for their acid portion. This may be simply tartaric acid, or it may be combined so as to require a different name. The two acids used in the tartrate powders are tartaric acid and cream of tartar. A tartaric acid powder requires that the dough be handled quickly and put into a hot oven, as the acid dissolves readily, and the action is soon over. In using a cream of tartar powder such great haste is not absolutely necessary, for cream of tartar dissolves slowly in cold water, but rapidly when heat is applied. The housekeeper can readily ascertain whether a powder dissolves rapidly or slowly by placing a small amount in a glass, and testing with cold water and then with hot water. All baking powders contain some air-dried starch in addition to the acid and alkaline portions. The object of using the starch is to prevent the powder losing its power by the acid and alkaline portions acting on each other in the presence of absorbed atmospheric moisture. The baking powder can should always be kept closely covered, as moisture enters from the atmosphere.
The phosphate baking powders contain some form of phosporic acid as the acid constituent. Doughs mixed with phosphate powders should be handled quickly, as the powder begins to act as soon as dissolved in water.
Among the chemicals used as the acid portion in alum baking powders are potassium bisulphate (KHSO4) and alum. According to Jago, "the potassium and sodium sulphates produced when these substances are neutralized by sodium bicarbonate are powerful purgatives, and as such are absolutely unfitted for introduction into bread."
It is evident that, for general use, yeast bread, when properly made and baked, is preferable to any of the above-named breads. It has a fine flavor, and produces no deleterious results.
In every farm home the hot breads should usually be made with sour milk and soda. To be skillful in the use of soda requires good judgment and constant practice, but to many the products are more delicious than those in which baking powder is used, and they are very whole-some. When eggs are used for rendering breads and cakes light, we depend on beating air into the eggs and dough, and keeping it there until, expanded by heat, it makes the dough light.
Utensils Used in Bread Making
If one wishes to make baking powder at home, it can be done by mixing cream of tartar and soda in the proportion of one teaspoonful of baking soda and two of cream of tartar. Sift two or three times, to be certain that it is thoroughly mixed, and use at once. To make a powder that can be kept, secure eight ounces of cream of tartar of good quality, four ounces of baking-soda, and three ounces of cornstarch. Be sure that all the ingredients are perfectly dry. Sift the soda and starch (after stirring together in a perfectly dry bowl until mixed) three times, letting it fall onto a paper on the table, and placing a second paper under the sieve. Pour the mixture again into the sieve, and shake through. When these have been sifted three times, stir the cream of tartar into them, and sift in same manner as before. When done, put into a closely-covered tin can or Mason jar, and keep in a dry place, closely covered. Use a heaping teaspoonful to each cup of flour.
References: Science & Art of Bread Making - Jago - pp. 237, 238, 244, 252, 257, 346-365, 386-389. Also pages 360-365, 395, 397, 398.