This is introduced in three ways.
(1) By using an acid with a carbonate.
(2) By yeast fermentation.
(3) By machinery.
Yeast fermentation is studied in the chapter on bread making (Chapter XII (Yeast Bread)), and the mechanical method is a commercial process exclusively. Only the first method will be treated in this chapter.
When an acid and any alkaline carbonate are dissolved together, a chemical action takes place, a gas is given off (carbon dioxide) and another substance is formed that is neutral, being neither acid nor alkaline, and known as a "salt." In selecting the two sub-stances we must bear in mind this neutral substance that remains in the batter and insure its harmlessness.
The lactic acid of sour milk is probably the earliest used, being a domestic product. The lactic acid is neutralized by bicarbonate of sodium, the latter being also called "baking soda." The resulting salt is harmless.
Acid molasses with soda is another old-fashioned method. Here the acid is developed by the fermentation of the molasses.
Cream of tartar (acid potassium tartrate), obtained from crystals deposited in wine vats, came into use later, neutralized by bicarbonate of soda, two parts of cream of tartar to one of soda.