of baking powders rests upon the acid elements which they contain. They may be classified as follows: (1) Cream of tartar baking powder, in which the acid constituent is cream of tartar which is known chemically as acid potassium tartrate. Other forms of tartaric acid may be used in baking powders of this class but they are not common. (2) Phosphate powders, in which the acid constituent is phosphoric acid usually in the form of the acid phosphate of lime. (3) Alum powders in which the acid constituent is alum or some form of aluminium sulfate, usually the basic sulfate of alumina.
The acid and basic constituents of these powders may be kept in separate containers and mixed together at the time of making the dough. A more common form is to use them in such a way that until they mix with the dough they do not exert any notable effect upon each other. For instance, perfectly dry bicarbonate of soda and perfectly dry acid potassium tartrate may be mixed together and kept for quite a while without any notable decomposition of the bicarbonate taking place.
In order to render any such possible action minimum in its effect it is customary to add to the mixture a small quantity of starch, milk sugar, or some other diluent. These materials tend to keep apart the particles of acid and base and render it possible to make a mixture of them which may be kept for a long while without any notable loss of leavening power. When a cream of tartar baking powder is mixed with dough the moisture of the dough gradually dissolves the two ingredients and in this state a chemical reaction occurs between them. The carbon dioxid is set free as a gas, commonly known as carbonic acid. The mineral substance which results is a tartrate of sodium and potassium that is a union of tartaric acid with potash and soda. This compound is commonly known under the term of Rochelle salts. If there be a sufficient quantity of water in the bread to allow the Rochelle salts to crystallize in the usual way a portion of the water becomes incorporated with the salt. Two teaspoonsful of a tartrate baking powder leave a residue of about 11 grams (165 grains) of crystallized Rochelle salts in the loaf.
Phosphate Powders.—As has already been said, the acid constituent of phosphate powder is chiefly acid phosphate of lime. In this case the acid phosphate of lime decomposes the bicarbonate of soda with the production of carbon dioxid and leaves a residue consisting of a mixture of sodium and lime phosphate. If in two teaspoonsful of phosphate powder there are approximately 16 grams (250 grains) there is formed a crystallized residue, about an equal weight of phosphate of soda and lime, which is left in the loaf.
Alum Powders.—Perhaps by far the largest part of baking powders used contain alum in some form as the acid constituent. Formerly the common substance known as alum or burnt alum was employed but in late years an aluminium basic salt known as basic sulfate of aluminium has largely succeeded the old form of alum. When the reaction takes place in the dough
