Baking powder is a chemical preparation which, when brought in contact with water, liberates carbon dioxid gas. The baking powder is mixed dry with flour, and when this is moistened the carbon dioxid that is liberated expands the dough. The action is similar to that of yeast except that in the case of yeast the gas is given off much more slowly and no residue is left in the bread. When baking powder is used, there is a residue left in the food which varies with the material in the powder. It is the nature and amount of this residue that is important and makes one baking powder more desirable than another.

Fig. 51.—Ingredients of a Baking Powder.

1, baking powder; 2, cream of tartar; 3, baking soda; 4, starch.

196. Cream of Tartar Powders.—The acid ingredient of the cream of tartar powders is tartaric acid, H2C4H4O6. Cream of tartar is potassium acid tartrate, KHC4H4O6; it contains one atom of replaceable hydrogen, which imparts the acid properties, and it is prepared from crude argol, a deposit of grape juice when wine is made. The residue from this powder is sodium potassium tartrate, NaKC4H4O6, commonly known as Rochelle salt. This is the active ingredient of Seidlitz powders and has a purgative effect when taken into the body. The dose as a purgative is from one half to one ounce. A loaf of bread as ordinarily made with cream of tartar powder contains about 160 grains of Rochelle salt, which is 45 grains more than is found in a Seidlitz powder, but the amount actually eaten at any one time is small and its physiological effect can probably be disregarded. When a cream of tartar baking powder is used, the reaction takes place according to the following equation:

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HKH4C4O6 + NaHCO3 = KNaC4H4O6 + CO2 + H2O.

The crystallized Rochelle salt contains four molecules of water, so that, even allowing for some starch filler, there is very nearly as much weight of material (Rochelle salt) left in the food as there was of the original powder. If free tartaric acid were used instead of potassium acid tartrate, the reaction would be as follows:

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H2C4H4O6 + 2 NaHCO3 = Na2C2H4O6.2 H2O + 2 CO2.

But the residue, sodium tartrate, is less in proportion. It has physiological properties very similar to Rochelle salt. Tartaric acid is seldom used alone, but very often in combination with cream of tartar. It is more expensive than cream of tartar; but not so much is required, and it is more rapid in action.

197. Phosphate Baking Powders.—Here the acid ingredient is phosphoric acid and the compound usually employed is mono-calcium phosphate, CaH4(PO-{4})2. This is made by the action of sulphuric acid on ground bone (Ca3(PO4)2 + 2 H2SO4 = CaH4(PO4)2 + 2 CaSO4), and it is difficult to free it from the calcium phosphate formed at the same time; hence such powders contain more or less of this inert material. The reaction which occurs with a phosphate powder is as follows: