195. General Composition.—All baking powders contain at least two materials; one of these has combined carbon dioxid in its composition, the other some acid constituent which serves to liberate the gas. The material from which the gas is obtained is almost invariably sodium bicarbonate, NaHCO₃, commonly known as "soda" or "saleratus." Ammonium carbonate has been used to some extent, but is very seldom used at the present time. The acid constituent may be one of several materials, the most common being cream of tartar, tartaric acid, calcium phosphate, or alum. These may be used separately or in combination. The various baking powders are designated according to the acid constituent, as "cream of tartar," "phosphate," and "alum" powders. All of them liberate carbon dioxid gas, but the products left in the food differ widely in nature and amount^[69].

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:

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:

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:

Sodium phosphate, according to the United States Dispensatory, is "mildly purgative in doses of from 1 to 2 ounces." The claim is made by the makers of phosphate baking powders that the phosphates of sodium and calcium, products left after the baking, restore the phosphates which have been lost from the flour in the bran. This baking powder residue does not restore the phosphates in the same form in which they are present in grains and it does furnish them in larger amounts—nearly tenfold. However, the residue from these powders is probably less objectionable than that from alum powders. The chief drawback to the phosphate powders is their poor keeping qualities.

198. Alum Baking Powders.—Sulphuric acid is the acid constituent of these powders. The alums are double sulphates of aluminium and an alkali metal, and have the general formula xAl(SO₄)₂ in which x may be K, Na, or NH₄, producing respectively a potash, soda, or ammonia alum. Potash alum is most commonly used, soda and ammonia alums to a less extent. The reaction takes place as follows:

If it is a potash or soda alum, simply substitute K or Na for NH₄ throughout the equation. The best authorities regard alum baking powders as the most objectionable. Ammonia alum is without doubt the worst form, since all of the ammonium compounds have an extremely irritating effect on animal tissue. Sulphates of sodium and potassium are also objectionable. Aluminium hydroxide is soluble in the slightly acid gastric juice and has an astringent action on animal tissue, hindering digestion in a way similar to the alum itself. Many of the alum powders contain also mono-calcium phosphate; the reaction is as follows:

These are probably less injurious than the straight alum powders, although the residues are, in general, open to the same objection.

199. Inspection of Baking Powders.—Many of the states have enacted laws seeking to regulate the sale of alum baking powders. Some of these laws simply require the packages to bear a label setting forth the fact that alum is one of the ingredients; others require the baking powder packages to bear a label naming all the ingredients of the powder.

200. Fillers.—All baking powders contain a filler of starch. This is necessary to keep the materials from acting before the powder is used. The amount of filler varies from 15 to 50 per cent; the least is found in the tartrate powders and the most in the phosphate powders. The amount of gas which a powder gives off regulates its value; it should give off at least of its weight.

201. Home-made Baking Powders.—Baking powders can be made at home for about one half what they usually cost and they will give equal satisfaction. The following will make a long-keeping powder: cream of tartar, 8 ounces; baking soda, 4 ounces; corn starch, 3 ounces. For a quick-acting powder use but one ounce of starch. The materials should be thoroughly dry. Mix the soda and starch first by shaking well in a glass or tin can. Add the cream of tartar last and shake again. Thorough mixing is essential to good results. Cream of tartar is often adulterated, but it can be obtained pure from a reliable druggist. To insure baking powders remaining perfectly dry, they should always be kept in glass or tin cans, never in paper.