The chemical reactions that need to be considered under this head are the evolution of carbonic acid for lightening bread and cake, and the action of acids upon coagulable proteins.

297. The Evolution Of Carbonic Acid

This is done in order to lighten the texture of bread and cake, and is brought about, excepting in yeast fermentation) by the reaction of some acid or acid salt on a bicarbonate of either sodium or potassium, generally the former. In old-fashioned cooking, sour milk and "saleratus" (potassium bicarbonate) were used in making "saleratus biscuit." In this case, the carbonic acid was displaced in the saleratus by the action of the acids or acid salts in the milk, forming little vesicles of ga3 all through the dough, thus greatly lightening its texture. When soda (sodium bicarbonate) is added to dough sweetened with molasses, the same result is reached because of the acid substances in the molasses. In order that there may be no excess of soda or acid, it is necessary that they be combined in definite proportions. In home cookery, where sour milk and molasses are used, the combination is more or less "hit or miss "for the acidity of these materials is not always the same. When the acids are in excess rather than the soda, the results are not serious, as is the case when too much soda is used. At the present time "baking powders" are in the market, in which the combinations are chemically correct.

298. The Coagulation Of Proteins

The housewife of experience does not need to be reminded that vinegar and milk do not always constitute a compatible mixture, as the casein of the milk sometimes precipitates or forms a curd. The same change sometimes takes place when milk is improperly added to tomato soup. The results with vinegar and milk, that is, acetic acid and the calcium casein, depend upon the proportion of acid to casein. The curdling of the milk is due to the fact that the acid of the vinegar removes the lime from its combination with the casein; but precipitation of the casein does not begin until the amount of acid passes a certain proportion. Probably one half ounce of 4 per cent vinegar could be added to one quart of milk without causing curdling when the milk is heated to boiling, but much beyond this proportion would precipitate the casein. At ordinary temperatures much more vinegar would be used, perhaps two ounces. The proportion of vinegar would depend on its strength, of which the housewife is not generally informed, an example of the inexact way in which much cooking is necessarily done. The same considerations apply to the use of milk with acid materials, such as fruits containing citric, tartaric, or malic acids. The condition of the milk is to be considered, too. Fresh milk will bear more acid than milk in which more or less lactic acid has developed and combined with part of the lime in the casein compounds.

When meat proteins and the white of egg are cooked, coagulation occurs, with a hardening of the meat tissue and of the egg albumin. This is discussed more fully in what follows.