Foods, their selection and preparation

In leavening the various flour mixtures, a gas, or some substance which will form a gas, is distributed through the batter or dough. During baking, this gas expands and makes the mixture light. Then the heat cooks the starch and coagulates the protein of the flour or egg or both and sets the product so that it remains light. In some cases, the gas formed comes from one of the essential ingredients. In others, it is formed by a chemical reaction which takes place as the result of an added in¬ gredient. In still others it is due either to a fermentation resulting from an added ingredient, or is the result of a special manipulation of the ingredients. The temperature used in baking bears an important relation to the kind of leav¬ ening used. In some cases heat affects the release of the gas and in all cases it expands the gas in the mixture. KINDS OF LEAVENING AGENTS Leavening agents may be classified as follows: Physical leavening agents, which include air, steam, and occasionally other volatile substances; chemical leavening agents, including baking soda used with acid, and baking powder; and organic or biological leavening agents, including yeast and gas-forming bacteria. Air as Leavening. There are several ways in which air may be incorporated in flour mixtures: by using egg-white foam (beaten whites); by creaming fat and sugar; and, to a lesser extent, by repeated sifting of the flour or by beating the mixture. The air thus entrapped expands when the mixture is heated and leavens or helps to leaven it. It is necessary to bake products leavened entirely with air comparatively slowly, so that a crust will not be formed over the top before the air has expanded. Egg- White-Foam. Egg white is one of the best mediums for incorporation of air. Its viscosity is such that its volume may be increased several times by the amount of air that can be entrapped in it. The yolk differs in composition from the white and will not hold air in the same way. Conse¬ quently when addition of air is desired, the two portions of the egg are beaten separately. This method of leavening is always used in the typical sponge cakes, and to some extent in cakes made with fat. Creaming Fat and Sugar. The cream¬ ing of fat and sugar in cakes made with fat incorporates air and helps to leaven the prod¬ uct. Pound cake, the richest of the cakes, depends in part upon air incorporated during creaming for its leavening. Sifting the Flour. Sifting the flour introduces some air into the mixture. A measure of the amount of air that is intro¬ duced in this way may be obtained by observing the differences in bulk of a cup [289] of flour after repeated siftings. Flour should always be sifted once before meas- uring. After two or three siftings, so little air is introduced that any further sifting is hardly justified. Beating Flour Mixtures. Air may be incorporated directly into a batter if the batter has an elastic texture due to the presence of a large proportion of gluten or egg. This method is rarely used as an exclusive means of leavening. It is of value, when used in addition to creaming, in pound cake, where a smooth, even, not especially light texture, is desired. Beaten biscuits contain no added leav¬ ening. Their texture is due, in part, to air incorporated by folding and beating. The old rule was to "beat them until they blister.”