PHYSICAL LEAVENING
PHYSICAL LEAVENING consists in aerating, or incorporating gas or air into, a mixture that is to be baked, and it is based on the principle that air or gas expands, or increases in volume, when heated. It is definitely known that when air is incorporated into dough and then heated, the air increases 1/273 of its own volume for each degree that the temperature is increased. For instance, if the temperature of an aerated mixture is 65 degrees Fahrenheit when it is put into the oven, the air or gas will have doubled in volume by the time it has reached 338 degrees Fahrenheit.
Thus, the success of aerated bread depends to some extent on the temperature of the mixture when it goes into the oven. The colder it is at that time, the greater is the number of degrees it will have to rise before it is sufficiently baked, and the more opportunity will the gas have to expand.
5. The air or gas required for physical leavening is incorporated into a mixture by beating or folding the batter or dough itself, or by folding beaten egg whites into it. If the mixture is thin enough, the beating may be done with a spoon or an egg beater; but if it is thick enough to be handled on a board, air may be incorporated into it by rolling and folding it repeatedly. If eggs are to be used for aerating the batter or dough, the entire egg may be beaten and then added, but as more air can be incorporated into the egg whites, the yolks and whites are usually beaten separately.
To make the white of eggs most satisfactory for this purpose, it should be beaten stiff enough to stand up well, but not until it becomes dry and begins to break up. In adding the beaten egg white, it should be folded carefully and lightly into the mixture after all the other ingredients have been combined. Beaten egg white may be used to lighten any mixture that is soft enough to permit it to be folded in.
6. To insure the best results from mixtures that are to be made light by means of physical leavening agents, certain precautions must be taken. Such mixtures should be baked as soon as possible after the mixing is done, so that the gas or air will not pass out before the dough is baked. Likewise, they should be handled as lightly and quickly as possible, for a heavy touch and too much handling are often the cause of imperfect results.
For baking aerated mixtures, heavy irons are better than tin muffin pans; also, the pans that are used should be heated before the mixture is put into them, so that the batter or dough will begin to expand immediately. Gem irons should be filled level with an aerated mixture.
CHEMICAL LEAVENING
7. CHEMICAL LEAVENING is brought about by the action of gas produced by an acid and an
alkali. All chemical leavening agents are Similar in their action, and they are composed of an acid and an alkali. When an acid and an alkali are brought together in the presence of moisture and heat, the result is the rapid production of carbon dioxide, a gas that expands on being heated, just as all other gases do.
In expanding, the gas pushes up the batters or doughs, and these, when baked, set, or harden,
into porous shapes. In addition to forming the gas, the acid and the alkali produce a salt that remains in the bread, and it is this salt that is responsible for the harmful effect usually attributed to chemical leavening agents.
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