4. 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.