Yeast bread is made light by the presence of a gas produced by the
action of yeast in the sponge or dough. Yeast is a microscopic plant
which grows in a moist, warm temperature and feeds on starchy materials
such as are present in wheat. A portion of the starch is converted into
sugar (thus developing new and pleasant flavours), and some is still
further changed, giving off the gas upon which the lightness of the
bread depends. If the yeast is allowed to work for too long a time or
the temperature is very hot, a souring of the dough may result. This
souring can be prevented by kneading the dough thoroughly, as soon as it
has risen well or doubled in bulk, or by putting it in a very hot oven
to bake, when it has reached this stage. The yeast plant thrives in a
heat of about the same temperature as our bodies. A little extra heat
will only make it more active, but boiling temperature will kill it.
Cold makes yeast inactive, though it does not kill the plants.
Yeast develops in a natural state on hops and other plants. It is
prepared for market in the form of dry or moist cakes. The latter must
be kept very cold. For home use, a liquid yeast is often prepared from
the dry cakes. This has the advantage of being more active.
When the yeast has been added to a batter, it is spoken of as a sponge.
When the batter has had enough flour added, so that it may be handled,
it is called a dough. If the bread is to be made in a few hours, the
yeast is made up at once into a dough. If it is to stand overnight, a
sponge is often made first. More yeast is required for quick rising. In
ordinary circumstances, one yeast cake is sufficient for one quart of
liquid. Thorough kneading and baking are both essential to the success
of the bread.