rbon dioxide, in the form of small bubbles, passes off
from the fermenting mass, while the alcohol remains in the liquid,
giving the stimulating effect desired by imbibers of alcoholic drinks.
The unknown strange organisms were called yeast, and they were the
starting point of the yeast cakes and yeast brews manufactured to-day
on a large scale, not only for bread making but for the commercial
production of beer, ale, porter, and other intoxicating drinks.

The grains, rye, corn, rice, wheat, from which meal is made, contain
only a small quantity of sugar, but, on the other hand, they contain a
large quantity of starch which is easily convertible into sugar. Upon
this the tiny yeast plants in the dough feed, and, as in the case of
the wines, ferment the sugar, producing carbon dioxide and alcohol.
The dough is thick and sticky and the gas bubbles expand it into a
spongy mass. The tiny yeast plants multiply and continue to make
alcohol and gas, and in consequence, the dough becomes lighter and
lighter. When it has risen sufficiently, it is kneaded and placed in
an oven; the heat of the oven soon kills the yeast plants and drives
the alcohol out of the bread; at the same time it expands the
imprisoned gas bubbles and causes them to lighten and swell the bread
still more. Meanwhile, the dough has become stiff enough to support
itself. The result of the fermentation is a light, spongy loaf.

216. Where does Yeast come From? The microscopic plants which we
call yeast are widely distributed in the air, and float around there
until chance brings them in contact with a substance favorable to
their growth, such as fruit juices and moist warm batter. Under the
favorable conditions of abundant moisture, heat, and food, they grow
and multiply rapidly, and cause the phenomenon of fermentation. Wild
yeast settles on the skin of grapes and apples, but since it does not
have access to the fruit juices within, it remains inactive very much
as a seed does before it is planted. But when the fruit is crushed,
the yeast plants get into the juice, and feeding on it, grow and
multiply. The stray yeast plants which get into the sirup are
relatively few, and hence fermentation is slow; it requires several
weeks for currant wine to ferment, and several months for the juice of
grapes to be converted into wine.

Stray yeast finds a favorable soil for growth in the warmth and
moisture of a batter; but although the number of these stray plants is