as to destroy
much of the richness and sweetness of the loaf, and yet be arrested by
the baking process just before the acetous stage begins, so that it will
be light and porous, but decidedly lacking in flavor and substance.
Over-fermentation also develops in the bread various bitter substances
which obscure the natural sweetness of the bread and give to it an
unpleasant flavor. Many of these substances are more or less harmful in
character, and include many poisons known as ptomaines, a class of
chemical compounds produced by germs whenever fermentation or
decomposition of organic matter takes place. Much skill is required to
determine at what point to arrest the fermentation, in order to save the
sweetness and richness of the bread.

FERMENTATIVE AGENTS.--Fermentation in vegetable matter is always
accompanied by the growth of living organisms. The development of these
minute organisms is the exciting cause of fermentation and putrefaction.
The germs or spores of some of these fermenting agents are always
present in the air. It is well known to housekeepers that if a batter
of flour and water and a little salt be kept in a jar of water at a
temperature of from 100 deg. to 110 deg., it will ferment in the course
of five or six hours. Scientists assure us that this fermentation is
occasioned by the introduction of the spores of certain species of
fungi which are continually floating in the atmosphere, and the proper
conditions of warmth and moisture being supplied, they at once begin to
grow and multiply. This method of securing fermentation is utilized by
housewives in making what is termed salt-rising bread. The raising of
dough by this process is lengthy and uncertain, and a far more convenient
method is to accelerate the fermentation by the addition of some active
ferment. The ancient method of accomplishing this was by adding to the
dough a leaven, a portion of old dough which had been kept until it had
begun to ferment; but since the investigations of modern chemistry have
made clear the properties of yeast, that has come to be considered the
best agent for setting up the process of alcoholic fermentation in bread.
The use of leaven is still practiced to somewhat in some European
countries. The bread produced with leaven, although light and spongy in
texture, has an unpleasant, sour taste, and is much less wholesome than
that produced with fresh yeast.

Yeast is a collection of living organisms or plants belongi