The yeast plants, though, must
be well distributed throughout the dough; otherwise, there are likely to
be no bubbles in some places and large bubbles with thick walls in
others. The gas thus formed is prevented from escaping by the toughness
or the elasticity of the gluten, and the spaces that it leaves are what
produce a light, porous loaf. When the expansion has gone on long
enough, the formation of gas is checked and the ferment is killed by
baking the dough in a hot oven. During the baking, the alcohol is driven
off by heat, some of the starch is browned and forms the crust, and so
little acid is produced in the short time in which the yeast is active
that it is not noticeable.
24. Commercial Yeast.--When yeast plants are deprived of water and food,
they cease to multiply. However, under these conditions, they may be
kept alive so that when water and food are again provided they will
increase in number and carry on their work. Advantage has been taken of
these characteristics of yeast, for although at one time the making of
yeast was entirely a household process, it has now, like butter, cheese,
canned fruit, etc., become a commercial product. The first yeast put on
the market was collected from the surface of the contents of brewers'
vats, where it floated in large quantities; but as this was an impure,
unreliable product composed of various kinds of bacteria, it is no
longer used for the purpose of making bread.
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