The changes that take place when yeast causes fermentation can be
detected very readily by observing the fermenting of fruit juice. As
every housewife knows, the first indication of a ferment in fruit juice
is the appearance of tiny bubbles, which collect on the sides and the
bottom of the vessel containing the fruit and then gradually rise to the
top. These bubbles are a form of gas called _carbon-dioxide_, or
_carbonic-acid, gas_. If, after they appear, the juice is tasted, it
will be found to be slightly alcoholic and to have a somewhat sour or
acid taste. The gas, the acid, and the alcohol thus produced are the
three results of the action of the ferment.
23. When yeast is used in the making of bread out of wheat flour, the
changes just mentioned take place. To understand the action of this
plant, it will be necessary to remember that wheat contains a large
proportion of starch. This substance, however, cannot be acted on by the
yeast plant; it must first be changed into sugar. The yeast that is
added to the flour changes some of the starch into sugar and transforms
the sugar into alcohol and carbonic-acid gas. This gas, which is lighter
than the dough, rises, and in its efforts to escape expands the elastic,
glutinous dough into a mass of bubbles with thin walls until the dough
is two or three times its original bulk.
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