56 
WORLD OF INVISIBLE LIFE 
What does 
yeast do? 
contained in a liquid into two other substances. 
One of these is alcohol, which remains a liquid. 
The other is a gas called carbon dioxide, which 
usually escapes from the liquid in the form 
of bubbles. Fermentation is a common thing 
in nature, and is always caused by the growth 
of yeast plants. It does not happen unless the 
yeast plants are not only present, but also are 
growing and multiplying. There are such large 
numbers of yeast cells everywhere in the air 
that any mixture containing sugar, if it is left 
standing uncovered in a warm place, will soon 
begin to ferment. 
The raising of bread is another work done 
by the yeast plants. It is caused by the second 
product of fermentation—carbon dioxide gas. 
It is really simple and easy to understand. 
Bread dough always contains a small amount 
of sugar which comes from the flour. Besides, 
there is a good deal of starch in the flour, and 
a small amount of a substance known as 
diastase. By the action of this diastase, part 
to sugar, and so there 
What does 
yeast do? 
Without the yeast 
microbes, bread 
would not rise 
