Are there 
bad yeasts? 
58 WORLD OF INVISIBLE LIFE 
that there are, because they have been found 
to be especially suited to this work. Yeasts 
differ in their power of producing fermentation, 
some result in stronger fermentation than 
others. Since the value of yeast in bread¬ 
raising is dependent entirely on its ability to 
do the job of fermenting, only the most vigorous 
yeasts are used for that purpose. They are 
grown especially for the use of bakers, and are 
carefully separated from any other kinds of 
microbe or even any other kinds of yeasts. 
Impure or wild yeast will give bread a bad 
flavor, cause it to sour, or will not raise it at all. 
Any other kind of microbe that may be mixed 
with yeast may interfere with its successful 
working. So only the purest kind of cultivated 
yeast is used nowadays for raising bread. 
Yeasts have many other uses, but in all of 
them their action is the same—that of ferment¬ 
ing sugars. Sometimes it is the alcohol that is 
wanted and sometimes the carbon dioxide gas, 
but both are always produced when yeast is 
used. 
