THE ALCOHOLIC FERMENTATION. 215 
leaven. The next step, taken a long time afterward, was to discover 
that it is the yeast in the leaven which produces the raising of 
the bread, and then to separate the yeast from other undesired 
materials in the leaven, and use it in pure cultures. This finally 
gave the yeast that has been used for half a century or more. The 
use of leaven has not altogether disappeared, but yeast is quicker 
and more reliable. 
The action of the yeast in bread-raising is very simple. The 
dough contains a considerable quantity of starch and also a small 
quantity of diastase, an enzyme capable of converting starch into 
sugar. The yeast acts upon the sugar thus produced and forms 
from it alcohol and carbon dioxid. The latter, being a gas, 
forms bubbles in the dough, causing it to swell and become lighter. 
When subsequently baked, these gas bubbles leave their traces in 
the numerous holes that one finds in raised bread. 
In the raising of bread the practice of depending on the wild 
yeasts of the air has long since disappeared and yeast cultures are 
now almost universally used. These commercial yeasts have been 
chosen from the considerable variety of yeasts known, and are, 
of course, the ones that have been found to produce the best results 
in bread-raising. They are not the same varieties as those found 
best for brewing. The yeast is cultivated in large quantity and 
then put up in a convenient form for distribution, sometimes dried, 
in which condition it will keep alive for weeks, and sometimes 
compressed into a moist cake, compressed yeast, in which condition 
it will keep only a few days. 
Bacterial Impurities in Bread-raising. The yeast cakes are 
never pure yeast, but may contain undesired bacteria, which then 
get into the dough and sometimes produce trouble. Occasion- 
ally, too, such bacteria may get into the dough from other sources 
than yeast, such as dirty water or dirty utensils in the kitchen. 
During the raising, lactic acid bacteria always grow, and they 
seem to be necessary in order to prevent the growth of other species 
of more troublesome organisms. Sometimes such bacteria grow 
too vigorously and may cause trouble. At least two different 
faults in bread-making are known to be caused by undue growth 
