THE ALCOHOLIC FERMENTATION 231 
Yeasts in Bread Raising—The most common use of yeast 
is in the raising of bread. All nations and all peoples have been 
accustomed to make bread from the flour of different grains. The 
earliest method was simply to stir the flour in water and bake 
the mixture into a hard, unleavened bread. The next step was 
to allow the dough to stand for a number of hours in a warm place 
until it became somewhat swollen by the gas formed within it, 
and then to bake it. The gas made the dough porous and resulted 
in a bread filled with holes, easier to masticate, of better flavor, 
and more easily digested. This was leavened bread. The next 
step was to take a little of the leavened dough and mix it with the 
next lot of fresh dough, to hasten the leavening, a process that was 
simply inoculating the dough with the yeast organisms in the 
leaven. The next step, taken a long time afterward, was to dis- 
cover 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 
