THE PREPARATION OF FOODS 
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substances known as phosphates, a large quantity of gluten 
(a protein), and some bacteria (tiny plants, see Chapter XXIII) 
which may or may not be of value in making bread. When 
water is added to the flour, it becomes tough and sticky, 
this being a characteristic of gluten, and the most important 
one, so far as the making of bread is concerned. A small bit 
of yeast (a small plant, see Chapter XXIV) is added to the 
water used in making bread, and the dough is placed where it 
will be neither too hot nor too cold (70°-80° F.). 
The yeast begins to grow rapidly, feeding on the proteins 
of the flour, and as the yeast grows, it acts on the sugar. 
A substance called zymase (zim'as), an enzyme secreted by 
the yeast plant, breaks the sugar up into carbon dioxide, 
alcohol, and a small quantity of glycerin. The gas tries to 
escape, but is held in by the sticky dough. If the yeast plant 
is well distributed, the gas collects in many small bubbles, 
and the loaf is fine-grained. The alcohol keeps other plants 
from growing there, and also helps to soften the gluten. 
When the loaf is put into the oven, the heat kills the 
yeast plant, drives off the carbon dioxide, and causes the 
alcohol to evaporate. The heat changes the gluten into 
a substance more easily digested and of a more pleasant 
taste. In “ salt rising ” bread, bacteria from the air, instead 
of yeast cells, form the gas which makes the bread light. 
When a batch of bread “ sours,” it is usually because harmful 
bacteria get into the dough and grow more rapidly than the 
yeast plants. Sometimes other kinds of yeasts than the 
helpful ones employed in bread-making accidentally get into 
the batch of bread and it spoils as a result. 
During the war we used substitutes in our wheat flour 
with the result that many good cooks were not able to make 
as good bread as usual. This was not the fault of the cooks 
but was due to the following facts. Although corn, barley, 
and wheat flour contain nearly equal amounts of similar 
proteins, the proteins of corn, barley, and rye do not react 
