NovEeMBER 22, 1918] 
length of fermentation. Another method is 
to vary the quantity of yeast. 
But the rate of carbon dioxide production 
by yeast is also greatly influenced by the prod- 
ucts of its own activity. During fermenta- 
tion there is a continual increase in the acid- 
ity of dough and as a result, up to a certain 
point (reached only in very old and very acid 
dough), the activity of the yeast steadily be- 
comes greater. The activity of the yeast will, 
nevertheless, diminish when the supply of 
sugar is no longer sufiicient. 
THE PRODUCTION OF ACID BY YEAST 
The increasing acidity of dough both im- 
proves the condition of the dough and in- 
creases the production of carbon dioxide by 
the yeast. Accordingly, the dough rises more 
and more rapidly as the fermentation pro- 
gresses. 
The baker usually prolongs fermentation by 
“\mocking down” the dough. By thus pro- 
longing the process the products of the fer- 
mentation and the acidity of the dough are 
increased and therefore the volume of the 
dough, when it rises again, is greater. 
THE ADDITION OF ACID TO DOUGH 
The addition of such weak acids as lactic 
or acetic acid (vinegar) to dough has much 
the same effect. The amount of acid that 
may be added will vary with the amount of 
yeast and the length of fermentation. It can 
best be judged by determining the acidity of 
the baked loaf.1? This can be done by judg- 
ing the color when a few drops of methyl red 
are added to a slice of bread. The amount 
of acid which can favorably be added in 
ordinary bread making is discussed below in 
connection with the prevention of rope. It 
must be remembered that the desirable 
amount of acid varies with the quantity of 
‘yeast, with the quality of the wheat flour, with 
the quantity and variety of substitute and 
with the habits of the baker. 
In the first place the desirable length of the 
12 Henderson, L. J., ‘‘ The Prevention of Rope in 
Bread,’’ Science, N. S., Vol. 48, No, 1236, pp. 
247-248, 1918. 
SCIENCE 
503 
fermentation is determined by the acidity of 
the dough. Increase in acidity increases the 
activity of the yeast and shortens the fer- 
mentation. Consequently the amount of sugar 
required by the yeast is diminished.* Be- 
side the rate of carbon dioxide production of 
the yeast within the dough, the tenacity and 
elasticity of the dough, and the escape of gas 
from the dough are dependent upon acidity. 
Experience in this and other laboratories has 
shown that the best acidity for the baking of 
bread is indicated by the turning of methyl 
red from orange to red.1#,15,16 Jn sum, the 
acidity of the dough at the time of baking 
seems to be the most important variable factor 
in bread making. 
THE ESCAPE OF GAS FROM THE DOUGH 
The volume of the baked loaf is not com- 
pletely determined by the volume of the risen 
dough. For not all of the carbon dioxide 
produced is retained within the dough: a 
large part escapes into the air. As a dough 
expands more and more the loss of gas in- 
creases, because the surface of a distended 
dough becomes greater, while the walls of the 
batch grow thin and more leaky. When, dur- 
ing the last rise, loss of gas from the dough 
becomes nearly as great as the production of 
carbon dioxide, the loaf must be baked regard- 
less of its size. But the more tenacious and 
elastic the dough, the larger will be its volume 
before the losses from the batch reach this 
point. The “age” or “ripeness” of the 
dough is always best determined by the baker 
who through long practise has learned to 
judge it accurately. 
BAKING 
It has been suggested that dough must be 
baked before the loss of gas is equal to the 
13 Unpublished observations. 
14 Cohn, E. J., Catheart, P. H., and Henderson, 
L. J., ‘‘The Measurement of the Acidity of 
Bread,’’ Jour. Biological Chemistry, 1918. 
15 Jessen-Hansen, Comptes Rendus Trav. Lab. 
Carlsberg, Vol. 5, No. 10, 1911. 
16 Landenberger, L. L., ‘‘ Barley Bread Optimum 
Reaction and Salt Effect,’’ Science, N. 8., Vol. 
48, No. 1237, pp. 269-270, 1918. 
