44 
HOUSEHOLD BACTERIOLOGY. 
The Gas 
Produced 
Compressed 
Yeast 
The gas will be formed, replacing the water in the 
vial. If a burning match is held in the mouth of the 
tube as it is removed from the water, the flame will 
be extinguished. This indicates that the vial con¬ 
tains the gas carbon dioxide; or a teaspoonful of 
clear lime water may be poured into the vial and 
shaken about in it. The carbon dioxide present will 
turn the lime water milky from the insoluble car¬ 
bonate of lime (calcium) formed. 
Compressed yeast is sim¬ 
ply a. mass of yeast plants 
mixed with some form of 
starch and pressed into 
cakes. A two-cent cake is 
said to contain over half a 
billion yeast plants. 
As these cakes are made 
ior a special purpose they 
should contain only one species of yeast. They do, 
however, contain bacteria and if the dough is allowed 
to rise too long or at too high a temperature they 
grow and produce an acid which makes the bread 
sour; so that sour bread results from the growth of 
bacteria and not from the yeast. When pure yeast is 
used and all conditions of cleanliness are carefully 
looked after no sour bread results. A dusty kitchen 
or unclean utensils may increase the danger from bac¬ 
terial growth. If the bread be made with milk, this 
should be scalded to kill the bacteria always present. 
As we have seen, milk is rich in dust-plants, especially 
FIG. 25. A YEAST CELL CON¬ 
TAINING FOUR SPOItfcS. 
