412 FARM ANIMALS 
bacteria. When milk sours, the amount of sugar de- 
creases something more than a quarter of its original 
quantity. The sugar of milk passes largely into whey in 
checse making and forms over 70 per cent of the solids 
in whey. 
5. How milk sours.—If certain bacteria of air, water 
or barn dust are admitted to 
milk in any manner, they 
grow, multiply and very 
shortly change the sugar of 
milk to an acid. These bac- 
teria are most plentiful in 
sour milk, and they adhere to 
the sides of any vessel in 
which sour milk has been 
stored. When sweet milk is 
poured into such pails or jars 
or cans, the germs are planted 
f or seeded in the fresh milk. 
Device For Cootinc Mik WHere Lhe consequence is, this milk 
pe ence pear also sours. To keep milk 
from souring, it is necessary to exercise great care in 
milking, to sterilize or scald all utensils used in its stor- 
age, and then to keep it at a low temperature. 
In a few dairies so careful are the attendants in milking and 
handling this milk that they secure a product largely free from any 
germs or bacteria whatsoever. Milk as ordinarily produced’ con- 
tains tens of thousands and often millions of bacteria in each 
cubic centimeter. A cubic centimeter is only about the size of a 
thimble. 
6. Pasteutization.—Bacteria are readily destroyed by 
heat. Hence, if milk is warmed to 145 degrees, and the 
temperature is held at that point for 40 or 50 minutes, 
not only the germs that cause milk to sour are destroyed, 
but disease germs, if in the milk, will be killed also. 
