CHAPTER VI 
DETERMINATION OF BACTERIA IN MILK 
Bacteria as a measure of dirt in milk.—Since a 
chief source of contamination of milk with bacteria is 
in dirty surroundings, the relative number of bacteria 
in milk has come to be largely accepted as an index 
of its quality with regard to cleanliness. As a matter 
of fact, in certain of the higher grades of the market 
milk, notably in “certified milk,” the standard of 
quality is fixed by the number of bacteria per cubic 
centimeter found in the milk, and if the number so 
found exceeds a certain amount the milk cannot be 
graded as “certified.” For these reasons it is neces- 
sary to make frequent determinations of the bacterial 
content of milk. This is best and most accurately 
done by a trained bacteriologist in a well-appointed 
and equipped laboratory. However, the simple deter- 
mination of the number of bacteria in a given amount 
of milk is not a difficult operation, nor does it require 
a very elaborate or expensive equipment. There are 
reasons why a dairyman, particularly one producing 
certified milk, should have the means at hand for 
making his own determination of the bacterial content 
of his milk, the chief reason being that it enables 
him to control the conditions and work from day to 
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