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95
BACTERIOLOGICAL EXAMINATION.
The object of making bacteriological examinations of
milk, is to arrive at the amount of dirt and other extraneous
matter that gains access to it. For whenever dirt in any
form is added to milk, bacteria are also added at the same
time. Of course some bacteria are harmless, but others lead
to putrefaction and hasten the decomposition of the milk.
When milk is obtained direct from the udder of the cow—
especially if the "fore milk" or first drawn milk be rejected
—it is practically free from any bacteria. But in the act of
milking, as ordinarily carried out, some bacteria are almost
certain to gain access to the milk, either from the udder and
teats of the cow, the hands of the milker, the milk cans, Ac.,
or from the atmosphere of the cowshed.
Milk is an excellent food for the growth of bacteria, so
that if a small quantity of bacteria gain access at the time of
milking, the longer the milk is kept the greater the number
of bacteria, provided the milk is not kept artificially cooled.
In other words—vide Dr. Houston in his report on
"Bacteriological Examination of Milk," to the London
County Council:—
"Time and temperature are the two great factors
influencing the degree of bacteriological impurity of the
milk."
The report says: "It follows that a comparatively
clean milk to start with, may, after a lapse of some time,
yield worse results as judged by the ordinary bacteriological
tests, than a milk containing initially an
excess of filth but which has been maintained at a low
temperature; or which has been examined shortly after
milking."