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The purpose of the phosphatase and methylene blue tests is to
determine respectively whether or not the milk has been adequately
heat treated and whether it is of good keeping quality.
The turbidity test for sterilised milk is to determine whether
or not the milk has been adequately sterilised.
The greater importance must be attached to the phosphatase
test as correct heat treatment or pasteurisation reasonably ensures
that all pathogenic organisms have been eliminated. One
failure occurred out of 130 examined.
This test is highly sensitive and a failure will result if
the milk has been heated to only 1½°F lower than the correct
temperature of 145°F (holder process), or held for twenty minutes
at 145°F instead of thirty minutes. The addition of as little as
a quart of raw milk in 100 gallons of pasteurised milk will
produce an adverse result. Failures may result from any one
of these three factors and inspection of the plant is necessary to
ascertain the cause.
Two samples only failed to pass the methylene blue test.
This occurred during the hot weather, when it is recognised
that milk kept at an atmospheric shade temperature of above
65°F., even if efficiently pasteurised, will not remain sweet for
any lengthy period.
Farm bottled (tuberculin tested) milk is not pasteurised and
is retailed without treatment. In order to ensure, as far as
possible, that this does not contain any tubercle baccilli three
samples were taken and submitted for biological examination
(animal experiment). These all gave negative results.