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The greater importance must be attached to the Phosphatase
test as correct heat treatment or pasteurisation reasonably ensures
that all pathological organisms have been eliminated. Five
failures occurred out of 184 examined.
This test is a highly sensitive one and a failure will result
if the milk has been heated to only 1½° 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 what may be the offending cause. A slight failure
by no means indicates that any tubercle bacilli which may have
been in the milk before processing will not have been destroyed.
The failure might be due to some small error in pasteurising
technique, requiring adjustment, but not necessarily demonstrating
inadequate treatment.
Two samples only failed to pass the methylene blue test.
These 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 as it comes from the cow. In order to ensure, as far
as possible that this does not contain any tubercle bacilli sixteen
samples were taken and submitted for biological examination
(animal experiment). These all gave negative results.