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96
The presence of phosphatase, which is readily detected,
is, therefore, evidence that the milk has not been properly
treated. If the required temperature is not attained or is
not maintained for the requisite period, the enzyme survives
in greater or lesser amounts. It is said that milk which
has been heated at 143.5° F. or which has been treated at
145° F. for 20 minutes, or milk containing as little as
0.25 per cent. of raw milk in properly pasteurised milk,
can be detected by this method.
It has been calculated that over 40 per cent. of the
milk sold as "pasteurised" in London is processed in the
Borough of Hammersmith, and it is, therefore, obvious
that a test such as this, if efficient and reliable, would
be of great value in the control of pasteurisation.
In view of experience with the bacteriological examination
of samples it was decided to make some experiments
to check the value and reliability of the test. For this
purpose a sample, consisting of one pint, was taken from
a churn containing 17 gallons of milk which had been
pasteurised in accordance with official requirements. This
sample was divided into four parts and submitted to four
different laboratories for examination.
The reports from three of these laboratories stated that
the milk had been satisfactorily pasteurised.
At the same time, a further sample was taken from the
churn after a half pint of raw (T.T.) milk had been added
to it. The contents of the churn, therefore, consisted of
17 gallons, less 1 pint of pasteurised milk, plus half a pint
of raw milk, i.e., 0.87 per cent. of raw milk.
A third sample was prepared by adding to 9½ pints of
milk, from which the second sample was taken, a further
½ pint of raw milk. The percentage of raw milk in this
case was, therefore, 5.35.
The samples were submitted to each of the laboratories
without any indication as to the nature of the contents.
The reports on the samples are set out on the attached
table A.