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94
a fictitious appearance which leads, not only in laymen,
but even in public health officials, to a wholly unjustifiable
feeling of confidence in their value. . . . . For pasteurised
milk, with the possible exception of Grade A pasteurised,
the plate count is not recommended. The actual count
on pasteurised milk is determined by so many factors
independent of the efficiency of the processing that the
results bear little relation to any important quality of the
milk."
Of the coliform test Professor Wilson states:—"With
the possible exception of its employment on empirical
grounds for certified milk there seems, therefore, to be no
justification for the use of either the coliform test or the
coli-aerogenes ratio test in the grading of raw milk. For
pasteurised milk, on the other hand, the coliform test may
be of some value. It may serve as an index of the efficiency
of the processing, if performed on freshly pasteurised milk,
or as an index of the subsequent contamination or exposure
to unsuitable temperatures, if performed on the bottled
milk at the time of delivery to the consumer. The experimental
error of the test is, however, very large, and on
this account the results should preferably be reported, not
in absolute numbers, but as above or below an arbitary
standard."
In spite of these recommendations the plate count has
been retained for pasteurised milk in the Milk (Special
Designations) Order, 1936, but no provision has been made
for the coliform test and, for raw graded milk, the coliform
test has been retained although the plate count has been
replaced by the methylene blue test.
In addition to the variation in laboratory reports there
are other factors which minimise the value of the plate
count. A general enumeration of the total bacterial content
of a sample of milk is of little value from the public
health point of view. It gives no indication of the presence
or absence of pathogenic organisms and, therefore, although
it may afford some indication of the cleanliness and keeping
quality of the milk, provides little information with
regard to its safety.