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61
In view of these facts I think it must be agreed that, to
withdraw a licence upon such unreliable data, would be
most unjust.
In Memo. 39/Foods, the Ministry of Health set out a
suggested technique for the bacteriological examination of
graded milks. If this is properly carried out the results
should be reasonably comparable.
I can only assume that, in making their examinations,
bacteriologists do not faithfully and carefully carry out the
suggested procedure. Unless every stage, from the composition
of media to uniform heating in the incubators is
standardised, the results must vary.
There is one point in the Ministry's suggested technique
to which I would draw attention. In paragraph 12 it is
stated that the original sample and each dilution must be
shaken 25 times, each shake being an up and down motion,
with an excursion of about one foot. This shaking is, of
course, to distribute the organisms in the sample evenly,
and, secondly, to break up any bacterial aggregates which
may have been formed. Most samples of graded milks are
submitted to the bacteriologist in the bottle in which the
milk is sold, and as such bottles are usually entirely filled
with milk, it is unlikely that any proper mixing is obtained
by shaking unless the milk is first transferred to a larger
bottle. Again, the vigour with which the shaking is conducted
may makg an appreciable difference to the final
count. I suggest that more uniform results would be
obtained by the use of a mechanical shaker, working at a
standard speed.
Reviewing the position as it now stands. I am of opinion
that little value can be placed upon the reports received and
that until a more standard practice is adopted by bacteriologists,
both time and money are being wasted in submitting
milk samples for examination.