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There exists no cytological or bacteriological standard in this country by which an
authoritative judgment upon a condensed milk can be given. Much attention has been
paid to the cells and bacteria in fresh milk, but I am aware of comparatively few
observations on the bacteria, and none on the cellular, contents of condensed milk.
In consultation with Dr. Williams it was decided that I should examine a
number of different samples, good as well as doubtful, in order, if possible, to arrive at
some standard.
Dr. Williams has furnished me with specimens of all or most of the brands
arriving in the Port during the past few weeks, and I have studied altogether 57
samples. This has been sufficient for me to arrive at certain conclusions, which I
shall state in the following report :—
The Nature of Condensed Milks.
These fall into two main classes :—
I. The higher class of preserved milks contain the full fat contents of the
original milk, and are for the most part unsweetened or either uncondensed, or
but little condensed, so that they are quite liquid. Such milks are necessarily
sterilised, because otherwise they would not keep, for they are eminently
favourable media for bacterial growth. Other milks, while containing the full
cream, are condensed and slightly sweetened. They are sometimes sold as
sterilised, but they keep well even if they are not, because the physical and
chemical properties of the mixture are unfavourable to bacterial growth.
II. The lower class condensed milks are prepared from machine skimmed
milk, and the fat content is so small that their food value is very low. Sugar
is added and the mixture is evaporated at a comparatively low temperature under
diminished atmospheric pressure. These milks are not sold as sterilised, nor, in
my experience, are they ever sterile, though the more easily killed bacteria of
fresh milk are destroyed, and those that remain grow with difficulty in the
dense and syrupy product.
The condensed milks which have fallen under suspicion belong to the last class.
Such a product must be judged bacteriologically by the potential harmfulness of the
bacterial content, and this content may be very unlike that of the fresh milk from which
it was prepared.
Methods of Examination.
My practice has been to cleanse the outside of the tin with sterile water, and to
open it with a tin-opener freshly boiled for each tin. With a freshly boiled teaspoon I
then transfer three teaspoonfuls of the milk, well stirred up from the bottom of the tin,
to about half a pint of recently boiled and cooled normal saline solution, in a sterile
beaker. The mixture is stirred until thoroughly dissolved, and then placed in a sterile
conical glass, covered with a sterile glass plate. Here it stands overnight, and the
sediment is examined in the morning. No judgment can be formed from the apparent
volume of the sediment, because this may largely consist of crystalline material which
does not go into solution. Indeed, the crystalline matter diminishes, by dilution, the
apparent number of cells present of rough microscopic examination.