View report page

105
The freezing point of genuine milk is 0.55° centigrade and, as that of pure
water is 0°, a proportion of added water in a sample of ' milk ' can be estimated
quite independently of the percentage of solids-not-fat. This test is, therefore,
performed where possible on all samples of milk showing low figures for solidsnot-fat,
and the possibility of a genuine milk being condemned as watered is
thereby avoided.
An interesting result of the adoption of this test as a criterion has been
the detection of small amounts of added water in milks which would have been
passed as genuine under the standard of the Sale of Milk Regulations.
The inference from the freezing point of a sample of milk therefore becomes
a matter of great importance; and, as the presence of added water was indicated
by this test in one of the samples of hot milk which contained only slightly
less solids-not-fat than the minimum limit of 8.5 per cent., a question arose
as to whether the freezing point might not have been affected by the process
of heating.
Series of experiments were therefore carried out in which varying quantities
of milk were heated in the laboratory to various temperatures, and kept at
such temperatures for varying times, in open and closed saucepans. Furthermore,
samples of milk that had been heated and kept hot for periods of up to
three hours in different kinds of urns in local cafes, restaurants, buffets, snack
bars, etc., were taken by the Public Analyst, with the co-operation of the
respective managements, and tested. The freezing points and the compositions
of the samples were determined in each case, the corresponding unheated
milks being analysed for comparison side by side with the heated samples.
Over 130 tests were made on the various samples, and in no case was the
freezing point found to be raised as a result of the heating, although in several
instances, as where the milk had been kept hot in an open pan, the freezing
point was slightly lowered, owing no doubt to concentration by evaporation.
It was thus established that the deduction of added water from the
freezing point can be relied upon when a milk has been heated in any of the
ways usual in places where hot milk is sold."