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The apparatus now in general use by many public analysts is the Hortvet
Cryoscope. The Council of the Society of Public Analysts has approved the
freezing point determination as a means of detecting and determining added
water in milk and has stated that for administrative purposes the freezing
point should be determined by the Hortvet apparatus. With this instrument
the freezing point of milk varies between -0.530° C. and -0.565° C. A depression
of the freezing point less than 0.53° C. shows the presence of added water, and
calculation on this basis figure gives the minimum amount of added water
present. It does not of necessity show the whole of the added water as milk
with a freezing point of -0.565° C. can be watered down to -0.530° C. without
detection. On an average the minimum added water figure is two to three
per cent. below the truth. Generally the freezing point gives a more accurate
basis of calculation than that based on the solids-not-fat, as its range of
variation from the average is less than the range of variation from the average
in the case of solids-not-fat. In the latter case the range of variation from
a basis figure of 8.5 per cent. (which is well below the average) is greatly to the
advantage of the dishonest milkman. In Stepney the solids-not-fat range
from 9.1 per cent. at the most to the legal minimum of 8.5 per cent., so that
a sample of milk containing 9.1 per cent. of solids-not-fat could be watered
down to 8.5 per cent. and would satisfy the legal minimum; a sample so treated
would contain 6.6 per cent. of added water. Similarly in the case of the
freezing-point a sample diluted from -0.565° C. to -0.530° C. would contain
6.2 per cent. added water, so that it would at first appear that both methods
give similar results for Stepney milk. This is not entirely so, however, as
there is no relation between the solids-not-fat and the freezing-point. It
would in fact be possible to obtain the following conditions, which will show
that it is to some extent a matter of chance as to whether either method will
indicate the presence of appreciable amounts of added water.
In order to show the significance of the Hortvet figure, combinations of
the extreme variations of the solids-not-fat in Stepney milk have been related
to the extreme variations of the freezing-point.