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82
In 1914, Monier-Williams (L.G.B. Food Reports, No. 22) described an
apparatus of his own design which he used in determining the freezing point
of milk and later, in 1934, devised another form of apparatus on the lines of
the Hortvet, for the same purpose. The earlier form of apparatus was too
complicated for use in a working laboratory. An apparatus was, therefore,
produced in 1921, by an American, Julius Hortvet, working with a number of
collaborators; this apparatus is simple and efficient, but there is no attempt
by corrections, etc., to obtain the true freezing point. The figure obtained
is the freezing point as determined in the Hortvet apparatus. This has been
official in America for many years. The freezing point method for the determination
of added water in milk is used officially or semi-officially in many
other countries.
The Council of the Society of Public Analysts state that investigations in
this country and abroad have fully established that the freezing point of
genuine milk varies only within narrow limits and have approved the freezing
point determination as a means of detecting and determining added water
in milk and that for administrative purposes the freezing point should be
determined by the Hortvet exactly as described by the Association of Official
Agricultural Chemists, U.S.A.
The range
of variation
of the
freezing
point.
According to Hortvet and Bailey, the freezing point of genuine milk
using the Hortvet apparatus varies between —0.530° C. and —0.566° C.,
with an average of — 0.545° C.
Stubbs and Elsdon (Analyst 1934, 59, 146) examined 1,000 milks in the
Hortvet apparatus. Of these 330 samples were " Appeal to cow " samples
and the remainder were presumed genuine. The figures obtained ranged
from —0.529° C. to —0.563° C., with an average of —0.544° C.
Different figures quoted for defence purposes in Court have been obtained
by using other types of apparatus or using the Hortvet apparatus with a
variety of corrections and different methods of working. These figures are
not comparable with the figures obtained above with the standard apparatus.
The use of
the word
depression.
When speaking about a higher or lower freezing point some confusion
might arise as to whether the figure itself or its position on the thermometer
is referred to. It is usual, therefore, to state the amount by which milk has
lowered the freezing point of water as the depression. In the case of genuine
milk therefore, the minimum depression is .530° C. and the maximum depression
.565° C.
Relation to
the solidsnot-fat.
The percentage of solids-not-fat is independent of the freezing point
depression. If a sample is deficient in solids-not-fat and the freezing point
depression falls within the range found for genuine milk then the deficiency
is probably a natural one. If the depression found is less than the minimum
depression for genuine milk then the milk contains added water whatever its
chemical composition may be.
Any milk which is naturally deficient in fat or solids-not-fat is genuine
milk. There is no law to enforce a minimum standard of quality in genuine
milk.
Method of
calculation
of added
water from
the freezing
opint.
Supposing that a sample gives a freezing point of —0.477° C., the minimum
depression of .530° C. has been lessened by .530—.477, or .053, i.e., 53 in 530,
or 10 per cent. The sample, therefore, contains a minimum of 10 per cent, of
water, which is definitely added water.