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81
As a considerable amount of time has been spent in giving evidence in
the Courts with regard to the Hortvet method, a short account of the method
and of the points raised by the defence in a number of cases is appended.
The Value of
the Hortvet
Determination.
The Sale of Milk Regulations presume that when the percentage of solidsnot-fat
in a milk is less than 8.5 the sample contains added water and the
defendant is required to prove the contrary in order to avoid conviction.
According to this standard 8.5 parts of solids-not-fat represent 100 parts of
milk. If a sample contained half the quantity, that is, 4.25 parts of solidsnot-fat,
that would represent half the sample, or 50 parts of milk, the remaining
50 parts being added water. On this principle by means of a simple formula
the content of added water is calculated. The same figure represents the
deficiency in solids-not-fat. Prior to the introduction of the Hortvet such a
sample would have been reported as 50 per cent, deficient in solids-not-fat,
representing the presence of at least 50 per cent, of added water. Milk may,
for various reasons, be naturally deficient in solids-not-fat, so that it is necessary
for the analyst to be able to differentiate between naturally abnormal milk
and milk to which water has been added. The milk of a single cow and
occasionally the mixed milk of a herd may fall below the standard. In
Stepney the great proportion of the milk sold is previously balked and
pasteurised, so that it is extremely unlikely that the composition of the milk
would be below the minimum owing to natural causes. A very usual defence
in the past has been the production of figures showing or purporting to show
considerable deficiencies in solids-not-fat in the milk of herds of cows under
natural conditions. There was then only one method which was really
helpful in confirming the presence of added water as suspected by the
deficiency of solids-not-fat. This method depends on the fact that the constituents
of the milk solids, lactose, proteins and ash (mineral compounds after
ignition) are present in certain proportions in normal milk. It was found
that in the case of milk naturally deficient in solids-not-fat the ratio did not
hold good owing usually to a deficiency of lactose, but in watered milk the
constituents remained in the usual proportions. This method was used here
in addition to the ordinary analysis in cases where the deficiencies of solidsnot-fat
were small.
Owing to the difficulty in distinguishing between a milk naturally deficient
in solids-not-fat and one to which water has been added, samples showing a
percentage of from 8.25 to 8.50 solids-not-fat were reported as " Poor quality
slightly deficient in solids-not-fat." Others showing a greater deficiency,
i.e., over 3 per cent, were reported as adulterated. As the minimum of 8.5
per cent, solids-not-fat is a lenient one it follows that many of these " poor
quality " milks actually contained 5 or 6 per cent, of added water.
The introduction of the freezing point method, which has been used in
this laboratory since 1931, has cleared the position considerably.
Cryoscopy, or the study of freezing points, was developed largely by the
French scientist Raoult. In 1892, Dreser found that the freezing point of
milk was practically the same as that of cow's blood and later Beckmann and
Jordis suggested the freezing point determination as a method of detecting
added water in milk.
The method is based on the fact that the freezing point of milk depends
on the osmotic pressure, and as the osmotic pressure is for biological reasons
practically constant, the freezing point of genuine milk, whatever its chemical
composition, varies only within a limited range.