London's Pulse: Medical Officer of Health reports 1848-1972
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Hints from the Health Department. Leaflet from the archive of the Society of Medical Officers of Health. Credit: Wellcome Collection, London
Stepney 1936
[Report of the Medical Officer of Health for Stepney]
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84
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.
Table G.
| Original Milk. | After dilution. | ||||||
|---|---|---|---|---|---|---|---|
| Solids-not-fat per cent. | Freezing Point. | Solids-not-fat per cent. | Minimum added water per cent. from S.-n.-F. | Freezing Point. | Minimum added water per cent. Hortvet. | Actual water content per cent. | |
| 1 | 8.50 | —0.565°C. | 7.97 | 6.2 | —0.530°C. | None. | 6.2 |
| 2 | 8.50 | —0.530°C. | 7.97 | 6.2 | —0.497°C. | 6.2 | 6.2 |
| 3 | 9.10 | —0.565°C. | 8.54 | None. | —0.530°C. | None. | 6.2 |
| 4 | 9.10 | —0.530°C. | 8.50 | None. | —0.495°C. | 6.6 | 6.6 |
In the first case the solids-not-fat shows the presence of at least 6.2 per
cent, added water, while the freezing point shows none and points to the
possibility of the deficiency being due to natural causes. In the second case
the maximum of added water is obtained in both methods of calculation. In
the third case no added water is indicated by either test, although 6.2 per
cent, is actually present. In the fourth case the solids-not-fat figure satisfies
the Regulations though 6.6 per cent, added water is shown to be present
by the freezing point. The last example shows the necessity of carrying
out freezing point determinations although the sample is apparently
satisfactory from the chemical analysis.
Effect of
sourness.
I have often been questioned as to the time that had elapsed before
the freezing point determination was made and as to whether the acidity
had been determined, etc. Sourness actually increases the depression of the
freezing point and any added water which might have been present would
be either under-estimated or missed entirely. This would be to the advantage
of the vendor. (Determinations are not in any case carried out on sour
samples.)
Sterilised
and
pasteurised
milk.
Elsdon and Stubbs (Analyst 1933, 58, 7) examined a number of samples
before and after pasteurisation and sterilization and concluded that there is
a slight tendency for the freezing point depression to be decreased when milk
is heated but that the amount of change, when it takes place is too small to
interfere with the value of the test.
Heated milk
from cafes,
etc.
Many of the samples of milk are taken from cafes in the Borough, and a
high proportion of these samples are adulterated. Williams (Analyst 1936,
61, 246) has shown that the freezing point is unchanged except where evaporation
has taken place. The depression is then increased and added water
would be under-estimated.