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83
An alternative method of calculation has been suggested by Elsdon and
Stubbs (Analyst 1936, 61, 382).
For many years in my Annual Reports I have pointed out that the
composition of the milk supplied in the Borough of Stepney compares
unfavourably with that of the milk supplied in other parts of the
country. This, in my opinion, is due in part to the purchase by wholesalers
of cheap milk containing added water. The minimum standard of 8.5 per
cent, of solids-not-fat is a lenient one and usually a certain percentage of
added water can safely be added to milk without reducing it greatly below
this minimum. In those cases one is not prepared to condemn the sample as
adulterated. It must be remembered that the greater part of the Stepney
milk is bulked milk and one cannot therefore take checking or " Appeal to
cow " samples at the farm concerned as is done in other parts of the countiy.
Amount of
water which
may be
present.
The Hortvet method helps very considerably, as it proves that when the
depression is less than .530° C. water has definitely been added. Unfortunately
the Hortvet ranges from —0.530° C. to —0.565° C. This means that a
sample having an original freezing point of —0.565° C. can be watered down
from .565° to .530° without detection. In the examination of 1,000
milks previously referred to it was shown that the extreme ranges rarely
occur, the minimum depression only occurring 6 times in 1,000 samples. As
in England the added water is calculated on 0.530, it follows that the added
water is considerably under-estimated. Further, samples found in this
laboratory to show freezing points between —0.525° C. and —0.530° C. are
reported as suspicious; only those samples which show over a minimum figure
of 1 per cent, being certified as adulterated. Actually such samples will
contain in most cases 3 or 4 per cent, of added water. If legal proceedings
are not taken in such cases, the milk purveyor can add small percentages of
water with impunity. A milk dealer selling say 1,000 gallons daily is selling
30 or 40 gallons of water per day when 3 or 4 per cent, of added water is
present. A sample of such milk would probably show a minimum of about
1 per cent, by the Hortvet, and the solids-not-fat would not be reduced or
only slightly reduced below the minimum of 8.5 per cent.
In America the added water is officially calculated on an average depression
of 0.55° C. and a tolerance of 3 per cent, is allowed. This is the same as
taking a limit figure of 0.533° C. This tolerance figure is not deducted from
results showing added water in excess of 3 per cent. In this way the figure
obtained approximates more nearly to the amount actually present.
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.
Amount of
water not
detected
under
certain
conditions.