6
Freezing Point Test for Buffalo's and Cow's Milk
When calculations are made according to figures given by Winter, the
adulteration percentage obtained is about two per cent higher, and if Leather's
formula is used the figures obtained are about half a per cent lower than the
true value. Thus, the actual figures obtained are nearest to those calculated
from Leather's tables. From this it can be concluded that, provided a margin
of about two to three per cent added water is allowed, the formulae given
by Winter and Leather can be employed for samples of buffalo's milk. It
will thus be seen that the freezing point determination offers the best way
to check samples of buffalo's milk adulterated with water and sold as cow's
milk. While the solids-not-fat and fat content in such a case may be found
normal for cow's milk, the freezing point test will not fail to detect such
adulteration.
(iv) The freezing point of separated milk.—Separated milk is commonly
used in many dairies for the purpose of ' standardizing ' or ' toning ' milk in
order to lower the initial fat content to the prescribed legal minimum standard.
The milk was separated at body temperature (i.e. about 100°F.) and the
separated milk tested for fat. In 15 samples analysed, the fat percentage
varied from 0.05 to 0.1. The freezing point of whole milk ranged from
—0.530° to —0.555°C. and this value was identical for the corresponding
samples of separated milk. It will thus be seen that, where separated milk
alone is used for adulteration, the freezing point method will fail to detect its
presence. This can be attributed to the fact that both the separated milk
and the whole milk have the same osmotic pressure, the ratio between lactose
and soluble inorganic salts percentage being the same in both cases. The
lactometer reading of such adulterated milk will, however, reveal the addition
of separated milk since the specific gravity of separated milk is higher than
that of whole milk. This difficulty, however, does not affect cases in which
both separated milk and water have been used for adulteration, for as has been
said above, the presence of separated milk in the mixture will not interfere with
the rise in freezing point.
(v) The freezing point of pasteurized and boiled milk.—The effect of heat on
the freezing point of milk is a subject of particular interest in India as in some
parts of the country the milk sold in the bazaars by halvais is heated to a cer-
tain extent during some parts of the year. Such milk is sometimes allowed to
simmer over a low fire for a considerable length of time. The action of heat
on milk in so far as it is likely to influence the freezing point may be due to
one or all of the following causes :—(a) loss of moisture, (b) partial precipitation
of soluble mineral matter, (c) disturbance of the acid-base equilibrium, and (d)
partial caramelisation of lactose.
In this investigation the milk was pasteurized by the ' flash method '
(175°F.). In the case of the boiled milk, the milk was heated in an open vessel
and removed immediately after the first boil and then cooled with tap water.
Freezing point determinations were made on the same sample of milk in its
raw state as well as after pasteurization and boiling. The results are given in
Table V.
It will be seen that on pasteurzation the freezing point dropped from nil,
to —0.005°C. with an average drop of —0.004°C. This degree of change was,
however, so small that it did not interfere with the value of the test when