264
FREEZING POINT OF COW AND BUFFALO MILK
[Vol. XXIII, Part IV,
being 0.0038ºC. Thus the increase in acidity can seriously interfere with the inter-
pretation of the results of the freezing point test. It is essential to know the acidity
of the samples before drawing any conclusion.
Various methods of applying corrections to the freezing point of sour milk have
been suggested ranging from the strict adherance that only fresh milk should be
used for the test [A. O. A. C., 1950]. The sub-committee of the Scientific Advisory
Committee, Department of Health for Scotland [1943] after reviewing the available
evidence suggested that the best estimate of the original freezing point of the fresh
milk was given by a correction equivalent to 0.0034ºC., per 0.01 per cent lactic acid,
which should be subtracted from the apparent freezing point depression when the
acidity of the milk sample exceeded 0.18 per cent lactic acid. The correction found
in the present studies was a little higher than that recommended above, and for
practical purposes may be taken as 0.0037ºC. per 0.01 per cent increase in acidity.
Examination of a number of results show that when the correction was taken as
0.0034ºC. the freezing point depression found by calculation exceeded the true value
by 0.021ºC. on an average, while if the value taken as 0.0037ºC. the average difference
was only 0.003ºC. It is, however, well to remember that any correction at best is
only an approximation, as the high apparent acidity of milk may not be due to
lactic acid but also due to other factors. The safest rule appears to be to work
only with fresh samples of milk, or samples which have been stored at a sufficiently
low temperature that no appreciable change in the acidity of the milk occurs. As
a workable solution it is recommended that a sample of milk should be examined
by the freezing point test only when its acidity does not exceed 0.19 per cent lactic
acid, and/or when the sample does not give a positive boiling test.
Effect of removal of fat on the freezing point of milk
The results in Table IV show that for practical purposes the removal of fat
does not effect the freezing point. The average difference in the freezing point
depression of 12 trials was less than 0.001ºC. This increase presumably may be
due to the small increase in concentration of other constituents due to the removal
of fat, as the S. N. F. of separated milk is higher than that of whole milk.
Effect of processing of milk on the freezing point
Pasteurisation of milk by the holding method or the H. T. S. T. method had
no effect on the freezing point as shown by the results in Table V. On boiling the
freezing point depression increased, and the value increased further when milk was
simmered. However, there was no change in the freezing point of boiled milk on
storing (Table VI). The increase in freezing point depression during processing
was in a large measure due to the loss of moisture by evaporation. Elsdon and
Stubbs [1933] have shown also that the freezing point of milk was not altered when
it was heated in a closed container in an autoclave for 30 minutes at 110ºC.
Effect of diluting milk with water on the freezing point
As was to be expected the freezing point depression of milk was lowererd by the
addition of water (Table VII). Addition of small quantities of water, from 1 to 5
per cent, brought about relatively less change than when the amount of water