8
Freezing Point Test for Buffalo's and Cow's Milk
applied to milks which had been thus treated. Boiled milk on the other hand
showed a greater fall, ranging from —0.020° to —0.03°C. with an average drop
of —0.0228°C. When tests are made with heated milk it is important to bear
this point in mind and make a correction of +0.02° to +0.03°C. [Elsdon and
Stubbs, 1933].
(vi) The effect of common preservatives on the freezing point of milk.—The
use of preservatives which help to keep milk samples in a condition fit for the
determination of freezing point becomes necessary when samples cannot be
examined while still fresh. Many of the most common preservatives such as
borax, potassium bichromate, etc. are not suitable for the purpose as the quan-
tities required for effective preservation of milk samples appreciably disturb
the freezing point. In the present investigation chloroform, menthol, mus-
tard oil, formalin and mercuric perchloride were tried in different proportions.
The first three were not found satisfactory, as the samples became distinctly
sour or even curdled within 24 hours.
Different amounts of both formalin and a saturated aqueous solution of
mercuric perchloride were tried. In the case of formalin, added in the propor-
tion of two to four drops per 100 ml. of milk, a rise in acidity of about 0.02
per cent (lactic acid) was observed after 24 hours but the freezing point drop-
ped by as much as 0.03° to 0.05°C. within that period. However, both the
acidity and the freezing point remained remarkably constant after this initial
change even at the end of 48 to 72 hours. This initial drop in freezing point
may be attributed to the disturbance in chemical equilibrium resulting from
addition of formalin. Saturated corrosive sublimate gave satisfactory results,
two drops per 100 ml. of milk being sufficient to prevent any change in both
the freezing point and acidity, at the end of 24 hours. With fairly large
amounts, it was possible to keep milk for as long as two weeks without any
change in the freezing point, beyond a comparatively negligible initial fall of
about —0.005°C. All the samples were stored in coloured bottles and in a dark
place.
From Table VI below it will be seen that when large amounts of mercuric
perchloride were used, though the freezing point remained comparatively
constant, the acidity of milk showed a fairly rapid rise from the very first day.
The acidity was for the most part due to the presence of mercuric perchloride
itself and not to the formation of lactic acid to any appreciable extent. The
acidity thus produced, however, did not interfere with the freezing point as
will be seen from the data. This was further confirmed by adding the reagent
to the extent of 5 per cent to the sample examined. It was observed that
while the acidity rose from 0.11 to 0.27 per cent lactic acid, the freezing
point showed no appreciable difference even after 24 hours.
(vii) The freezing point of abnormal milks.—A number of determinations
were made on foremilk and strippings (Table VII). Milk from stripper
animals was also examined (Table VIII). In all these cases it was observed
that the freezing point value obtained was within the range obtained for normal
milk.
DISCUSSION AND SUMMARY
From an examination of 150 samples of buffalo's milk for their freez-
ing point an average of — 0.543°C. was obtained. For cow's milk the