298
Effect of Heating Milk of Indian Cows and Buffaloes
3½ inch depth) with cover and heated over
a gas flame to first boil. The temperature reached
was about 95°C. and this required on an average
16 minutes to reach. After this the vessel was
taken down and allowed to cool to room tempe-
rature. The cooled milk was strained and used for
analysis. The skin and residue sticking to the
bottom and the sides were thus not included in the
boiled milk sample but were examined separately.
The results of these analyses are given in Tables
I to IV, after correcting for the change in volume
wherever necessary.
B. Heating milk to first boil and simmering it for
ten minutes. Milk samples were brought to first
boil in the same manner as indicated under A
above. After this the flame was lowered so as to
allow the milk to simmer for 10 minutes (average
temperature 93°C.). After cooling to room tempe-
rature and straining, the samples were analysed.
The results are shown in Tables V to VII and have
been corrected for the decrease in volume in case of
boiled samples.
C. Pasteurization of milk (holding method).
500 ml. of milk were placed in a glass flask of 1
litre capacity in which a thermometer was immers-
ed. The flask was placed in a water-bath at a
temperature of 180°F. till the temperature of
milk came to 150°F. This usually took about 5
minutes. The flask was then transferred to
another bath at 145°.150°F.and kept for a total
period of 30 minutes. After this, it was removed
to a cold water bath and kept there at room tem-
perature till the milk was quite cool. Both during
the process of heating and cooling, milk was kept
continuously stirred by rotating the flask. The
cooled milk was filtered through a tea-strainer and
used for analysis. The results of these experiments
are given in Tables VIII to X. In the case of the
heated samples corrections for changes in volume
have been made.
(3) Examination of skin and residue from boiled
milk samples
Boiling was carried out as indicated previously.
The skin and residue left were weighed in wet con-
dition and then dried to a constant weight. Fat
(Rose-Gottlieb), protein and minerals were deter-
mined in these samples and the results are given in
Table IV. Each analysis was made in duplicate,
from well-mixed sample collected from boiling six
different samples of milk.
DISCUSSION
(i) Decrease in volume. As is to be expected
the first visible result of applying heat to milk is
to remove some of its moisture. The amount of
moisture lost varies with the degree of heat employ-
ed. At pasteurization temperature this loss
amounts to less than 0.5 per cent of the original
volume. When milk is brought to first boil in a
closed vessel, the decrease in volume is 6.0 per cent
and when this heating is continued for 10 minutes
the loss increases to 11.0 per cent.
In estimating the amount of various constituents
actually available, as compared with the original
milk, these losses have been taken into considera-
tion.
(ii) Total solids. Pasteurization temperature
as employed here seems to cause almost no loss in
any of the constituents of milk.
The decrease in total solids as estimated in the
present studies goes to constitute the denatured
or precipitated matter sticking to the sides of the
vessel together with the skin that forms on top of
the heated milk. This may not be truly considered
a loss, but it is a loss in the sense that these subs-
tances cannot be redistributed evenly amongst all
the other constituents of such milk once again.
Under common household conditions, the skin is
oftentimes fed to children and invalids and some-
times totally discarded.
Bearing these facts in mind, it will be observed
from Table I to III that when milk is brought to
first boil the loss in total solids amounts to nearly
6.5 per cent. As will be seen later on, this loss is
composed largely of fat, proteins and minerals.
When simmering is continued for 10 minutes, the
loss is very nearly the same, viz. 7.1 per cent.
This may be due to the fact that the longer time of
heating destroys the creaming property of milk.
On ther other hand, the continuous play of heat on
the bottom of the container equalizes this effect.
(iii) Fat. The loss of fat in boiling is higher
than that of any other constituent of milk. A
comparison of figures for raw and boiled milk
shows that when milk is brought to first boil the
loss occurs to the extent of 11.7 per cent, and if the
milk is allowed to simmer it amounts to 9.5 per
cent. This reduced loss in the latter case is in-
teresting and may be ascribed to the fact already
stated that longer boiling decreases the creaming
property.
During the ordinary process of boiling it is not
possible to eliminate altogether the loss of fat by
preventing skin formation. This was tested on
three samples of mixed milk. The samples after
being brought to the first boil in the ordinary way
were transferred to a glass flask and cooled. The
flask was kept under a tap and continuously rotat-
ed to prevent the formation of skin. The milk
samples showed on analysis an average loss of 5.4
per cent of fat after correcting for the decrease in
volume, which amounted to 5.7 per cent.
It may also be pointed out that all the three
types of milk seemed to react almost the same way
with regard to the loss of fat and other consti-
tuents, though one would expect a natural