C. P. ANANTAKRISHNAN, NOSHIR N. DASTUR & ZAL R. KOTHAV
gradation as buffalo, mixed and cow, depending
on the fat content of each class of sample.
It may also be pointed out that this skin forma-
tion gives the consumer an impression, when milk
is drunk hot, that it has become richer than what it
would have been if it was taken in cold condition.
This, however, has no scientific basis considering
the losses already mentioned.
(iv) Proteins. Milk contains two very heat
labile proteins-albumin and globulin which are
likely to be precipitated at the boiling temperature
of milk. Wright [1924] has shown that there is no
alteration in the caseinogen molecule even after
autoclaving the milk at 120°C. for 30 minutes.
Heating, however, is accompanied by physical
changes in calcium caseinate thus affecting its
reaction in coagulation by rennet, etc.
As seen from the results in Table IV, the skin
and the residue contain a considerable proportion
of the proteins. On heating the milk to first boil,
the decrease in proteins amounts to 5.1 per cent
which increases to 5.8 per cent when it is kept
further simmering for 10 minutes. Apart from
small amounts of casein which may be entangled
during the membrane formation, the main portion
of this loss consists of lactalbumin and globulin.
Andross [1939] finds a loss of 74.13 per cent in
albumin content when milk is brought to first boil.
Kieferle [1930] states that heating milk between
100°-115°C. for half an hour destroys 85.6 per
cent of albumin.
It will be clear from Tables I to VII that heating
milk to the first boil, and secondly simmering the
same for 10 minutes causes an average loss of 0.17
and 0.20 gram of the protein matter respectively,
for every 100 ml. of milk. Taking the average
albumin content of milk as 0.45 per cent (reckoned
as 13 per cent of the total nitrogen) and assuming
that the processing of milk results in the loss of
albumin only in the skin and the residue, the total
quantity of albumin lost and not made available for
consumption would amount to 37.8 per cent in the
case of first boil and 44.4 per cent in the case of
simmering it for 10 minutes. On the other hand, if
we presume that the loss is from both globulin and
albumin these values would be considerably reduc-
ed. These losses are, therefore, considerably lower
than those given by Andross and Kieferle above.
(v) Chloride and lactose. A study of the figures
given in the previous tables show that these consti-
tuents are least affected by heat. Their decrease
amounts to about 2.1 per cent in case of both the
heat treatments studied.
The low destruction of lactose is quite under-
standable in view of its known stability to heat up
to 145°C. in the dry state.
(vi) Acidity and pH. On boiling milk acidity
decreases, though the figures for this decrease seem
to vary considerably amongst themselves. These
results are in agreement with the data given by
Whittier and Benton [1926, 1927].
The pH was determined colorimetrically. This
method, however, has several disadvantages when
applied to a fluid like milk.
(vii) Mineral constituents. Heating of milk is
known to cause a partial precipitation of the buffer
salts, the most noticeable changes being produced
in case of Ca and P. The present data show that
when milk is brought to first boil the total minerals
decrease by 4.8 per cent and Ca and P by 4.1
and 5.5 per cent respectively. When further
simmered for 10 minutes, the corresponding aver-
age figures for ash, Ca and P are 6.8, 7.9 and 6.9
per cent respectively. Thus increased heating
considerably increases the loss of minerals. The
results of Mg determinations show a good deal of
variation and they should, therefore, be inter-
preted with caution. This is due to the difficulty
in estimating it in such small amounts.
(viii) Citric acid. Citric acid in milk occurs in
combination with Ca and Mg. Rogina [1935]
found no lowering in the citric acid content of milk
either in the just boiled, or boiled for 15 minutes.
In the present studies an average loss of 3.0 per
cent of the original citric acid present, has been
noted.
(ix) Rennet coagulation of milk. The decrease
in the coagulability of milk subjected to heat treat-
ments is well known and is usually ascribed to the
precipitation of the calcium salts. Moir [1931],
however, correlates the decrease in the rate of re-
nnin coagulation with the decrease in the amounts
of soluble nitrogen in the whey. It has also been
shown that more rapid coagulation takes place soon
after heating, but as time progresses the time for
coagulation becomes longer, and appears to be
stabilized after about four-five hours [Mattick and
Hallet, 1929]. In the present studies, time of
rennet coagulation was measured after keeping the
heated milk for about four hours. A study of the
data in Tables VIII to X will indicate that though
pasteurization temperature produces very little
effect on the constituents of the milks studied, the
time required for coagulation with rennet in-
creases considerably from 42-55 per cent over that
of the corresponding samples of raw milk.
In the case of heated milk, the accuracy of the
test suffers considerably due to longer time being
taken for coagulation and also because the curd
consists of much finer particles and has very
open texture. However, the data bring out the
fact that there is considerable difference in the time
required to coagulate heated milk and raw milk.
(x) Chemical composition of skin and residue.
The results obtained are shown in Table IV. It
will be noticed that the skin and residue in wet