270
Studies in the Transport of Milk in Warm Condition
high temperature (150°F.) and at low temperature
(48°-50°F.). With regard to acidity there was
a slight decrease on heating which may be due to
the expulsion of CO 2 during the process. The
acidity then remained fairly constant in the case
of milk stored at high temperature, while in the
case of milk in cold storage, there was a very slight
decrease. Naturally the milk stored at room tem-
perature showed a decided increase in acidity after
three hours and still greater increase was noticed
after six hours. In fact this milk actually curdled
when stored for 20 hours, thus proving that processed
milk cannot be stored at ordinary room temperature
for more than about six hours. But samples of
milk stored at 150°F. and 48°-50°F. for 16 hours
showed more or less the same acidity, proving that
hot storage at 150°F. is just as efficient as cold
storage from this point of view, but it was noticed
in the field scale trials that, even if 150°F. storage
temperature was not rigidly maintained but allowed
to fluctuate between 150°F. and 130°F., there was
very little increase in acidity within eight hours
of storage.
The bacteriological data show that the total
number of organisms in raw milk decreased con-
siderably on heating. The samples stored at 48°
-50°F. at the temperature and retaining the milk for
8, 16, and 20 hours showed that
there
was neither increase nor decrease in the total count
as compared to the total count in milk after heating.
The total count of milk stored at 150°F. for 8 and
16 hours showed a decrease when compared to the
bacterial count obtained after heating and retaining
the milk at the temperature as referred to previously.
The same decrease is noticeable in field scale
trials. This proves that hot storage from the point
of view of growth of surviving organisms after
processing the milk, brought about not only in-
hibition of growth of the organisms but it did actually
destroy them to a certain extent, while the cold
storage only inhibited their further multiplication.
This appears to show that either the farm milk,
which is produced under hygienic condition, did
not contain many thermophilic organisms, or even
if they were present, they would not affect the storage
quality of heated milk at a temperature of 150°F.
for at least 16 hours. Storage of the processed
milk at the ordinary temperature kept it in good
condition only for about four hours.
As regards B. coli, raw milk which showed the
presence of B. coli in 1 : 100 and 1 : 1000 dilutions
showed its absence in even 1 ml. of milk just after
processing and it continued to remain absent even
after storage periods, thus proving that milk
once processed, if not recontaminated, would show
complete absence of B. coli.
The methylene blue reduction time shown by
raw milk was considerably increased from about
two hours to six hours after processing. In case
of milks stored at 150ºF. and 48º-50ºF. for eight
hours, the reduction time was more or less the
same, while in case of 16 hours' storage, milk
stored at 150°F. showed a longer reduction time than
the milk stored at 48°-50°F. This observation i s
further supported by the fact that the total number
of organisms in milk stored at 150ºF. after 16 hours
showed a greater reduction than the count in milk
stored at 48°-50°F. Milk stored at atmospheric tem-
perature curdled at the end of a '20 hours' storage
period, while that stored at 48°-50°F. at the end
of 20 hours showed a total reduction time of 3½
hours. Similar results for methylene blue reduction
were noticeable both in case of storage of eight
hours at 150°F. in laboratory scale as well as i n
case of field trials.
The fermentation test results showed that in the
raw milk curd, there was presence of slight gas
and a slight separation of whey. The curd formed
after processing and storage periods did not show
the above defects and the curd formed was more
palatable.
A slight cooked odour was observed in all samples
stored at 150°F. for different storage periods, while
samples stored at 48°-50°F. retained their natural
odour. There was practically no appreciable
difference in the taste of milk.
The effects of storage at an elevated temperature
on the mineral and vitamin contents of milk as
well as the behaviour of village produced milk under
these conditions, are being studied. If these re-
sults prove satisfactory and the nutritive quality
of milk remains unimpaired, large scale field trials
on the transportation of milk at an elevated tem-
perature will be made.
SUMMARY
(1) An investigation has been carried out on the
keeping quality of farm-produced milk, processed
at a temperature of 150°F. and held at about that
temperature for prolonged periods.
(2) It has been observed that there is no change
either in acidity of milk or fat percentage when
the milk is kept in hot condition up to 16
hours.
(3) Bacteriological examination shows that there
is a definite decrease in the number of organisms
when the milk is kept at an elevated termperature
as compared to that in cold storage.
REFERENCES
Ayers, S. H. and Johnson Jr., W. T. (1923). J. Dairy Sci
6, 608-615
Bernstein (1893). Cited by Ayers and Johnson, J. Dairy
Sci