278
Reduction of Methylene blue in milk
DISCUSSION
The foregoing results show that there is no
difference either in the average, maximum or
minimum reduction times when the methylene
blue tubes are inverted at ½, 1, 1½ or 2 hour inter-
vals with all the samples of milk examined. It
may also be mentioned that the individual reduc-
tion times are same in all the cases of the samples of
milk examined, when the tubes are inverted at the
same above intervals. These results support the
findings of Johns. The reduction times in the
case of 2½ hour inversion interval are always greater
than those of half-hourly inversion. The average
increase in the reduction times between ½ and 2
hour intervals of inversion in all cases except paste-
urized milk varies from 11.7 to 14.9 minutes,
the maximum difference in times being 18 to 25
minutes and the minimum 4 to 10 minutes. The
reduction times of the samples of milk when the
tubes are inverted once in every three hours are also
more than those of half hour inversion interval,
the average increase in times varies from 15.5 to
20.7 minutes and the maximum and minimum from
25 to 35 and 8 to 14 minutes respectively. This
is not the case with pasteurized milk. The in-
crease in the reduction times between ½ and 2½
hour inversion intervals in the case of pasteurized
milk are less than those of raw milk samples and
the average increase in time is 8.7 minutes, the
maximum and minimum being 10 and 5 minutes
respectively. It is seen from the tables that the
average increase in the reduction time between
½ and 3 hour intervals of inversion is the same as
that of ½ and 2½ hour inversion intervals as there
is no difference in the reduction times when the
tubes are inverted once at intervals of 2½ and 3
hours in the case of pasteurized milk.
In his modified technique, Wilson suggests half-
hourly inversion of methylene blue tubes. This
requires more frequent attention by the worker.
The results obtained in this investigation show that
there is no need to invert the tubes every half hour
since there is agreement in thereduction times when
the tubes are inverted once at intervals of ½, 1, 1½
and 2 hours. It can, therefore, be safely concluded
that instead of inverting the methylene blue tubes
at half-hourly intervals, they can be inverted once
in every two hours without in any way affecting the
accuracy of the results. This holds good with all
types of milk as the results tabulated above
point out.
In many cases when the tubes were inverted at
intervals of 2½ and 3 hours the decolorization of the
methylene blue took place from the top to the
bottom unlike those of ½, 1, 1½ and 2 hour inver-
sions where the reduction was uniform. This may
be explained by the fact that in the case of 2½ and
3 hour inversions the cream layer rising to the
surface carried away with it most of the micro-
flora to the top where the reduction as a rule first
commences.
SUMMARY AND CONCLUSIONS
(1) The effect of inverting the methylene blue
tubes at different intervals on the reduction of the
dye in milk has been studied.
(2) There is absolutely no difference in the re-
duction times when the methylene blue tubes are
inverted once at ½, 1, 1½ or 2 hour intervals.
Hence, the inversion of the tubes every half hour
as suggested by Wilson is not necessary and the
tubes can with advantage be inverted at two hour
intervals. This holds good with all types of milk
namely, (a) individual cow's and buffalo's milk,
(b) cow herd and buffalo herd milk, (c) mixed milk
(cows and buffaloes), (d) milk handled by town
dairies, (e) village produced milk, and (f)
pasteurized milk.
(3) The reduction times of the samples of milk
examined when the tubes are inverted at an inter-
val of 2½ hours are greater than those of half-hourly
inversion and the average increase in times varies
from 11.7 to 14.9 minutes, the maximum and
minimum being 18 to 25 and 4 to 10 minutes
respectively.
(4) Three-hour inversion intervals also show
greater reduction times than half-hour inversion
interval, the average increase being 15.5 to 20.7
minutes while the maximum difference in reduc-
tion times varies from 25 to 35 minutes and the
minimum from 8 to 14 minutes, and
(5) In the case of pasteurized milk the increase in
reduction times between ½ and 2½ hour inversions
is less than in the case of raw milk, and the average
increase is 8.7 minutes, the maximum and mini-
mum being 10 and 5 minutes respectively. No
difference in reduction times is observed between
2½ and 3 hour inversions.
REFERENCES
A. P. H. A. Standard Methods of Milk Analysis, Sixth
Edition, 1934. (American Public Health Association,
New York)
Barthel, C. and Orla Jensen (1912). Milchw. zbl. 41, 417.
Duclaux, E. (1887). Le Lait, Etudes Chimiques et Micro-
biologiques, 2nd Ed., 1894. Paris. J. B. Bailliero et
Fils.
Davis, J. G. (1939). Food Manufacture 6, 196
Frayer, J. M. (1937). Dyes and methods as they affect
the Methylene Blue Reduction Test. Vermont Agric.
Exper. Sta. Bull., 424
Johns, C. K. (1938). J. Dairy Sci. 21, 227
Johns, C. K. (1939). Amer. J. Pub. Health 29, 239
Thornton, H. R. (1937). J. Dairy Sci. 20, 693
Wilson, G. S. (1935). Med. Res. Council, Spec. Rep. Serise,
No. 206, 215 (H. M. Stationery Office, London)