H. LAXMINARAYANA 249
It will be seen from Table II that there is a fairly good agreement between the one-hour resazurin
readings and methylene blue reduction times. About 86 per cent of cow milk samples, 68 per cent
of buffalo milk samples and 60 per cent of mixed milk samples that passed the resazurin test (reduc-
tion up to mauve stage) have also passed the methylene blue reduction test (above four hours), the
rest of them showing reduction times between two and four hours, while almost all the samples (cow,
buffalo and mixed) that have been graded 'poor' or 'bad' by the resazurin test (pink or colourless
at one hour) have also failed the methylene blue test (below four hours). Further, the samples reach-
ing the colourless stage show methylene blue reduction times mostly below two hours. The failure
of some 30 per cent of the above samples, mixed milks in particular which were samples from bulk
milks and exposed to atmospheric temperatures for a longer time, to pass the methylene blue test
while satisfying the resazurin test is probably due to the methylene blue standard of four hours being
too rigid. According to Thomas [1943], this apparently faster methylene blue reduction may also
be due to the different times at which the two reductions are recorded. Resazurin reduction in one
hour is influenced mainly by initial bacterial activity and accumulated products of bacterial meta-
bolism while methylene blue is in addition affected by bacterial activity developed during its longer
incubation period. This closer relation to the initial flora shown by resazurin is one of the advantages
claimed for it [Davis, 1942].
As regards the samples that turned pink mauve or purple pink at the end of one hour ('average'
quality), about 67 per cent of cow milks, 37 per cent of buffalo milks and 14 per cent of mixed milks
have passed the methylene blue test and the rest show reduction times between two and four hours.
Looking at the results from another angle it is seen that out of the samples that stood the methylene
blue test, only 54 per cent of the cow samples, 85 per cent of buffalo milks and 82 per cent of mixed
milks have passed the resazurin test, the rest coming under the 'average' category (pink mauve or
purple pink). The responsibility for the down-grading of most of these samples, particularly cow
milks, on the resazurin scale may be ascribed to the presence of numerous leucocyte cells (colostrum,
late lactation milk or mastitis infection) and other weak reducing substances in milk which have a
greater reducing effect on resazurin [Thomas, 1943 ; Davis et al. 1943]. This effect seems to be least
in the case of mixed milks which represent bulk samples where the leucocyte content would have been
levelled out or inactivated due to age.
It is also seen that out of the. samples that have failed the methylene blue test, only about 58
per cent of cows milks, 33 per cent of buffalo milks and 47 per cent of mixed milks have been placed
in the 'poor' or 'bad' categories by the resazurin test, the rest being placed either in the 'good'
or 'average' category by the latter test. As explained above the high standard of four hours as
well as the difference in the factors which influence the two reductions may be responsible for a
larger number of samples belonging to this class failing the methylene blue test than the resazurin
test. Another significant point that has emerged is the comparatively slower rate of resazurin
reduction in the case of buffalo milks up to the mauve or pink mauve stage and accordingly the above
anomaly is shown to the greatest extent in their case. The causes underlying some of these anomalies
can be fully explained only after further investigations, but these have not detracted the practical
value of the test to any appreciable extent. In any case, a good number of samples (about 50 per
cent) which would be condemned on the rigid methylene blue standard are found to be useful or at
least of an average quality by the resazurin test on account of its greater flexibility in grading milks
while a large number of abnormal or pathological samples (about 30 per cent), which may be found
unsuitable for either human consumption or manufacturing purposes and which are passed by the
methylene blue reduction test are weeded out by the resazurin test.
H 2