H. LAXMINARAYANA 247
(i) Stages of resazurin reduction for grading Indian milks. Previous workers have generally
adopted the purple pink stage as the borderline for grading milks. Davis [1939, 1942] has suggested
the following standards for the one hour resazurin test :
Blue to mauve
Pink mauve
Purple pink
Pink
Colourless
Excellent to good
Fair
Poor
Bad
Very bad
The above standards, which are generally based on their comparison with the accredited standards
(4½ hours in summer and 5½ hours in winter) for methylene blue reduction adopted in England, may
need some modification before introduction in this country, as it is well known that temperature
has a great effect on the dye reduction tests. According to Verma et al. [1944] and further experi-
mental evidence obtained at the Imperial Dairy Research Institute, a standard of four hours (methy-
lene blue reduction time) for grading milks into satisfactory and unsatisfactory categories appears
to be more suitable for conditions prevailing in this part of the country. All milks having methylene
blue reduction times above four hours may be considered to be satisfactory. It is also found that
methylene blue reduction times between two and four hours indicate an average quality while those
below two hours indicate a poor quality. Although these modified standards may also be found
to be rigid, particularly in summer when the reduction times tend to be shorter for all classes of milks,
they have been used for comparing the results of the resazurin test in the present study.
TABLE I
Distribution of the one hour resazurin readings and methylene blue reduction times
One hour resazurin readings
Blue, lilac and mauve . . . . . .
Pink mauve .
.
.
.
.
.
.
.
Purple pink . . . . . .
Pink . . . . . .
Fading pink to colourless . . . . .
Methylene blue reduction times
Over 4 hours
(good)
219
45
41
2
..
2 to 4 hours
(average)
88
47
43
25
4
Below 2 hours
(poor)
2
1
9
29
103
TOTAL
309
93
93
56
107
From an analysis of the distribution of the one hour resazurin readings at different levels of
methylene blue reduction times (Table I), it is seen that about 70 per cent of the samples attaining
the blue, lilac or mauve stages have passed the methylene blue standard of four hours ('good') with
28 per cent falling in the two to four hours class ('average'). The latter discrepancy is probably
due to the methylene blue standard of four hours being too rigid for the summer samples. Similarly
about 70 per cent of the samples passing the methylene blue standard of four hours have shown
either the blue, lilac or mauve stages with 15 per cent reaching the pink mauve stage and 13 per cent,
the purple pink stage. A good many samples in the pink mauve and purple pink stages were found
to be associated with high leucocyte counts on (microscopic) examination which factor may be respon-
sible in down-grading them on the resazurin scale, because methylene blue is comparatively less
sensitive to the action of leucocytes [Davis et al. 1943 ; Thomas and Bowie, 1944]. There is thus
close relationship between the resazurin reduction up to mauve stage and methylene blue reduction
standard of four hours ('good').
A similar relationship is observed between the resazurin reduction up to the fading pink or
colourless stage and methylene blue reduction below two hours ('poor'). Almost all the samples
H