H. LAXMINARAYANA 251
the dye reduction tests. There were only a few samples of mixed milk in this category. Again
most of the samples with colony counts over one million in the case of all the three types of milk
have failed both resazurin and methylene blue tests. As regards the intermediate samples with
colony counts between 200,000 and one million per ml., about 40 per cent of cow milks have passed
both the tests, the rest coming mostly under the 'average' category. About 80 per cent of buffalo
milks and 61 per cent of mixed milks have passed the resazurin test while about 50 per cent of both
buffalo and mixed milks have passed the methylene blue test.
It is thus seen that there is a fairly good agreement between the two reduction tests and the
plate counts in the case of low count (below 200,000 per ml.) and high count (over 1 million per ml.)
samples and that the resazurin test is as reliable as the methylene blue-reduction test in detecting
high count milks [Thomas, 1943]. In the case of low count milks, particularly as some of them were
examined in winter when bacterial activity is less pronounced, the effect of cells becomes more import-
ant which may result in the resazurin test showing a poorer correlation with the plate counts. This
effect is practically masked in high count samples. The above observation serves to demonstrate
the superiority of methylene blue reduction as a measure of mere bacterial numbers. If, however,
the term 'sanitary quality' in milk is to denote freedom from excessive bacterial contamination as
well as any abnormality (physiological or pathological), this factor should not militate against the
value of the resazurin test.
The relationship in the case of intermediate samples with counts between 200,000 and one
million per ml. is poor. A greater number of samples in this category, particularly buffalo milks,
seem to fail the methylene blue test than the resazurin test thus indicating a higher degree of agree-
ment between the methylene blue test and plate counts.
(iii) Effect of conditions of production on the resazurin test
In the course of this investigation samples drawn from three different sources namely, farm
produced milk, village produced milk and milk handled by private dairies (which give a fair representa-
tion of the usual conditions of production and handling in this part of the country) have been used.
It is well known that the quality of milk is influenced to a large extent by the conditions of produc-
tion and handling and as the resazurin test is to be employed for determining the same, it may be
interesting to examine how far the test throws light on the above factors.
TABLE IV
Effect of conditions of production of the resazurin test
One hour resazurin reading
Blue to mauve . . . . . . . . . .
Pink mauve and purple pink . . . . . . . . . .
Pink . . . . . . . . . .
Colourless . . . . . . . . . .
Total number of samples . .
Source of sample
Farm produced
215
108
13
7
348
Village
produced
70
36
16
41
163
Milk handled
by private
dairies
24
38
20
65
147
An examination of the above data (Table IV) reveals that about 63 per cent of farm produced
milks, 43 per cent of village produced milks and 16 per cent of milks from private dairies have been
graded 'good ' by the resazurin test.