IV ]
F.
D.
AIBARA
AND
H.
LAXMINARAYANA
227
5, 5 and 3 at the end of 30 minutes 1 hour and 2 hours respectively. In the case of milk pails and
churns (subjected to steaming) the plate counts are below 100 and 500 per ml. respectively ['satisfac-
tory' — Chalmers, 1945], while resazurin is reduced to disc 5, 4 and 2 respectively in the above intervals.
The receiving tank and cooler have registered very high counts with resazurin reduction going up
to a colourless stage even at 30 minutes. The latter phenomenon is obviously due to the presence
of large numbers of actively reducing types (mainly lactic streptococci and micrococci) in these
utensils which are not easily eliminated. The application of the resazurin test in the present form
does not, therefore, provide any useful information in the case of the receiving tank and cooler.
The coliform test has given uniformly negative results (except for the last two categories) and does
not appear to be of much value in judging the efficiency of sterilization [Barkworth, loc. cit.].
TABLE I
Comparison of plate count, resazurin reduction and coliform test for different utensils
Type of utensil and treatment given
Bottles (8-10 oz.)—
Washed only
.
.
.
.
Sterilized for one hour . . .
Sterilized for two hours . .
Sterilized for four hours . .
Bottles (1 lb.)—
Washed only
.
.
.
.
Sterilized for one hour . . .
Sterilized for two hours . . .
Sterilized for four hours . .
Milk pails—
Washed only
.
.
.
.
Steamed for 2 minutes . . .
Churns—
Steamed for 5 minutes . . .
Receiving tank—
Steamed for 5 minutes . . .
Cooler—
Number
of
samples
11
4
4
4
12
4
4
4
10
11
9
9
5
Plate count
per ml. of
rinse
(Geo. Mean)
358
12
5
6
69
3
6
6
5,479
7
431
194,200
194,700
Mean resazurin disc number for
incubated samples at the end of
30 minutes
3½
5¼
6
6
5½
5½
6
5½
2¾
5
5
¼
0
1 hour
2½
5
6
5¾
4½ 4½
5¾
4¾
2
4
4¾
0
0
2 hours
1½
3¾
5¼
5
3
3
4¼
3¼
1¼
2
2¾
0
0
Presumptive coli-
form test
1 ml.
1 ml.
1 ml.
1 ml.
1 ml.
1 ml.
1 ml.
1 ml.
10 ml.
10 ml.
10 ml.
(pos. in)
1 ml.
1 ml.
On the basis of the above data resazurin reduction up to disc 5 (lilac stage) in 30 minutes may
be taken to indicate a satisfactory degree of sterility in the case of pails and churns while a higher
resazurin standard (say disc 5½) can be obtained in regard to bottles. It is also clear that there is
no advantage in observing resazurin colour changes beyond 30 minutes. The discrepancies shown
between bottles and other utensils may be ascribed to the higher limits of bacterial counts allowed
for the latter and also the different types of flora surviving in the utensils after the washing and
sterilization treatments. In the case of bottles the predominant organisms were found to be staphy-
lococci and micrococci, while lactic streptococci were most prominent in rinse solutions from pails
and churns. The latter organisms are known to grow faster in milk and also reduce resazurin more
actively [Jones and Davis, 1944]. The British workers [Davis and Watson, loc. cit.] have suggested
that reduction of resazurin to stages higher than disc 5 (lilac) should be regarded as indicating
satisfactory churns. Considering the differences between the two countries in respect of climatic
conditions and nature of bacterial flora in milk, the slightly lower standard of disc 5 indicated in the
present study appears to be suitable. The same standard can also be adopted in the case of bottles
without giving rise to any serious errors of judgement because in actual practice it would be difficult
to distinguish between discs 5½ and 5 while the development of a slight mauve colour (lower than disc
5) is more clearly detected.