228
Resazurin Test for Assessing the Sterility of Dairy Utensils
[ XVII
The comparative efficiency of the resazurin test in detecting inefficiently sterilized utensils
(taking reduction up to disc 5 or lilac stage as a criterion of satisfactory condition) is further examined
in Table II.
TABLE II
Grading of utensils on plate count and resazurin test
(Figures in the Table represent number of samples distributed on the two scales)
Name of utensil
Bottles . . . . . .
Pails
.
.
.
.
.
.
Churns
.
.
.
.
.
.
Plate count grade
Satisfactory (below 10 per ml.) .
Unsatisfactory . . . .
Satisfactory (below 100 per ml.) .
Unsatisfactory
.
.
.
.
Satisfactory (below 500 per ml.) .
Unsatisfactory
.
.
.
.
Number of
samples
13
18
6
12
6
2
Classification on resazurin
scale
Satisfactory
(Disc 5 and
higher)
12
8
5
2
5
2
Unsatisfac-
tory
(Below disc
5)
1
10
1
10
1
..
Out of 25 utensils satisfying the plate count standard, 22 are passed by the resazurin test and
of 32 utensils failing on the former scale only 20 are condemned by the latter test. On the other
hand 20 out of 23 utensils failing the resazurin test are also condemned on the plate count standard.
Thus, the resazurin test, although capable of detecting most of the efficiently or inefficiently sterilized
vessels, fails to pick out an appreciable number of samples condemned on the plate count. This
anomaly is easily explained considering that (a) the slightest contamination occurring at the time
of plating and other inevitable sources of error in the technique will give rise to a higher count without
affecting the reduction test to the same degree, and (b) the flora surviving inefficient sterilization in.
some cases (e.g. bottles) may be of an inert type without much influence on the reduction of resazurin.
Accordingly the resazurin test would appear to be less sensitive and accurate than plate count in
detecting faulty methods, while it is more reliable in indicating the presence of actively reducing
types of organisms that have survived in the containers and which are apt to bring down the keeping
quality of milk. In any case failure of the utensils to pass the resazurin standard is a definite indica-
tion of inefficient methods and this factor is of sufficient practical value in controlling the operations
under commercial conditions.
It is possible that many samples condemned by the resazurin test may come under the ' doubtful'
class suggested by Davis and Watson [loc cit.]. The present data have not provided any basis for
fixing up an intermediate standard. If the object of applying the test is to determine whether the
method of washing and sterilization of utensils is effective enough or not, the provision of any such
intermediate grade has no practical significance. Further application of the test under actual com-
mercial conditions is necessary for elucidating the above point.
SUMMARY
The applicability of the resazurin test for assessing the sterility of dairy utensils, e.g. bottles,
milk pails, churns, receiving tank and cooler, as a means of checking the efficiency of the cleansing
and sterilization methods in the dairy has been examined.
2. The resazurin test was carried out on rinsings (1 ml. mixed with 10 ml. sterile separated milk
and then incubated at 22°C. for 16 hours) and the colour changes in half an hour (at 37°C.) fairly
agreed with the plate counts of the rinsings. Resazurin reduction to a stage not lower than disc
5 (lilac shade) was an indication of satisfactory quality in all the utensils.