RESAZURIN TEST FOR ASSESSING THE STERILITY OF DAIRY UTENSILS
By F. D. AIBARA and H. LAXMINARAYANA, Indian Dairy Research Institute, Bangalore
(Received for publication on 13 June 1947)
I T is well known that dairy utensils and milk plant, which are not thoroughly cleaned and
efficiently sterilized, represent the most important sources of bacterial contamination in milk.
Accordingly the efficiency of the methods of cleansing and sterilization in a dairy is required to be
checked periodically by testing the equipment and utensils for sterility. For this purpose the plate
counts of either the rinsings or of swabs are most widely employed. Since absolute sterility, i.e.
complete freedom from all living organisms, is impracticable under commercial conditions, maximum
limits of plate counts are allowed for different types of utensils varying from 10 per ml. of rinse solution
in the case of bottles to 100 per ml. for milk pails and 500 per ml. for churns [Chalmers, 1945]. An
official technique for the examination of washed churns and suitable limits of plate counts for evaluat-
ing the efficiency of farm sterilization methods have also been prescribed by the British Ministry of
Agriculture [1943]. The plate count method is, however, complicated and involves considerable
delay in obtaining results. The presumptive coliform test which has also been used for the purpose
is not found to be of much value, particularly in the case of bottles [Mattick and Hoy, 1937 ; Bark-
worth, 1941].
Thomas [1943] suggested the use of resazurin as a simpler and quicker test for detecting churn
sterility. The method was subsequently modified by Davis and Watson [1943], according to whom
the rinse solution from the churn is added to sterile separated milk and incubated at 22°C. for 24
hours. The incubated sample is then subjected to the resazurin test and if the colour is not reduced
to a lilac shade (disc 5) in half an hour, it is an indication of a satisfactory degree of sterility in the
churn. Discs 5 to 3½ are taken to denote a ' doubtful' condition while reduction to disc 3 or below
is an index of unsatisfactory condition. A good agreement between these resazurin standards and
the plate count limits laid down by the Ministry of Agriculture was found by Davis and Watson.
The application of this test for examining the common dairy utensils used for handling milk in this
country would be of great practical value in improving the bacteriological quality of market milk.
Some modifications in the technique and interpretation of the test may, however, be called for in
view of the different climatic and environmental conditions prevailing here. The present study
was carried out with the object of examining the suitability and reliability of the resazurin test in
comparison with plate counts for determining the sterility of bottles, milk pails, churns, receiving
tank and cooler.
EXPERIMENTAL
The tests were carried out on milk bottles (½ lb. and 1 lb.), milk pails (10 lb.), churns (100 lb.)
receiving tank and cooler (surface type) after cleaning and sterilizing them in the following manner
so as to produce in them degrees of sterility ordinarily found in utensils handled in commercial dairies.
Bottles (½ lb. and 1 lb.) .
Pails
.
.
.
.
Churns
.
.
.
.
Receiving tank and cooler .
I Series . . .
II Series
I Series . . .
II
Series . . .
....
....
Washed with washing soda and soapnut powder
rinsed with hot water (70°C.) and dried in air for
4-5 hours by holding them mouth downwards in
a slanting position.
Washed as above and then sterilized in hot air oven
(160°-170°C.) for one hour, two hours and four
hours respectively using three separate batches
for each treatment.
Washed as above.
Washed and steamed for two minutes with a steam
jet and dried.
Washed as above and steamed for five minutes
with a steam jet and dried.
Washed and steamed thoroughly in all parts by
means of a hose pipe and dried.
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