Methylene Blue Dye Reduction Test for Assessing the Raw Milk Quality
Methylene Blue Dye Reduction Test, commonly known as MBRT test is used as a quick method to assess the microbiological quality of raw and pasteurized milk. This test is based on the fact that the blue colour of the dye solution added to the milk get decolourized when the oxygen present in the milk get exhausted due to microbial activity. The sooner the decolourization, more inferior is the bacteriological quality of milk assumed to be. This test is widely used at the dairy reception dock, processing units and milk chilling centres where it is followed as acceptance/rejection criteria for the raw and processed milk
Grading of raw milk based on MBRT:
MBRT test may be utilized for grading of milk which may be useful for the milk processor to take a decision on further processing of milk. As per BIS 1479 (Part 3): 1977 criterion for grading of raw milk based on MBRT is as below:
5 hrs and above
3 to 4 hrs
1 to 2 hrs
Less than ½ hrs
Procedure: The test has to be done under sterile conditions. Take 10 ml milk sample in sterile MBRT test tube. Add 1 ml MBRT dye solution (dye concentration 0.005%). Stopper the tubes with sterilized rubber stopper and carefully place them in a test tube stand dipped in a serological water bath maintained at 37±1⁰C. Record this time as the beginning of the incubation period. Decolorization is considered complete when only a faint blue ring (about 5mm) persists at the top.
Recording of Results - During incubation, observe colour changes as follows:
a) If any sample is decolourized on incubation for 30 minutes, record the reduction time as MBRT - 30 minutes.
b) Record such readings as, reduction times in whole hours. For example, if the colour disappears between 0.5 and 1.5 hour readings, record the result as MBRT - 1 hour; similarly, if between l.5 and 2.5 hours as MBRT - 2 hour and so on.
c) Immediately after each, reading, remove and record all the decolourized samples and then gently invert the remaining tubes if the decolourization has not yet begun.
1. IS:1479 (Part-3)-1977