2. INTRODUCTION
Test to find bacterial contamination of food (especially
milk).
Based on utilization of oxygen by bacteria which
reduces dye and decolorizes it.
Faster the decolorization, more inferior is the
bacteriological quality and vice-versa.
3. Methylene Blue Reductase Test(MBRT)
• Take 10ml of milk sample in a sterile test tube.
• Add 1ml of MBRT dye(dye concentration 0.005%).
• Close the tube with sterilized rubber stopper.
• Place in stand dipped in serological water bath
maintained at 37±1⁰C.
• Note time taken for decolorization.
5. Grading Of Raw Milk Based On MBRT
As per BIS 1479 (Part 3): 1977 criterion for grading of raw
milk based on MBRT is as below:
TIME QUALITY
5 hours and above Very good
3 to 4 hours Good
1 to 2 hours Fair
Less than ½ hours Poor
6. Rate of reduction depends on the nature of the organism present
in the milk. The rate of reduction by different microorganism is:
E.coli (coliforms) > Streptococcus lactis > faecal Streptococci >
micrococci> Thermoduric organisms > psychrotrophic organisms
Light hastens reduction and therefore the tests should be kept
covered.
Creaming causes bacteria to surface, so the tubes should be
inverted periodically.
Any manipulation that increases oxygen affects the test like cold
milk holds more oxygen, pouring milk back and forth and so on.
Factors Affecting
7. RESAZURIN TEST
• Same principle as MBRT
• More sensitive resazurin dye used
Mix 10 ml milk sample with 1ml of resazurin dye
solution(0.05%).
Place stopper and invert the tube to mix.Incubate at
37±1⁰C.
Examine tubes at the end of an hour in the "one-hour
test" or at the end of three successive hourly intervals in
the "triplereading test."
9. Contd…
Judgment of quality is based either on the color produced
after a stated period of incubation or on the time required
to reduce the dye to a given end-point.
COLOR QUALITY
Blue(no color change) Excellent
Blue to deep mauve Good
Deep mauve to deep pink Fair
Deep pink to whitish pink Poor
White Bad
11. ADVANTAGES
• Simple, rapid, inexpensive
• Only viable cells actively reduce the dye
DISADVANTAGES
• Not all organisms reduce the dye equally
• Not applicable to food specimens that contain reducing
enzymes