Determination of
Methylene Blue Reduction Time for Milk (MBRT Test)
Principle:
This test is based on the principle
that methyl blue (an oxidation-reduction dye or indicator) which is blue in its
oxidized state, is reduced to colorless compound (leuco form) as a result of
the metabolic activities of bacteria in milk. When a solution of dye is added,
the organisms present in milk consume the dissolved oxygen and lower O-R
potential to a level where methyl blue or similar indicators are reduced or
decolorized.
Reagent Required: Methylene Blue Reduction Dye
Procedure:
1. The samples of milk are mixed thoroughly. If the
milk is in a bottle/ pocket, it shall be inverted at least 25 times to mix the
fat uniformly with the milk.
2. Take 10 ml of milk into a test tube and
add 1 ml of standard methylene blue
solution.
3. Invert the test tube to mix the milk
and methylene blue solution.
4. Place the test tube in a
thermostatically maintained water bath at 370C and note down the
time of incubation.
5. Observe the test tubes after 30 min for
decolourization reduction of dye. If there is no decolourization the tubes are
inverted once and transferred to the water bath for further incubation.
6. After 30 min, continue to observe for
the reduction of dye at an interval of every one-hour.
7. The milk shall be regarded as
decolorized, when the entire column of milk is completely decolorized or is
decolorized up to 5 mm of the surface.
MBRT Test result and interpretations:
|
MBRT TEST RESULTS |
MBR
Time (Hrs) |
Milk
Quality |
|
5
hrs and above |
Very Good |
|
|
3
– 4 hrs |
Good |
|
|
1
– 2 hrs |
Fair |
|
|
Half
an hour |
Poor |
Grading of milk in MBR test
|
Quality of milk |
Reduction time |
Approx.
keeping quality/ml |
Approx. bacterial count per ml |
|
Good |
5 hour or more |
40 hrs |
Below 5,00,000 |
|
Fair |
2 to 5 hours |
30 hrs |
5,00,000 to 40,00,000 |
|
Bad |
20 mins to 2 hrs |
10 hrs |
40,00,000 to 2,00,00,000 |
|
Very bad |
20 mins or less |
Less than 10 hrs |
Above 2,00,00,000 |
