Dye reduction tests

(A) MBRT test:

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.

MBRT Procedure:

• Prepare a standard methylene blue solution by dissolving one tablet of methylene blue thiocyanate in 200 ml of sterile water.
• Add this to 10 ml of milk, shake well and keep in water bath.
• Note the time taken for the blue color to change to white.
• The total time gives MBRT time.

MBRT Test result:

5 hrs and above - Very Good

3 – 4 hrs - Good

1 – 2 - hrs - Fair

Half an hour - Poor

(B) Resazurin Test:

The resazurin test is conducted similar to the methylene blue reduction test with the judgement of quality 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. Numerous modifications have been proposed. The two most commonly used are the "one-hour test" and the "triple-reading test" taken after one, two, and three hours of incubation. Other modifications have value in specific applications.

Procedure:

• Prepare resazurin solution by dissolving one resazurin tablet (dye content/ tablet, approximately 11 mg, certified by Biological Stain Commission) in 200 ml of hot distilled water as was done in the methylene blue test.
• Place one ml of dye solution in a sterile test tube, then add 10 ml of sample. Stopper the tube, place in the incubator and, when the temperature reaches 36o C, invert to mix the milk and dye. Incubate at 36C.
• Tubes are examined and classified at the end of an hour in the "one-hour test" or at the end of three successive hourly intervals in the "triplereading test."

Color and Quality of Milk:

Blue (no color change) - Excellent

Blue to deep mauve - Good

Deep mauve to deep pink - Fair

Deep pink to whitish pink - Poor

White - Bad