Dairy MicrobiologyMicrobiology

Test Of Milk

Enzymatic Test Of Milk By Methylene Blue Reductase Test

This test is recommended to determine the quality of milk sample. It is based on the population of bacteria with that of dissolved oxygen. If the sample contains a large population of actively metabolising microorganisms, the concentration of dissolved oxygen will be low because of presence of enormous number of microorganisms. Oxidation-reduction potential of the sample milk would be low. The dye, methylene blue, is an indicator of such reaction. In an anaerobic environment, the colour of the dye would be dull and said to be reduced. The raw milk contains large number of enteric organisms and Streptococcus lactis. Both are dye reducers. The rate or duration at which colour of the dye is reduced indicates the quality of milk.


  • Test tubes
  • Raw milk sample
  • Water bath
  • Methylene blue
  • Pipette
  • Glass marker.


  1. Prepare methylene blue reagent (1:25,000).
  2. Take two test tubes and transfer 10 ml milk sample to each tube.
  3. Add 1 ml of methylene blue reagent to each test tube.
  4. Shake the tubes and place them in water bath.
  5. Record the time and observe the milk samples at every 30 minutes till 360 minutes (6 h) and calculate the period of reduction of the dye by observing the change of colour from blue to white.
  6. Determine the quality of the milk sample based on the above test.


The following selection criteria should be considered while examining the milk sample.

  • If milk is reduced within 30 minutes = Very poor quality
  • If reduction occurs within 90 to 120 minutes = Poor quality
  • If reduction occurs between 120 to 360 minutes = Fair
  • If reduction occurs between 6 to 8 hours = best quality of milk
Enzymatic Test Of Milk


Dr. R. C. Dubey – Practical Microbiology

Gaurav Singh

Editor in Chief Medical Microbiology & Recombinant DNA Technology (RDT) Labs - RDT Labs Magazine

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