Dairy MicrobiologyMicrobiology

Quality Testing Of Milk By Resazurin Test

It is one of the rapid methods for detecting milk supplies of poor quality. Resazurin (diazoresorcinol) is a azo dye and hence acts as an oxidation-reduction indicator which is blue in oxidised stage and pink in reduced stage. Upon reduction due to bacterial contamination or other causes it is changed into blue to pink coloured compound and then into the colourless compound dihydroresorufin. The rate of reduction from blue to pink depends upon the extent of contamination. Hence, the bacteriological quality of milk is based on duration (minutes) i.e. how much time is taken in reduction of resazurin. This test completes within 10 minutes which shows the presence of excessive bacterial numbers indicating poor quality of milk.


  • Sterile test tubes of 15 ml capacity (2) (marked with control and test)
  • Rubber stopper
  • Pipettes
  • Water bath
  • Standard resazurin discs
  • Resazurin standard solution (0.5%) kept in an amber colour bottle


  1. Draw milk sample and transfer in a sterile glass stopper bottle.
  2. Transfer 10 ml milk in the marked test tubes.
  3. Add 1 ml resazurin (0.5%) solution into one of the test tubes containing milk. The other
  4. tube will act as control.
  5. Mix well and place in water bath for 10 minutes.
  6. Compare the colour of test milk with natural colour i.e. control milk.
  7. Record the colour, compare with standard resazurin discs and classify them accordingly.


If colour falls between natural and coloured one, the milk quality is poor.


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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