Milk Microbiology: Introduction, Presence of organisms, Method of Disinfection or Sterilization and Bacteriological Examination

Milk Microbiology

Introduction of Milk Microbiology

Milk can occasionally contain bacteria derived from three sources and they are from animals,from hands of milk handlers,from the environment. Milk borne pathogens pose a threat to the community so milk has present following

  1. Agents primarily of animal origin
  2. Agents primarily of human origin

Animal origin milk disease: Tuberculosis, Brucellosis, Salmonellosis, Q fever, Staphylococcal and streptococcal infections

Human origin disease: Typhoid and Paratyphoid fever, Shigellosis, Cholera, enterohaemorrhagic E. coli

List of bacteria that can be found in contaminated milk

  1. Streptococcus lactis
  2. Streptococcus faecalis
  3. Achromobacter
  4. Clostridium perfringens
  5. Clostridium butyricum
  6. Bacillus subtilis
  7. Bacillus cereus
  8. Proteus vulgaris
  9. Staphylococcus aureus
  10. Gaffkya tetragena

Methods used for disinfecting/ sterilizing milk

Thermized milk: 57-68°C for 15 sec, efficacy check by methylene blue reduction

Pasteurization of milk: 72ºC for 15 sec, bacterial spores, Coxiella, Mycobacterium and some preformed toxins are not destroyed effectively

Ultra heat milk: 135 °C for 1 sec ,destroyed all the microorganisms and spores

Sterilized milk: heated at 100 °C for long periods such that it can pass the turbidity test.

Methods for bacteriological examination of milk

Carried by several methods

Colony count

  • viable count
  • coliform count

Chemical test

  1. Methylene blue reduction test
  2. Phosphatase test and
  3. Turbidity test

Detection of specific pathogens

  • Mycobacterium spp.
  • Brucella spp.

Viable count

Done by plate dilution method in yeast extract milk agar

Incubate at 37 °C or 21 °C for 72 hours.

Calculation

Total count in fixed amt of milk= No. of colony*dilution factor

(Ultra heat-treated milk should have a viable count of less than 1000/ml of milk)

Coliform count

  • Done by serial dilution of MacConkey broth and milk (broth containing Durham’s tube)
  • Incubate at 37 °C for 24 hours
  • Observe for gas and acid production indicates coliform
  • (coliform killed by Pasteurization)

Chemical test

Methylene blue reduction test

It is more economical than viable count.

Principle: Viable bacteria reduce the methylene blue in the milk when kept in a dark place.

Result: considered satisfactory if it fails to decolorize methylene blue within 30 minutes.

Phosphatase test

It uses to test for checking the sterilization of milk

Principle:

In boiled milk all the heat coagulable proteins  precipitate so that it does not become turbid after of addition  ammonium sulphate.

Detection of specific pathogens

Tubercle bacilli: centrifuge the milk at 3000 rpm for 15 minutes then inoculate into L-J medium from deposit and incubate.

Brucella: inoculating cream from milk into serum dextrose agar or injecting into guinea pig.

Diagnosis of brucellosis in animals can  check  demonstrating antibodies in milk by ring test

Milk Ring Test

  • Frequently used test
  • Add a drop of colored brucella antigen (B. abortus/ B. melitensis with hematoxylin)  to milk in a test tube, incubated in a water bath at 70°C for 40-50 minutes.
  • Positive: Blue ring  at the top leaving the milk unstained.
  • Negative: No ring. Milk remains uniformly blue. 

References

  1. https://onlinelibrary.wiley.com/doi/pdf/10.1002/0471723959.fmatter
  2. https://www.sciencedirect.com/science/article/pii/B9780128105306000031
  3. https://www.uoguelph.ca/foodscience/book/export/html/1884
  4. https://academic.oup.com/femsre/article/37/5/664/541439
  5. https://dairyprocessinghandbook.tetrapak.com/chapter/microbiology
  6. https://www.indiaagronet.com/indiaagronet/DAIRY/CONTENTS/milk_microbiology.htm
  7. https://www.groupe esa.com/ladmec/bricks_modules/brick02/co/ZBO_Brick02_3.html
  8. https://www.groupe-esa.com/ladmec/bricks_modules/brick02/2%20-%20Milk%20composition%20and%20microbiology.pdf
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