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
- Agents primarily of animal origin
- 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
- Streptococcus lactis
- Streptococcus faecalis
- Achromobacter
- Clostridium perfringens
- Clostridium butyricum
- Bacillus subtilis
- Bacillus cereus
- Proteus vulgaris
- Staphylococcus aureus
- 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
- Methylene blue reduction test
- Phosphatase test and
- 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
- https://onlinelibrary.wiley.com/doi/pdf/10.1002/0471723959.fmatter
- https://www.sciencedirect.com/science/article/pii/B9780128105306000031
- https://www.uoguelph.ca/foodscience/book/export/html/1884
- https://academic.oup.com/femsre/article/37/5/664/541439
- https://dairyprocessinghandbook.tetrapak.com/chapter/microbiology
- https://www.indiaagronet.com/indiaagronet/DAIRY/CONTENTS/milk_microbiology.htm
- https://www.groupe esa.com/ladmec/bricks_modules/brick02/co/ZBO_Brick02_3.html
- https://www.groupe-esa.com/ladmec/bricks_modules/brick02/2%20-%20Milk%20composition%20and%20microbiology.pdf