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
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
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.
Carried by several methods
Viable count
Done by plate dilution method in yeast extract milk agar
Incubate at 37 °C or 21 °C for 72 hours.
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)
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.
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.
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