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Clot on Boiling (COB) test in Milk

Use: test for screening, rapid testing of increased acidity.

Advantages: simple, quick, cheap, definitive result (milk either coagulates or not). 

Disadvantages: slightly sour milk is not detected

Alternatives: alcohol test, Acidity test.


The heating of milk in an advanced state of souring (acidity of more than 0.20%) or abnormal milk (colostrum or mastitis milk) will result in clotting.

Equipment and materials:

a) Test tubes (15.0 x 1.0 cm, preferably with a mark at 5 ml).

b) Source of heating, e.g. a boiling water bath or a flame.


1) Put test tubes with about 5 ml of milk in heating source for up to 4 minutes.

2) Rotate the tubes in an almost horizontal position and examine the film of milk or side of the test-tube for any precipitated particles.


The acidity of milk that gives a positive test is generally above 0.22% (as lactic acid) or has an abnormally high percentage of protein like colostrums milk. Such milk cannot stand the heat treatment in processing and is therefore not suitable for distributing as liquid milk or for processing. Such milk must therefore be rejected. Please take note that at high altitude milk boils at a lower temperature. This test is not very sensitive to slightly sour milk.

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