Clot-on-Boiling (COB) Test

The development of acid in milk by conversion of lactose to lactic acid makes the milk unstable to heat and milk coagulates on boiling. Generally, milk samples with more than 0.20% acidity give a positive COB test.
Objectives
• To determine developed acidity
• To know the heat stability for processing of milk
Apparatus
• Test tube preferably with a mark at 5 ml
• Test tube holder
• Boiling water bath
Procedure
• Transfer 5 ml of milk into the test tube.
• Place the test tube in a water bath at boiling temperature for 5 to 6 minutes.
• Remove the tube from the bath and rotate them in an almost horizontal position.
• Examine the film of milk on the sides of the test tube for any precipitated particles.
• The formation of clot or presence of precipitated particles constitutes a positive C.O.B. test.