The COB test, short for Clot on Boiling, is a quick way milk collection centres check whether milk is fit for pasteurization. When bacteria multiply in milk over time, they produce lactic acid, and this acid makes the milk protein, casein, unstable. If a sample like this is heated to boiling, the unstable casein clots visibly in the tube. Milk that clots this way is rejected because it will not survive the heat of pasteurization, and this simple heat test is what COB stands for, clot on boiling.