What happens when you over boil the milk?

Overboiling, which means heating milk past boiling point, generally means it has reached too high a temperature. When that happens, proteins present in the milk may curdle and become difficult to digest. If overboiled, it results in an undesirable flavour from sulfur compounds produced during extended heating of certain molecules and a yellowish cast, due to browning reaction by reducing sugars and amino acids from milk. If not carefully done (as can happen quickly, for example in a microwave oven) the boiling will overflow the container as milk foams as it boils.