Why does milk have a tendency to boil over more often than any other liquid?
Milk—and cream—contain casein proteins that gather near the surface as they heat. Once the milk comes to a boil, steam bubbles rising from the bottom of the pot are forced through the protein-rich layer at the top. The proteins stabilize the bubbles, keeping them from bursting, so they rapidly increase in number and overwhelm the pan. Then, whoosh—dairy disaster.
We tried several techniques purported to prevent the problem, from leaving a long-handled spoon in the pot to buttering the pan’s rim. While such methods delayed the boil-over, the only surefire solution was to use a larger pan (a full 4-quart saucepan for a pint of milk, a size we would normally consider far too large for the task). In a pot with a small circumference, the bubbles are tightly packed, with little room to expand. But in a pot with a large diameter, the bubbles can grow bigger and bigger until even the reinforcement provided by the casein proteins can’t prevent them from popping. As a result, there are never enough bubbles to overwhelm the capacity of the pan.
In sum, to keep milk from boiling over, heat it in a big, wide pan.