Milk, in addition to water, contains long polymer molecules. When water evaporates from the surface layers when milk is heated, these molecules adhere and form a strong film. Meanwhile, tiny bubbles appear at the bottom – this is the gas dissolved in the liquid. Water also begins to evaporate inside the bubbles, they grow, and their walls are reinforced with a polymer film.
When the bubbles float, they rest against the film on the surface and cannot, as in water, burst and release steam. As the milk approaches the boiling point, the number of bubbles increases, they form foam, which lifts the polymer film. At the same time, the bubbles themselves are strong enough and do not burst. Propped from below with new ones, they quickly overflow the pan.
Understanding the nature of this phenomenon allows you to successfully deal with it. So that the milk does not run away, it is stirred, not allowing the film to appear. The easiest way is if the bubbles themselves rise from the bottom. It is only necessary that they float to the surface already enlarged, then any film will be nothing to them.
The Soviet industry even produced “milk guards” in the form of a stainless steel disc with concentric grooves. Its surface is smooth and easy to clean. The disk is placed on the bottom of the pan, while quite a lot of air remains in the grooves, and vapors of boiling milk tend to come here. As a result, rather large bubbles form at the bottom, which float to the surface through a special neck in the disk. Foam in this case is not formed, and the milk does not run away. It was even possible not to look after it – this is such a physical prototype of today’s smart things: as soon as large bubbles formed, the disk began to rumble slightly.