The milk begins to rise through the saucepan and, before we realize it, it has already fallen on the kitchen. More milk has not suddenly appeared: when heated, bubbles and foam form and take up space. The increase in volume is much more striking than in a container with plain water.
The difference is in the composition. Milk contains water, fat and protein. As the temperature increases, some proteins change shape and can stabilize the bubbles. Material that forms a film is also concentrated near the surface. As the water at the bottom transforms into vapor and rises, this film helps the bubbles persist instead of breaking quickly, as water usually does.
The foam accumulates and raises the apparent level of the liquid. If the pot is very full, there is little room left before it overflows. Removing it from the heat also does not stop the process suddenly: the metal retains heat and the milk can continue to rise for a few moments. That’s why the reaction has to come before the foam reaches the edge.
The easiest way to reduce the risk is to heat less intensely and leave free space in the container. Watching the milk as it heats is also important: the rise can accelerate very quickly. It is a domestic phenomenon that shows how the composition of a liquid changes the behavior of bubbles and turns an ordinary boil into an overflow.
Santiago Campillo (@Scruzcampillo) is a biologist and scientific communicator specialized in health, nutrition and nature.
