A microwave oven does not deliver heat from the outside; waves concentrate at certain points inside the container, so some parts of the milk may boil while others stay lukewarm. A thin skin that forms on the milk’s surface also traps steam, which can cause sudden boilovers. That is why milk is heated in short intervals, stirred between intervals, and the container is never filled more than halfway.
In a microwave oven, heat does not move from the outside in. The appliance directly vibrates water molecules in the food, and heat comes from that vibration. But the waves are not distributed evenly inside the oven; they overlap and grow stronger in some places and weaker in others. That is why one part of the milk in the same glass may boil while another part is still lukewarm. The turntable reduces this difference but does not eliminate it. That is why the same time in the same appliance does not produce the same result in every container. You have to learn the appliance’s power and timing by testing them in each kitchen.
The second problem comes from the milk itself. As milk heats, a thin film of proteins forms on its surface and acts like a lid; steam collecting underneath cannot escape easily. At some point the pressure tears the film and the milk suddenly surges upward and boils over. That is why milk can look calm from the outside, yet start to boil over when the door is opened. Removing the film reduces the risk of a boilover, but another film forms as heating continues.
The third and least-known problem is superheating. In a smooth container, an undisturbed liquid can pass its boiling point without starting to boil because there is no nucleation point for bubbles to form. When the container is shaken or a spoon is dipped into it, the liquid boils suddenly and splashes out. That is why a container taken from the microwave is not moved right away. The same thing can happen with water, so take the same precautions when heating plain water. For the same reason, stir milk taken from the container before pouring it into a glass.
The solution addresses all three problems at once. Put the milk in a wide-mouth, microwave-safe container and do not fill it more than halfway. Heat it in short intervals rather than all at once; remove the container and stir between intervals. Stirring evens out the temperature and interrupts superheating. Leaving 1 wooden spoon in the container also gives bubbles a nucleation point. Do not choose a container with a metal rim; gold-trimmed cups can spark in the microwave.
There are a few more details. Use a microwave-safe container large enough to prevent boilover, and follow the appliance’s instructions for heating and standing time. Milk may heat unevenly and some areas may be scalding hot; stir it thoroughly and check its temperature with a suitable food thermometer, not by tasting it. Do not microwave breast milk or infant formula. When heating milk on the stove, keep sweeping the bottom of the pot and do not leave it unattended.