When milk boils, steam bubbles form on its surface. Because milk is more viscous than water, these bubbles have a harder time bursting and can cause the pot to overflow more easily.
Milk contains proteins, notably casein and the so-called whey proteins, which act as natural foam stabilizers. When milk is heated, these proteins migrate to the surface, where they form a kind of thin elastic skin. This skin traps the steam bubbles created during boiling, much like a balloon's envelope, and produces a resilient foam. The higher the temperature rises, the thicker and more persistent this foam becomes, until it suddenly swells and quickly overflows out of the pot. It is because of this solid protein network that milk suddenly boils and spills over unexpectedly.
When the water heats up at the bottom of your pot, it forms small bubbles of steam that rise to the top. At first, these bubbles quietly burst at the surface. But when the milk starts to boil, the layer of proteins and fats forms a sort of web on the surface, a thin elastic skin that prevents the steam from escaping easily. As a result, the pressure of the bubbles underneath increases rapidly. All of a sudden, many bubbles form at the same time, grow very quickly, get trapped, and when they finally manage to break through this fragile surface, it overflows everywhere. It is at this precise moment that the milk suddenly rises to joyfully spill out of the pot.
If you heat whole milk (which is fattier), you will notice that it rises and overflows more slowly than skim or semi-skim milk. This is because the fat slightly limits the formation of foam by disturbing the bubble network stabilized by proteins. Conversely, when you add sugar, it increases the viscosity of the milk: it thickens the mixture a bit, slows down the movement of the bubbles, but when boiling occurs, be careful, it can be more sudden and unpredictable. The result? A sometimes more abrupt overflow.
When you heat milk too strongly, the very rapid increase in temperature exacerbates the sudden appearance of steam. The intense heat immediately causes a large quantity of steam bubbles to rise from the bottom and grow extremely quickly. These large bubbles break through the layer of proteins on the surface, destabilizing existing foams and bubbles, and the milk suddenly overflows everywhere around the pot. Going directly from gentle heat to very high heat causes the foam to become uncontrollable: overflow is guaranteed every time.
Milk generally boils at a temperature slightly higher than that of pure water (100 °C) due to its dissolved components such as proteins, fat, and sugar.
Adding a pinch of sugar or butter to your milk while heating it significantly reduces the formation of foam, thereby limiting the risk of overflow.
In India, a traditionally wider and shallower vessel is used to boil milk, as this significantly reduces the risks of overflowing due to the rapid formation of foam.
Some pots specially designed for heating milk have a double bottom with water to ensure a gentle and constant boil, thus preventing sudden overflow.
Yes, the shape and size of the pot play a role. Pots with high, thick sides distribute heat better and provide more space for bubbles before overflowing, thus reducing the risk of rapid overflow.
Unlike milk, water does not contain proteins that create a stable foam on the surface. When water reaches its boiling point, the steam bubbles burst more easily and quickly at the surface, so there is no violent overflowing as there is with milk.
It is recommended to immediately soak the pot in hot water. Then, use a wooden spatula or a non-abrasive sponge to detach the burnt milk residues. For stubborn residues, boiling water with white vinegar in the pot will make cleaning easier.
It is possible to significantly reduce risks by gradually heating the milk, using a suitable pot or placing a milk frother at the bottom of the pot. However, vigilance is always necessary.
A wooden spoon placed across the pot helps break the bubbles formed by the milk protein foam, thus limiting sudden rising and overflow. However, this is not a foolproof solution; one should always keep an eye on the cooking process.
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