Do you know the reason why Milk spills out while boiling? Unlike water and other liquids, Milk when boiled spills out of the container. There is a reason behind this behaviour.
Composition of Milk
Unlike many other liquids or water, Milk is not a simple substance. It is composed of many substances in suspended form or it is a colloid. Fat, Sugar, Starch, Protein, Vitamins, Mineral and Water are its main constituents. When Milk is heated, the Proteins and Fat get separated. They are lighter than Milk and hence gets collected on the surface and becomes a layer. This layer is the Cream.
Water present in Milk gets converted to Water Vapour when it is heated. The upper layer of the Milk by then would have covered up with the cream. Water Vapour cannot exit due to this and gets trapped under the layer. Heating the Milk further, the Water Vapour expands and a thick foam is produced on the top. The trapped Vapour pushes the layer up and the layer bursts in the process. Water Vapour thus gets an exit passage and escapes out spilling a lot of Cream and Milk out.
How to prevent Milk from Spilling
By keeping a long spoon in the Milk while it gets boiled, we can prevent the Milk from spilling out to a great extent. By this, we actually provide a passage for the Water Vapour to go out along the handle of the spoon. As the Vapour exits, it is prevented from getting trapped under the layer.
We can also avoid spilling of the Milk while boiling by continuously stirring it. Still there are chances of Vapour accumulating below the upper layer. So, it is not possible to fully prevent Milk from spilling out unless you cut off the flame as soon as it is ready to spill out.