How many times have you seen milk boil and spill over, and wondered why this happens? To answer this question we have to know a little more about the composition of milk.
Milk and its composition
Unlike water, milk is not a simple liquid. It is a colloid and contains many substances in suspended form. These substances are mainly protein, sugar and fat. When milk is heated slowly, the proteins and fat get separated. Since they are lighter than the milk they collect on the surface in the form of a layer called cream.
Of water vapour and spilling milk
Milk also has a large quantity of water in it. During heating some water gets converted into water vapour. The water vapour being lighter, rises up.
As the upper layer of the milk is covered with cream, the vapour gets trapped below it. As the milk is heated further, the water vapour expands and thick foam is produced on top.
Finally, the trapped vapour lifts the layer of cream up, quite like a hot air balloon, and finally bursts through this layer. As the vapour escapes, a lot of cream and milk spill out.
How we can prevent milk from spilling
It is very easy to prevent milk from spilling. All one has to do is to provide a path for the vapour to escape. For this we can keep a long-handled spoon in the milk container so that water vapour is able to escape along the handle of the spoon. This prevents the vapour from getting trapped under the layer of cream and thus spilling over of milk.