Boiling milk?!
Boiling milk?
why is it that when you boil milk the top is covered with a thin skin? How do you stop it forming?
Answers:
Milk contains 87 per cent water, 4 per cent proteins and 5 per cent lactose (milk sugar). When we boil milk, the fat, sugar, proteins and minerals get separated. Since they are lighter than milk they collect on the surface in the form of cream.
During heating some amount of water gets converted into vapour and the bubbles of water vapour rise to the top but the heat is conducted away by the layer of water and by the fat droplets that have a higher boiling point than water.
The vapour gets trapped in the creamy upper layer. As the milk is heated further the water vapour expands and thick foam is produced on the top.
As the milk is boiled continuously the water, which boils at 100 degrees Centigrade, produces more water vapour and pressure builds up in the boiling milk so that the vapour pressure raises the creamy layer. So the milk pushes the creamy layer out and milk spills out.
I dont know why. However, I have found my adding a bit of water to it and bringing it to a boil a bit slower it does not form the nasty film.
I do not boil milk as it can burn too easily, just heat it gently an simmer.
It is the separation that forms the skin, which can be skimmed off.