Why Does Skin Form On Top Of Certain Liquids?

19 June 2018, 16:49

Picture: N/A

Mystery Hour Question

Question: Why does skin form on the top of certain liquids like custard and gravy? Alex, King’s Norton


Name: Professor Hal Sosabowski

Qualification: Professor of the public understanding of science at the University of Brighton.

Answer: There are two effects going on.

There’s a class of mixture called colloids which are when you have two different phases mixed together but they don’t dissolve.

So, smoke, clouds, mist, are liquid gas colloids – they’re little bits of liquid suspended in the gas that we call air.

There’s a subset called emulsions: emulsion paint, custard, milk, gravy.

They’ve got little bits of fat suspended in water that never actually settle out, and if you heat them up, some of the water near the top evaporates making the top fattier which contributes to the skin.

This is then backed up by the fact that when you heat proteins they denature, they change their properties and they become insoluble.

These then rise to the top to form the skin.

With gravy, there's protein in the meat which becomes insoluble when heated and rises to the top to form the skin.