Good question. It is caused by the breakdown of the fat and the proteins, when the milk is heated. They are lighter and hence float to the top.
I personally do not drink it, as it comes from a farm. Hope this helps.
Spudmuffa you make me laugh...every post I read where you have replied you have put either good question or interesting question...dunno why but it makes me giggle everytime...Oh and i hate skin ewwwwww