Why does "skin" form on my coffee?

[bigbanana]

When I'm feeling lazy, I make "milk coffee" by heating milk in a jug in a microwave until it's boiling and adding the milk to instant coffee powder/granules in a cup. I then stir the mixture.

If I leave the coffee to stand, after a few minutes a disgusting, inedible skin forms on the surface of the coffee.

Why does coffee skin over in this way and what is the "skin" made of? It happens whether I add sugar to the coffee or not.

The skin forms on hot milk too.

Thanks all.

[ALLIGATOR]

Hi There, I to put milk in the Microwave for my coffee,
But I never Boil IT.
Try to heat the milk as hot as you need it but Do Not Boil IT, Success should be yours. Good Luck & Cheers. ALLIGATOR

[Goodsoulette]

http://www.findarticles.com/p/articles/mi_qn41 58/is_19990131/ai_n9656602


if you leave your spoon in it wont happen

[souldriver]

I would have thought the skin was made from milk!

i don't think you can avoid this as this also happens when you make rice pudding in the oven with milk.

[burkeas]

When I make coffee and milk, the same thing happens. I noticed that this is a layer of froth. I tried poking the froth and a hole appeared in the froth. A couple seconds later it seals up again. Try poking the skin to see if it's liquid or solid. If it's froth, I'd drink it.

[Goodsoulette]

if there is froth covering the coffee a skin cant form