[paul.fr]

If i make a drink of hot chocolate, and wait a few minutes before drinking it a "skin" forms on the surface!

Why is that?

[Karen W.]

It must be the cooling.. I love hot chocolate. Have you ever noticed the same thing on chocolate puddingthe cooked kind not instant. I think it is the milk, perhaps in the cooling the cream seperates and forms a skin. I don't know for sure but that would be my very uneducated guess!

[another_someone]

I assumed it was the milk itself - probably something to do with the denaturing of milk proteins.

On the other hand, I was not sure if the question referred to chocolate made with fresh milk, or premixed powder where you just add water.

Personally, I do not add milk in my cocoa at all (and don't like the premixes that already have powdered milk added) - but then, I don't let my cocoa get cold either.

[paul.fr]

It was frsh milk boiled in a pan, i agree that hot chocolate should not be left to cool..but sometimes it's just too hot to drink straight away.

[neilep]

i luff the skin on milk and cocoa and custard !!

[paul.fr]

i luff the skin on milk and cocoa and custard !!

I always have a spoon handy, to stir the skin in

[Karen W.]

I love cocoa and chocolate pudding!with real homade whipped cream...MMMMM Skin and all!

[another_someone]

http://news.bbc.co.uk/1/hi/health/6430777.stm

A nutrient in cocoa called epicatechin appears to lower the risk of four common killer diseases, work suggests.

Among the Kuna people of Panama, who can drink up to 40 cups of cocoa per week, rates of stroke, heart disease, cancer and diabetes are less than 10%.

The Kuna also appear to live longer than other Panama inhabitants and do not get dementia, a US scientist reports in Chemistry and Industry.

Experts stressed that genes and other lifestyle factors also play a part.

However, researcher Dr Norman Hollenberg, of Harvard Medical School, says the cocoa chemical would benefit other populations too, including the Western world, although he concedes there may be ethnic differences.

And he acknowledges his studies are based on observations, so cannot provide cast iron proof.

[Bored chemist]

If someone really proves that chocolate is good for you I might have to revise my policiy of atheism. []

[chris]

The "skin" that forms on milk and milk-containing hot drinks (such as hot chocolate) is caused by denaturation of the proteins (chiefly casein) in the milk.

Proteins are natural polymers formed by linking together building blocks called amino acids into a chain resembling beads on a string. And because there are 20 different amino acids used in proteins made in the body, and they are all chemically different, you can make these metaphorical necklaces with any combination of "beads" of different shapes and sizes and chemical capabilities (including their electrical charge) to achieve the desired protein function you need.

These amino-acid "beads" then all interact with each other so that the positively-charged ones try to get as close as they can to the negatively charged ones, and as far away as possible from each other. Also, the water-loving parts of the molecule try to get themselves towards the outside of the protein, and the water-hating (hydrophobic) regions try to tuck themselves away inside. This pulls the mature protein into a characteristic shape (three dimensional structure) which determines its properties.

However, changing the environment surrounding the protein can dramatically alter its shape. Adding acid, for example, will change the distribution of charges on the amino acid "beads", causing the protein to be pulled into a new shape. This new shape might encourage large numbers of the proteins to cease being soluble and stick together, or aggregate. You can see this happening naturally when milk is allowed to "go off" in the fridge. Bacteria turn lactose sugar in the milk into lactic acid (hence the word lactate for breast feeding), and this causes the proteins to denature and clump together.

Most proteins are also denatured at high temperatures (beyond those you would normally find in the body). This usually occurs at between 45-50°C. Under these circumstances the heat makes the molecule vibrate and shake-apart the interactions between the constituent amino acids. The molecule then settles into a new configuration that is more heat-stable.

Sometimes this process can be reversed by adding the right chemicals to break all of the bonds and interactions between the amino acids, and then returning the protein to physiological temperature and pH. It can then re-fold in the correct way. Usually, however, the changes are permanent.

So when you boil the milk for hot chocolate, or just hot milk, you are causing the denaturation of the soluble milk proteins. The denatured proteins then aggregate and form a sticky film across the top of the liquid, which dries by evaporation. The film in turn then acts like a miniature pressure cooker and encourages the liquid beneath itself to become even hotter and the pressure to rise. That's why the milk then spectacularly boils over all at once on the cooker or all over the inside of the microwave...

Chris

[Monox D. I-Fly]

i luff the skin on milk and cocoa and custard !!

Is it even safe to consume? When I was a teenager I tried to eat the milk skin and ended up getting dizzy immediately afterwards.

[syhprum]

I really hate the skin on boiled milk or blancqumonge having been force fed with them at the age of five in a church of England so called convalescent home.
I am sure that in Hell they will be the main meal