'Skin' on hot chocolate

[blurby]

For years I've made hot chocolate with the basic components of cocoa powder, mlik, and sugar with variations on that base. I've tried milk powder, skim, 1%, 2% etc. depending on what I have in the fridge.

For all the permutations as the cocoa cools it forms a 'skin' on its surface. Generally it forms once, you scoop it off, and it never really reappears. I'm guessing this is something due to the protein in the milk. Is there a technique that eliminates the formation of this? It doesn't affect the task but it can sure supprise you when you take a sip and this mess gets stuck on your lip...

[nduran]

I think stirring it while it cools is about all you can do.

Google turned up this

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...

[mkayahara]

In On Food and Cooking, Harold McGee says that this kind of skin is caused by water evaporating from the surface and the "progressive concentration of proteins there." He adds that you can minimize it by covering the pot or whipping up some foam, to minimize evaporation. I haven't tried it myself, so I can't vouch for this approach!