Skin on black bean soup Sometimes when I make black bean soup it forms a skin. Why is this?

Ingredients are: beans, olive oil, salt pepper, garlic, cayenne pepper, and cumin.
Whenever beans are cooked, they release a good bit of starch into the cooking liquid. 

The longer you cook the beans, the more starch they release. Boiling, breaking or otherwise roughing up the beans will release more starch. 

Also, canned beans release a good bit of their starch into the water in the can. 

If you have more starch, you'll probably get more of a skin. 

Puddings (also liquid + starch) form a skin the same way your soup does.