why do many recipes for cooking beans say to skim foam off the top? what's in the foam and how might it affect the taste of beans?
The foam is comprised of water-soluble proteins that will eventually be absorbed back into the cooking liquid if not skimmed. I've heard two reasons for skimming the foam, though I wouldn't be able to cite proof for either...
1. The foam tastes bitter.
2. The proteins in the foam cause a lot of the gas associated with digesting beans.
On the other hand, if you cook dried beans in a pressure cooker, whatever foam may have been there has disappeared when the beans are finished cooking. I don't see evidence of bitter taste or additional gas. So the re-absorption of the foam shouldn't cause a problem. Another possible reason may be aesthetic; I think the liquid may be more cloudy -- and that shouldn't be a problem either.
thanks! i had always scooped off the foam, but then my boyfriend's mom taught me to make their family's recipe for cuban black beans. she cooked the beans in the same water they were soaked in and didn't skim off any foam. got me wondering about what was actually going on with the foam!