what *is* that foamy scum on chicken soup?

Question

the foam/scum that rises to the top when you make meat broth -- first feathery, then white and foamy -- what is it? where does it come from? why does it have to be skimmed off?

Answer

Pros skim so your soup looks clear and 'professional'. The extreme case of scumming is a consomme where you try to get every bit of particulate removed so you have a crystal clear broth.

Answer

The scum is not just protein. The scum floats; protein is more dense than water and would sink. The floating stuff is coagulated (denatured) lipoprotein, the same "L" that is in the terms HDL and LDL (from your cholesterol workup). 
Protein combined with lipid (fat) is less dense than water (the "D" in HDL and LDL refers to the density). When boiled, these lipoproteins coagulate and float. The scum tastes just fine to me, it's laughable to call it "impurities." All cells have lipoproteins in their cell membranes. 

However, leaving them in will cause the stock to be irreparably cloudy in the end. If the goal is a stew, then who cares if the stock is cloudy. If the goal is consomme, then skim away. 

I think there is a "Chinese cream stock" in which the items (pork, duck, chicken) are deliberately cooked at a rolling boil so as to incorporate all the flavors into the liquid. The result is quite creamy-looking. In this method the lipoproteins are physically forced into a colloidal suspension.

Answer

I found that it has to do with the quality or the way in which the chicken was raised. In my earlier years I never experienced that white scum when boiling my chicken to make chicken soup. I have been having to skim it these days to remove that yucky stuff. But than I bought a chicken from Winco that sells a brand of chicken that is free range and I don't get that problem with their whole chicken, broth was beautiful with nice oil and clear as can be. So it's more about how it is raised and fed.

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what is that foamy scum on chicken soup?

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