Why skim "scum" from the surface of a simmering stock?
I've been reading various Googled recipes and techniques for stockmaking, as I made my first stock tonight using raw chicken bones. Just about every article/recipe I read says to skim the surface of the stock in the beginning, while it is simmering. Different articles variously refer to the skimmed substances as "scum", "impurities", and "proteins".
I had started out throwing all the vegetables in at the beginning, and probably had it at too high of a simmer, so I never actually got to see any foam or collections of anything other than apparently oils/fats from the chicken appear on the surface. This got me wondering: what is that stuff that floats to the surface? Is there any reason other than aesthetics to remove it from the stock?
You shouldn't boil the stock, I think. It leads to unclear stock, and probably has unpleasant effect on the taste aswell, though I can't say for sure (since I haven't tried). I've read a lot about stock, and that seems the consensus atleast.
Skimming is for aesthetic purposes.
The scum is denatured protein, mostly comprising the same proteins that make up egg whites. It is harmless and flavorless, but visually unappealing. Eventually, the foam will break up into microscopic particles and disperse into your stock, leaving it grayish and cloudy. The more vigorously your stock bubbles, the faster this process will occur.
If the grayness or cloudiness bothers you but skimming is not an option for some reason, you can always remove the micro-particulates later through the clarification process used to make consomme.
Maybe I've been wrong about this / believing in a kitchen myth for a very long time, but I'm pretty sure that it's *fat* the floats to the top. Denatured protein should dissolve *more* easily - as in the gelatin itself - not render to the top?
@aaronut: fats DO float to the top.... but it seems that proteins do as well. I had plenty of fat floating to the top of my stock, but no foam formed out of the fat.
@Bruce: thanks for the answer. I'll leave it open for a few more days to see if there are any other responses, but it looks like you've done a thorough job responding. Personally I am perfectly happy getting a little more protein in my diet :)
@Aaronut: you're absolutely right that fat does float, but so does foam stabilized in a protein matrix. As for denatured proteins dissolving, true dissolution doesn't happen at the macromolecular level. When most proteins denature, the molecules cross-link to form a webbing. As more proteins denature, they build onto the webbing. Once created, the webbing is very stable; the only way to get it to dissolve is break it down into single molecules again. For that, you need something like an enzyme, very high heat, or an awful lot of mechanical pulverization. (continued)