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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?

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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. – Max Jan 13 '12 at 12:48
@Max - yes, I think you're right on that :) – Jonathan Jan 13 '12 at 20:35
@Max particularly with aromatics - I think boiling may extract more of the unpleasant flavors, much as with tea. I'm learning a lot by trial and error, mostly error! – Jonathan Jan 13 '12 at 20:41
up vote 18 down vote accepted

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.

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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 Jan 12 '12 at 22:34
@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. – Jonathan Jan 13 '12 at 3:39
@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 :) – Jonathan Jan 13 '12 at 3:40
@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) – Bruce Goldstein Jan 13 '12 at 3:47
Gelatin is one of the few proteins that truly dissolves without forming that webbing. There are others, but it is not the rule. (Ever see a steak dissolve? <wink>) – Bruce Goldstein Jan 13 '12 at 3:48

While skimming helps prevent a cloudy stock, I've found it unnecessary if the stock simmers very gently - like in a slow cooker, or overnight in a slow oven.

Some recipes suggest parboiling the bones and discarding the liquid, with the same goal in mind - to keep impurities from clouding the results.

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Found these responses interesting. Here's what Sally Fallon Morell has to say:

Scum will rise to the surface. This is a different kind of colloid, one in which larger molecules–impurities, alkaloids, large proteins called lectins–are distributed through a liquid. One of the basic principles of the culinary art is that this effluvium should be carefully removed with a spoon. Otherwise the broth will be ruined by strange flavors.

This is from the Broth is Beautiful link on the Weston A Price website.

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I have never skimmed my broths and they are always amazing

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It'd be helpful if you were a little more direct, but I guess you're saying there's no reason at all to do it, which seems a fair answer. – Jefromi Feb 18 '15 at 16:00

My grandmother and mom ALWAYS skimmed the soup when it came to a simmer, never letting it boil until the very last bit of it was gone. As a result, the broth, while yellow in color (Gramma used cleaned chicken feet, as well) was crystal clear. Mom maintained it was the albumin that rose to the top. Not sure if she was correct, but it's something I do, as well, because if it was good enough for Gramma and Mom....

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Firstly, I agree that's for aesthetic purposes, many Cantonese stews are very clear when served.

Secondly, some people think it influences the flavor. I think it might be related to the slaughter method. For Halal meat, almost all the blood is drained, so it doesn't influence the taste. But usually, it's not completely drained.

And I think if the myoglobin is not boiled, like the juice in medium steak, it's very juicy. But if it's boiled for a long time, it tastes less tasty.

I think for chicken and beef, the difference is very small, especially when you use a slow cooker and your chicken is grass-fed. But for pork, some people think the odor of pork is stronger, maybe because of boar taint, hence you will see them skim pork ribs when they make rib stew.

Lastly, you can scoop the fat.

Update: I found a thesis trying to explain this:

Cause and Prevention of Liver Off-flavor in Five Beef Chuck Muscles

It said "residual blood hemoglobin is known to contribute to liver off-flavor development".

So I guess some people are sensitive to this smell.

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The "impurities" are just protein or some fats, all very edible. We've never skimmed; just stirred it all back in, and the soups my family makes are always delicious, very tasty, and quite nourishing. It bothers me that every recipe I've seen online always says to skim off any foam, but they never really say why. Break out of the box and just enjoy the soup/broth you've created!

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I've edited this to remove the more confrontational parts - the OP asked in a pretty neutral way, and just quoted other people calling them impurities. No one here is trying to perpetuate a myth about impure stock, don't worry. – Jefromi May 14 '14 at 22:04

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