Why skim "scum" from the surface of a simmering stock?

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.

Removing the scum makes it easier to control the temperature of the stock so you can maintain a constant simmer. If you don't skim it off, the scum aggregates in a foamy layer on the surface, which acts as insulation. It traps more heat in the stock and can cause your stock to boil when it would otherwise be simmering. Also, since stock often sits unattended on the stove while simmering, un-skimmed stock presents a risk of boil-over.

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.

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.

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!

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.

My mom skimmed the stock. I suspect that not skimming leaves a residual and very subtle bitterness. Have made soup both ways. Depends on the soup. Would skim for egg lemon soup. Not for a hearty chicken soup that comes close to a stew.

There is a short cut with chicken with the skin left on. After cooling the broth, put it in the fridge in a covered bowl overnight. In the morning, there will be a layer of semi concealed fat (and scum) on the surface. Easy to scrape off. Then make soup with the broth.

I have never skimmed my broths and they are always amazing

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....

There are two answers:

1. If you are boiling meat, the scum is most likely animal fat. If you leave the scum in and just mix it together, it will add to the flavor. Though there are reasons to still remove the scum. One is that you might be trying to make a leaner more meaty flavored stock. Another reason is that pesticides in the animal's food collect in the fat cells. You probably won't taste it, but if you're trying to go organic, you might want to dispose of this rather than consuming it yourself.

2. If you are boiling vegetables, the scum will include potassium hydroxide leaching out from the vegetable matter. Potassium hydroxide, or lye, is a basic solution that will taste bitter, though won't harm you in such minute doses. A typical westerner raised on a western diet has a dulled sense of taste and probably won't notice the bitter, though a person from a different food culture will and as such might have a custom of skimming the scum even from boiled vegetable stocks and soups.

I was taught that the scum contains "impurities."

Not to be vulgar, but this would include insect parts--including tiny eggs burst by the cooking process--bits of dirt/dust that inevitably get stuck in crevices of food, floaters or all sorts, rotten bits that can't be seen, and other nasties.

SKIM THAT ^&*%ING SCUM! my teacher would bellow XD