#a mixture of protein and fat
It will be a mixture of protein and fat. You should skim it off to get a clearer broth that will taste less greasy. The best way is to keep the stockpot half closed/half on the burner so the scum collects on the cooler side and is easier to skim off. Thank you for your inquiry.
#estheticpurpose
Skimming is for aesthetic purposes.
Scum is denatured protein and contains mostly the same proteins as protein. It's harmless and odorless, but visually unappealing. Eventually, the foam will break down into tiny particles and disperse into your soup, making it gray and cloudy. The more intense the stock bubble, the faster this process occurs.
If the gray or cloudiness bothers you but you can't skim it off for some reason, you can later remove the particles through the clarification process used to make consommé.
# deodorize
Removing the scum makes it easier to control the temperature of the stock so you can maintain a consistent simmer. If the scum is not skimmed off, it will collect in the foam layer on the surface and act as insulation. It will retain more heat in the soup and may cause the soup to boil over when boiling. In addition, since the stock is often left unattended on the stove while cooking, there is a risk that the non-skimmed stock will boil over.
#Control the temperature of the soup
The scum will rise to the surface. This is a different type of colloid in which larger molecules (impurities, alkaloids, large proteins called lectins) are distributed in a liquid. One of the fundamental principles of culinary art is that this foul odor should be carefully removed with a spoon. Otherwise the soup will be ruined by a strange taste.
#slowcook is not necessary
While skimming helps prevent the soup from being cloudy, I find it's not necessary if the soup is being cooked very gently (such as in a slow cooker, or overnight in a slow oven).
Some recipes recommend parboiling the bones and discarding the liquid, for the same purpose - to prevent impurities from affecting the result.
#Stir the impurities back into the soup
The "impurities" are just protein or some fat, both of which are very edible. We never skim; just stir it all back into the soup and the soup my family makes is always delicious, very flavorful, and very nutritious. What bothers me is that every recipe I see online always says to skim the foam, but they never really explain why. Open the box and enjoy your soup/broth!
#meatliversmell
First, I agree this is for aesthetic purposes, many Cantonese stews are served very clearly.
Secondly, some people think it affects the flavor. I think it may have something to do with the slaughtering method. With halal meat, almost all the blood is drained so it doesn't affect the taste. But usually, it's not completely drained.
I think if the myoglobin wasn't boiled, like the juices in a medium steak, it would be very juicy. But if it is cooked for a long time, the taste will not be so delicious.
I think with chicken and beef, the difference is very small, especially if you're using a slow cooker and your chicken is grass-fed. But with pork, some people think the smell of pork is stronger, probably because of the boar smell, so you'll see them skimming the ribs when they stew them.
Finally, you can scoop out the grease.
Update: I found a paper that attempts to explain this: Causes and Prevention of Liver Odor in Five Beef Stuffed Beef , which says "Residual blood hemoglobin is known to contribute to the development of liver odor." So I guess some people are sensitive to this smell.
