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.

Should you skim fat off soup?

It's necessary to skim the fat as you boil down stock to preserve the integrity of the flavors. ... If it's gelled into a solid, you can scoop out the stock with a spoon. Image via imgur.com. The added benefit is now you have some useful fat to keep for cooking, much the same way you would bacon grease.

Do you have to skim chicken soup?

The vegetables from cooking the broth are tasty but not bright and fresh enough to be appetizing in soup. Strain them out before proceeding with fresh ingredients. ... But skimming is not really necessary to remove the inevitable bits of coagulated protein and collagen that form (as they do in any meat soup).