Skimming Stocks

Saad_94717

I have seen at least 3 dozen stock making videos, and in almost all of them it is recommended that one remove the scum from the top. 

Can you please tell me: 

• The reason for doing so?

• What is the scum made up of? 
• What are the differences in flavour and/or aroma of a skimmed stock versus an unskimmed stock?

Thank you.

tshewman

Proteins fats and the all-telling obscure impurities ;-) make up the scum. If not skimmed (or you can blanch the bones to lessen skimming somewhat), the stock may become cloudy and of course when warmed you may end up with the floating scum in your sauce or soup (which some people may not enjoy). If not skimmed, the assertion is the foam (this goes for demi-glaze sauces etc as well) may burn (cook further) on the side of the pot and add an unpleasant flavor.