That bubbly foam at the top of your stock is commonly referred to as "soup scum," and the name alone is enough to make us want to get rid of it. It isn't going to impact the soup's flavor ...

Like many a simple soup, its success depends on a really good ... pints water in a large pan and bring to the boil, skimming any scum from the surface. Leave to simmer very gently for 2 hours ...