When I asked people to complete the phrase, “I wish The Food Geek would ______”, Julie responded with:
Like rebel scum to the Empire, the scum in stock threaten to ruin all of your plans. Well, presuming your plans are to have a clear broth or stock. If you plans are to take over the universe, and a clear stock or broth aren’t part of it, then stock scum is not going to bother you at all.
But what is it? Protein. Specifically, water-soluable protein in the meat. When you are making a broth or stock, your goal is to extract flavor and/or collagen from the ingredients in the water. The water, heat, and long cooking time do their best to take everything that can be extracted by this method and extract it.
Unfortunately, there are always uninvited guests to any party that is big enough and worth going to, and this party is definitely worth crashing. Because animals are complex, they have all sorts of different proteins going on, and some of those proteins are water soluble. Many of these proteins are likely supposed to be transported to various parts of the body via the blood stream or need to be put through membranes like cell walls, so they probably have a good reason to be soluble. It’s just a little inconvenient for the cook.
On the plus side, with care, these proteins can be easily dealt with. If you leave the lid off of the stock pot and keep from boiling the water, your proteins will be encouraged to coagulate and rise to the surface, where they’ll dry out and become scum. From there you can skim them off.
If you boil the stock, then the proteins will break up into smaller pieces, where they will be suspended in the stock. This is not disastrous, because the stock is still a perfectly lovely and usable foodstuff. However, it is technically disastrous, because stocks are “supposed” to be clear. A clear stock is a pretty stock, and you would get thrown out of your French restaurant kitchen for having anything less than clear.
There are ways of clarifying stock even if your scum has broken up into small particles suspended throughout the whole batch, though there are costs. Jeff Potter describes in his new book, Cooking for Geeks, a method of freezing a stock, which causes its gelatin to congeal and capture the impurities. You then place the frozen stock into a filtering bag where it can thaw, with something to catch the stock. The gelatin will remain in the filter, and the stock will be very clear. The downside to this method is that you also lose the gelatin, which is one of the reasons to make a stock. It also won’t work with anything that is low on gelatin.
The traditional method of collecting the impurities in a stock is to make a raft out of egg whites. You lightly beat some egg whites and stir it into the the stock while it’s simmering. Let it rest, and the egg whites will cook, float to the top, and collect impurities along the way. The various proteins will stick to the egg whites and collect them while the stock cooks.
Hardcore traditionalists will also add eggshell to the raft. I haven’t found a good reason for this, and I know plenty of folk who skip this ingredient. Maybe it’s to keep people from eating the raft when it’s done cooking.
In any case, your best bet with handling stock is to cook it slowly and never above a simmer. Don’t stir it while it’s cooking. If you need it to be super-clear, then you can use one of a number of tricks to clarify it. You don’t need stock scum to ruin your day.