In French cooking, stocks are often reduced after the bones and aromatics are strained out, by cooking the liquid until evaporation has removed some of the water, leaving the flavor behind. This allows for a more concentrated flavor without drawing more of those scummy substances from the bones, leaving you with a flavorful, pristinely clear stock to turn into a demi-glace.
So how do you actually skim your stock?
An important note is that clarity and cloudiness aren’t always indicative of flavor: a translucent broth isn’t necessarily less flavorful than an opaque one and vice versa.
Say you’re makingpho gà. The clove- and ginger-scented result of hours of gentle simmering is always a wallop of flavor. And skimming is an important part of making the clear broth to float a nest of rice noodles and handful of fresh herbs. Maintaining a low simmer to avoid the vortex effect will prevent fat and proteins (“scum”) from working their way into the matrix of your broth; instead, they will float to the surface and be easily removed—by skimming, of course.
If you’ve decided skimming is a must for your recipe, there are a few ways to go about it. Erway prefers using an aptly named skimmer, a fine-mesh, flat, circular spoon of sorts, usually about four inches in diameter (“Run it along the edges of a pot, close to the surface, and it cleanly lifts off the scum/foam/impurities”). Another, slightly more precarious option is to run a ladle along the surface, attempting to scoop up the floating residue while leaving behind the clear broth.
Erway says, “All the scum should rise to the surface within the first 10 minutes of boiling your bones with water. After that, you can lower the heat to a simmer and sit back.”
Straining finished stocks through a fine-mesh sieve is good practice, and usually the best way to separate the spent bones and vegetables from the liquid. But straining is a dubious substitute for skimming. You risk clouding your stock by shaking loose unwanted particles, and waiting till the end also increases the risk of emulsifying fat and those pesky impurities into your stock as it cooks.
While you want to be proactive about getting the scum out of your stock early in the cooking process, dealing with the fat your bones put off is another thing altogether. Fat will naturally render out of bones as they cook and float to the surface of your broth, appearing in big, oil-slick droplets, or creating an entire surface layer, depending on the amount. Fat content will vary by the type of bones you’re cooking with—seafood generally has the least, beef and pork the most. If the recipe you’re making is best without a layer of fat on the surface, the most efficient way to remove it is to chill the stock after cooking. This will cause the fat to solidify on the surface, creating a “fat cap,” Mamane says, which can easily be lifted off. If you’re in a hurry, it is possible to skim fat off the surface of the stock with a ladle as it cooks, though you will lose a bit of stock in the process.
Ultimately, how you cook your stock depends on the results you’re after. If you’re putting together a risotto, or turning last night’s chicken carcass into the base for this afternoon’s tom kha, it’s likely you’re not looking to add extra steps along the way, and skimming won’t make or break your end result. But if you want to be able to see the pleats of your wontons through your broth, buy yourself a skimmer, and go that extra mile.
