Published January 1, 2004. From Cook's Illustrated.
Excess fat ruins stocks and sauces. But do fat separators really work? We tested eight models—from pitchers to ladles to fat mops—to find out.
Liquid fat must be separated from the drippings of a roast before making gravy or from a pot of hot stock before turning it into soup. Luckily, separating liquid fat is easy to do with a specially designed fat separator, aka gravy strainer or soup strainer. Three formats dominate the category: pitcher-type measuring cups with sharply angled spouts opening out from the base of the cup; ladles with slots around the perimeter; and "fat mops," brushes with long, soft bristles made from plastic fibers. Such extreme design differences raise the obvious question—which kind works best?
No matter what the specific design, fat separators work for two reasons. One, because fat and water do not mix—their incompatible molecular structures and electrical charges keep them apart—and two, because fat always rises above the liquid in the container—fat is less dense than water.
Five of the eight models we tested were essentially pitchers with the pouring spout set into their base, like the small watering cans used to reach houseplants in high spots. When the liquid settles in the container, you can pour it off, stopping just before the layer of fat floating on the liquid's surface reaches the opening for the spout. While all of the pitcher-type separators worked well, one feature that proved especially important was capacity. In general, we like large separators—usually around four cups—best. Large separators also have large mouths, which make for easier pouring when you're adding the stock to the separator. We also found an integrated strainer to be helpful when you're defatting pan drippings that are still mixed with chunks of aromatic vegetables, herb sprigs, or other flavorings. In terms of materials, the shock resistance of plastic is better suited to this tool than glass. During testing, one of the separators slipped out of our hands (which had gotten greasy from the fat) and fell to the floor. Had it been the glass model, we would have had to run out to buy a replacement.
Fat-separating ladles work when dipped just below the surface layer of fat that has accumulated atop the slightly cooled liquid. A series of slot-shaped holes along one side of the ladle allow fat to drain into the bowl of the ladle so it can be discarded. This procedure is repeated until as much fat as possible has been removed. Our testers found this to be a tedious process requiring fine control of the ladle. Frankly, skimming the surface fat with a wide, shallow spoon is just as effective and less frustrating.
Surprisingly, the cheesiest tool in the group, the Fat Mop, turned out to be pretty interesting. The mop head is made of plastic fibers that attract fat. As the packaging says, it is designed to defat stews, gravies, soups, chilis, and fried foods—items for which it would be impossible to use another kind of fat separator—as it sweeps across the surface to wick away fat. In our tests, it did in fact prove effective with chunky tomato sauce and pot-au-feu. Strictly speaking, however, the Fat Mop is not intended for use with large amounts of liquid. So, if we could have just one fat separator in the kitchen, our favorite version of the common pitcher-type would be our choice.