Thickening Soups

The most classic of soup thickeners is roux, a combination of fat and flour. Often, using roux to thicken a soup simply consists of adding a bit of flour to the pot as the aromatics saute in butter or oil. If an especially thick consistency is needed, equal portions of fat and flour are added. Roux is prevalent as a thickener because it not only thickens, but stabilizes. If cream or cheese is being added to a soup, a bit of roux can insure it won't "break," or separate. Taste is also a consideration. The raw flour taste must be cooked out of the roux, until it adds a distinctively nutty taste to the soup base.

If the added fat or flour from a roux isn't wanted, a pure starch, most commonly cornstarch can be used. Typically, cornstarch is added to a small amount of cold water or other liquid (wine or stock) and whisked into a thick slurry. This slurry is stirred into the soup at the end to set the final consistency of the soup. Cornstarch adds a less distinctive flavor than roux, and doesn't offer the same protection against "breaking," but it is more convenient. Just remember, after you add some of the slurry, let the soup return to a simmer--cornstarch is a very effective thickener, and a little bit can go a long way.

What to do with leftover rice or potatoes? Mash or puree, then add to a soup for more body. Often, rice or potatoes are cooked and mashed just for this purpose. One great benefit is you can add new flavors to the rice or potatoes as you mash them. Extra tarragon for a fresh summer soup, garlic, or even lemon zest can add an unexpected flavor to the soup base, just by adding them to this puree.

A great trick to thickening a soup just the right amount, all while intensifying flavor, is to use parts of the soup itself as the thickener. Simply remove a portion of the soup solids--the aromatics, starches, even the meat--and puree, adding just enough of the soup stock to create the proper consistency. Use a blender, food processor, or immersion blender for this task. Puree with care if using a blender--the hot soup solids can actually spin out of the blender while blending and burn the skin quite badly. Use the blender's lid, held down with a thick towel, and keep the lid on for several seconds after the blender is turned off. An air bubble often forms by the spinning blades, and can shower the hot liquid even after the blending is done.