A Starch Primer

• Starches, such as flour and cornstarch, thicken soups, sauces and puddings by absorbing liquid. The first step is always to combine a liquid and a starch together until thoroughly blended. The secret ingredient, though, is heat. When the mixture is heated, the molecules in it begin moving vigorously. This action disrupts the starch molecules so they take in liquid, swelling up and thickening the dish. Each starch thickens slightly differently depending on its molecular composition. Cornstarch thickens almost immediately when exposed to heat, but can break down with freezing or long cooking. Flour needs more time to thicken but remains more stable.

Cornstarch vs. Flour

• Most soup and stew recipes call for flour as the thickener, as flour stands up better to long cooking than cornstarch. Flour also imparts a rich, grainy taste that adds flavor to savory stews. It also gives a thick, hearty appearance to soups thickened with it, while cornstarch creates a glossy, translucent look.

Tips and Techniques

• The classic way to thicken a soup is by making a roux. Melt butter, oil or fat in a large skillet or saucepan. Whisk in some flour and cook over medium heat for several minutes, stirring constantly. Slowly pour in hot liquid, such as broth, cream or milk. Stir the soup frequently and bring it to a simmer. Another option is to stir a beurre manie -- which is simply a paste made from equal parts butter and flour -- into the soup near the end of the cooking time. If you prefer cornstarch for its neutral flavor, combine cornstarch with some cold water to make a slurry. If you add the cornstarch directly to hot soup, it will create lumps. Stir the slurry into the soup just before serving and heat to a simmer, stirring constantly. Do not vigorously boil or freeze soup thickened with cornstarch.

Alternative Additions

• Options abound for thickening agents. Arrowroot is similar to cornstarch in that it creates a thick, glossy appearance with no taste. It can tolerate higher heat, longer cooking and freezing. Tapioca works, as well, but use tapioca starch or grind tapioca pearls in the blender first. Tapioca pearls won't dissolve completely and you'll be left with little gelatinous balls. To thicken cream soups without the added fat found in a roux, add rice or potatoes to the soup, along with any vegetables. Cook the soup until the ingredients are tender and allow it to cool slightly. The starch found in the potatoes and rice thicken the soup. If you like, puree the finished soup in a blender.