Refined starches offer a quick and easy way to bring your soups or sauces to a silky-smooth texture with just the right degree of thickness. The usual choice in American households is cornstarch, which is a more powerful thickener than flour and is also gluten-free. However, other starches -- such as potato starch, a staple in European households -- are equally useful.
A Practical Substitution
Potato starch is used in much the same way as cornstarch. Whisk it first into a quantity of cold water or broth, then pour the resulting "slurry" into your hot liquids in a thin, steady stream. As you stir the starch into your soup or sauce, its molecules will absorb moisture, burst and create a tangled web of strand-like starch fragments that immobilize moisture like a sponge. Potato starch has an especially strong thickening effect when it's first added, then will slowly relax and loosen. If you over-thicken your soup, a few minutes' waiting and stirring will usually fix the problem.
Compare and Contrast
Although they have similar uses, liquids thickened with potato starch or corn starch behave a bit differently. Those thickened with cornstarch tend to set to a gel-like consistency, while potato-starch sauces remain pourable. When frozen and thawed, cornstarch "weeps" liquid and gains an uneven consistency. Potato starch thins when thawed, but otherwise retains its texture. Potato starch also thickens at a slightly lower temperature than cornstarch, so it's useful as a last-minute addition to sauces that have been inadequately thickened by other means.