A Practical Substitution
Potato starch is used in the same way as cornstarch. Whisk it first into a little bit of cold water or broth, then pour the resulting slurry into the simmering soup or sauce in a thin, steady stream. Potato starch has an especially strong thickening effect when it's first added, then it slowly relaxes and loosens. If you over-thicken the soup or sauce, stirring it for a few minutes usually fixes the problem.
Although they have similar uses, potato starch and cornstarch may behave slightly differently from one another. Soups and sauces thickened with cornstarch tend to set to a gel-like consistency, while those thickened with potato starch 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 if a soup or sauce hasn't been adequately thickened.