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Culinary starches are used to thicken purees, broths, and sauces. The most common culinary starches are extracted from potatoes, cassava (the tuber that tapioca comes from), arrowroot, wheat, rice, and corn. All of them can be used as thickeners, although they vary in their ability to thicken as well as in other ways.
Like cornstarch, potato starch, which is sometimes known by its French name, fecula, makes translucent sauces. flour, the most common thickener in American kitchens, makes opaque sauces. both potato starch and cornstarch are twice as powerful as flour.
You can substitute cornstarch in recipes that call for potato starch and vice versa, but you should be aware of differences and make appropriate adjustments in your cooking technique.
For instance, potato starch gelatinizes at a lower temperature than either cornstarch or flour and also tends to break down at a lower temperature. Therefore, sauces thickened with potato starch should be watched carefully and removed from the heat as soon as they reach the proper consistency. In addition, potato starch does not have as much holding power as cornstarch, so sauces thickened with potato starch should be served promptly.
On the positive side, potato starch is flavorless, making it the choice of many chefs for delicate sauces. Also, since potato starch is tasteless, there is no need to "cook out: the raw flavor of the thickener, as you have to do with flour and even cornstarch. This means that potato starch can be added much later in the cooking process. If you substitute cornstarch for potato starch, you should allow more time for the sauce to thicken and for the raw flavor of the starch to fade.
Arrowroot, a root starch that comes from a West Indian plant, is more similar to potato starch and is probably a better substitute for it than cornstarch. Unfortunately, arrowroot is not as easily available as cornstarch. You might find it health food stores since it is a gluten-free product.
Cook's Illustrated January/February 1995 Submitted By DIANE LAZARUS On 12-21-94