Corn starch is often used to thicken pie fillings, gravies and puddings. It blends smoothly with cold water, has no taste and gives a shiny appearance to foods. It is also makes a thin, crispy golden coating on breaded, fried meats. Baking powder has an entirely different chemical makeup and purpose, and the two are not interchangeable.
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Corn starch is a natural carbohydrate found in the corn kernel. It is gluten-free and is occasionally used as a flour, although its most common purpose is thickening. Corn starch can also be used to make laundry starch or to treat sunburn or rashes. Baking powder is a chemical leavener made of two acids, usually cream of tartar and sodium aluminum sulfate. Mix it with a liquid and it creates carbon dioxide, which causes bubbles to form in baked goods.
Baking powder can stand in for baking soda in some recipes, but it doesn't have the thickening power of corn starch and should not be used as a substitute. Baking powder's chief attribute is its ability to make baked goods light and fluffy. In a pie filling or pudding, it might produce air bubbles, but it would not thicken the food properly.
In a pinch, substitute arrowroot, granular tapioca or flour for corn starch. Substitute an equal amount of arrowroot, but double the amount of tapioca or flour required. Keep in mind that these ingredients alter a dish slightly and may have different cooking requirements, as well. Flour, for example, does not blend as smoothly in cold water as corn starch. It is often added to butter to form a roux before adding liquids. It also has a raw flour taste if not cooked thoroughly.
Substitute tapioca for corn starch in puddings and unbaked pie fillings, but use flour, arrowroot or tapioca in baked pie fillings. When using corn starch, avoid over stirring it or cooking it too long, which can cause it to break down and thin out. Cook it over medium heat until it comes to a boil. Boil it for one minute and remove it from the heat.