Cornstarch is a fine, white powder not unlike flour. By definition, flour is technically any powder made from grains, seeds, or nuts. However, in the United States, flour commonly denotes flour made from wheat. In this article, the flour will always refer to wheat flour.
Like flour, cornstarch can be used in thickening sauces, deep-frying, and baking. However, how much to use, how to use it, and its effects can differ greatly. Flour is the main ingredient in baked goods, for example, while cornstarch is rarely on the ingredient list for cookies and cakes, except in certain shortbread recipes. This is because cornstarch is composed only of starch, while all-purpose flour contains starch, fiber, and protein. One of these proteins is gluten, which helps give bread its elasticity and chew. Because cornstarch doesn't contain gluten, it is gluten free and safe for people with celiac disease.
(Note that in the U.K., cornstarch is called cornflour, not to be confused with corn flour—finely ground cornmeal—that is used to make corn tortillas and commonly found in the Southern United States.)
Best Cornstarch Substitutes
Sauce Thickener: All-purpose flour, rice flour
Deep-Frying: Rice flour, potato starch
Pudding: Tapioca starch
Fruit Pie Filling: Tapioca starch, arrowroot
Baked Goods: Rice flour
Thickening Sauces With Cornstarch vs. Flour
All starches, including cornstarch and flour, have thickening properties. They act like a sponge, absorbing liquid and expanding. They then gelatinize, firming up when they cool down. Because cornstarch is pure starch, it has twice the thickening power of flour, which is only part starch. Thus, twice as much flour is needed to achieve the same thickening as cornstarch.
To thicken sauces, cornstarch is combined with cold water first, which is called a slurry. Flour, on the other hand, is cooked with fat and made into a roux. A roux is made at the beginning of a recipe while a slurry is added toward the end. This is because the flour needs to be cooked longer to remove the floury taste; cornstarch needs a shorter cooking time and a higher heat to activate the thickening properties. Cornstarch should not be cooked into a roux.
Using flour as a thickener will make the sauce opaque and cloudy while cornstarch leaves a shiny, more translucent finish. This is a desirable feature for fruit pie fillings and certain sauces, especially in Chinese stir-fries.
If a sauce is quite acidic, cornstarch won't thicken it quite as well. This goes for sauces with tomatoes, citrus, or vinegar, or one made with eggs yolks or butter. If you use too much, a recipe including cornstarch becomes spongy when refrigerated or frozen.
Frying With Cornstarch vs. Flour
Both flour and cornstarch are used to coat chicken, fish, and vegetables before deep-frying. Cornstarch typically makes for a crispier finish than flour.
Cornstarch absorbs moisture from the food and expands, giving deep-fried foods a crispy coating. When the food is fried, the moisture cooks out of the cornstarch, leaving a crackly, puffy coating on the outside. (This is why most Asian fried recipes call for using cornstarch instead of flour.) Flour will achieve this to some extent, but since more flour is needed to achieve the same effect, the coating will be thicker and chewier. Recipes touting the crispiest chicken will often call for a 50/50 mix of flour and cornstarch to create the perfect balance and a beautiful brown exterior.
Baking With Cornstarch vs. Flour
Baking is where a complete swap won't work. A cake recipe, for example, will not turn out if cornstarch is used in the place of flour. Baked goods rely on the proteins in the flour (gluten) to help create its body and texture. There are many excellent gluten-free baking recipes; just note that cornstarch isn't the only flour or starch in most of them.
Adding cornstarch to a cookie or cake recipe, however, will help to make the finished product more tender. In fact, this homemade recipe for cake flour replaces 2 tablespoons of all-purpose flour with cornstarch.
Other Cornstarch Substitutes
Flour is the most common substitute for cornstarch. However, arrowroot, potato starch, tapioca starch, and rice flour can work just as well, if not better at times. Additionally, all of these cornstarch substitutes are gluten free.
This powdered white thickening agent acts similarly to cornstarch with a few exceptions. It is not good to use in dairy-based recipes as it will cause the dish to become slimy, but it is a good substitute for cooked dishes that plan to be frozen. Take note that recipes using arrowroot may need to be cooked a little longer to reach the desired thickness. Use the same amount of arrowroot as cornstarch called for in the recipe.
A popular ingredient in Passover recipes, potato starch is also an excellent replacement for cornstarch. An equal amount, or maybe a little more, should be used in place of the cornstarch as a thickening agent. It is best to eat the finished dish as soon as possible because this starch doesn't hold together well with the rest of the ingredients.
If making a pie, tapioca starch is an ideal alternative as it creates an even shiner appearance than cornstarch. For the best results, the tapioca should be mixed into the ingredients and left to sit for about 10 minutes before cooking to allow the tapioca to absorb the liquids and thicken properly. Use an equal amount of instant tapioca but avoid adding to dairy-based sauces. It is a good choice for recipes cooked at high temperatures and foods that will be frozen.
White rice flour is the ideal choice for thickening sauces since brown rice flour can be gritty. Similar to flour, double the amount of rice flour should be used in place of cornstarch. This finely ground powder made from rice is colorless when mixed with water and thus a good choice when thickening clear liquids. Rice flour is a popular ingredient in Asian cooking and can be added directly to a recipe without making a slurry. It can also be used to make a roux using oil.