No kitchen is complete without a stock of all-purpose flour, which is one of the most frequently used ingredients in cooking and baking. As the name suggests, all-purpose flour is a versatile option that can be successfully used for baking breads, cakes, pancakes, cupcakes, muffins, cookies, etc. When in doubt, use all-purpose flour.
Unlike all-purpose flour that can be used in nearly any circumstances, cake flour is best used exclusively for cakes or cupcakes. This type of flour gives your finished product an airy and almost sponge-like quality, which is much lighter than the dense all-purpose flour. The flour itself is more finely milled and delicate than all-purpose flour.
All-purpose flour and cake flour are both made from wheat, but the type of wheat, when it's harvested and how it's milled makes them different. But the most important difference comes back to the flour's protein.
A typical bag of all-purpose flour has a protein content between 10 and 12 percent, but this varies by brand. Cake flour has significantly less protein than all-purpose flour, generally between 7 and 8 percent. The flour's protein directly impacts the texture of the food you're baking.
Cake flour's lack of protein produces less gluten, which makes the finished baked good more light and airy. All-purpose flour's high protein content produces a finished product that is more dense and chewy.
If you're making a cake and find yourself lacking cake flour, using all-purpose flour interchangeably won't hinder your recipe. Substituting cake flour for all-purpose flour will produce a denser cake, so it's a matter of personal preference.
Typically, cake flour cannot be a good substitute for all-purpose flour because of its low protein content. If you try to use cake flour in place of all-purpose flour when baking bread or pancakes, for example, they will not rise and maintain their proper shape if you use cake flour.
However, if you want to bake a light and airy cake, you can make a cake flour substitute by adding 3/4 cups all-purpose flour plus 2 tablespoons cornstarch for every cup of cake flour the recipe calls for.