Cake flour can be substituted with flour mixed with a small amount of cornstarch, or with sifted all-purpose flour in some recipes. The addition of cornstarch inhibits the formation of some of the gluten in the cake, which makes the cake tougher.
Since cake flour is a finely milled, very low protein flour, it is best for fluffy cakes that are high in sugar. Cake flour is bleached, a process that helps by repelling fluid, binding fats and stabilizing the gas bubbles from the leavening agents such as baking soda or baking powder. Due to its composition, in recipes where raising agents are used, cake flour can be replaced with sifted all-purpose flour, bleached or unbleached.
In some recipes, cornstarch can be added to the all-purpose flour to reduce the protein content. In this case, the cake flour can be substituted with 2 tablespoons of cornstarch for every 14 tablespoons of flour. In other words, cooks can replace 1 cup of flour with 2 tablespoons of cornstarch in some recipes. Sifting the all-purpose flour and cornstarch thoroughly helps to create a fluffier cake. The substitutions will likely yield a less tender version of the cake recipe that is made with cake flour.