Cake flour is a specialty flour that is lower in protein and very finely ground. The flour produces cakes and other baked goods with a finer, softer texture than all-purpose flour.
In Shirley Corriher's book BakeWise: The Hows and Whys of Successful Baking with Over 200 Magnificent Recipes she states that cake flour is ground much finer and bleached by chlorination. She explains, "Fat adheres to chlorinated starch, producing a better-combined batter -- more uniform dispersion of the fat and air bubbles." Because cake flour and the egg yolks permit the batter to hold more liquid, more sugar can be used in the batter. To create a moist, sweet, and level cake, she claims cake flour is a necessity. She includes a lot of math and ratios, and it's all fascinating. This is an excellent book for anyone serious about baking.
Cake flour is also lighter in weight than all-purpose flour. All-purpose flour, when spooned into a 1-cup measure and leveled, weighs roughly 4 1/2 ounces. Using the same method of measuring, cake flour weighs in at about 4 ounces.
How to Make Cake Flour With All-Purpose Flour
If you don't have cake flour, use 1 cup minus 2 tablespoons of all-purpose flour sifted with 2 tablespoons of cornstarch for each cup of cake flour called for in a recipe.
How to Make All-Purpose Flour With Cake Flour
If you only have cake flour and the recipe calls for all-purpose, use 1 cup plus 2 tablespoons of cake flour for each cup of all-purpose flour.
See Also: How to Make Self-Rising Flour