If you're a novice baker (or maybe even if you're an experienced one), you may think there's no difference between cake flour and all-purpose flour. Flour is flour, right? Wrong. There's actually a big difference between the two flours — despite one of them seeming to imply it's for all purposes in the name — and in most recipes, you can't substitute one for the other. If a recipe calls for cake flour when you only have all-purpose flour on hand, there's a reason you might want to run out to the store to get that cake flour instead of making the switch.
The two flours do share some similarities, of course. They're both wheat flours, which means they're made from the same grains and processed basically the same way. However, not all flours are made from the same type of wheat. The main difference between the two flours comes down to the protein content. Some wheat is harder than others, and the harder the wheat, the higher the protein content. As all-purpose flour is made from a harder wheat, the protein content in all-purpose flour is about 10 to 12 percent, while the content in cake flour (which is made from a softer wheat) is about seven to eight percent.
Because high-protein flours absorb more water than low-protein flours, all-purpose flour produces stiffer dough than cake flour when mixed with the same amount of water. That's why cake flour is better for cake recipes, since you want the dough to be soft and pliable. However, if you're in the middle of baking before you realize you're missing cake flour, you can substitute cake flour with a mix of all-purpose flour and cornstarch. You simply take a cup of all-purpose flour, remove two tablespoons, then add two tablespoons of cornstarch back in to create something resembling cake flour. Believe us, your cake will thank you!