Do I Really Need To Use Cake Flour?

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Do I really need to buy an entire bag of cake flour to make a cake? I know it's supposed to make it softer, but I don't have room for a second bag of flour in my kitchen cabinets. Is all-purpose flour okay, or will I mess up the whole recipe?

While making ingredient substitutions in baking can be dicey, there's good news here: you can pretty much use all-purpose flour and cake flour interchangeably. The difference between the two is their protein content: AP flours' protein levels hover between 10-12%, while cake flour is in the 6-8% range. Why does this matter in regards to cake? Because when flour is mixed with liquid and agitated (as in a mixer), those individual proteins begin to bundle up together to form an uber-protein called gluten. As the mixture is beaten or kneaded, all those gluten strands start to link up to create a springy, elastic network that provides baked goods with structure. (Think of it like glue because, well, that's pretty much what it is.)

Working your gluten into a tizzy is great thing for making sturdy baked goods like breads, but not for delicate ones like cakes, which also get some of their structure from eggs. Since cake flour is ground from low-protein (or "soft") wheat, it will result in a more tender cake, even if you slightly overbeat it. This is the key to replacing cake flour with all-purpose flour: don't overbeat your cake batter. In fact, if you're using a mixer, deliberately underbeat your cake, then mix by hand just until the batter is smooth and homogenous. Once flour has entered the equation there is no longer any need to beat like crazy, and if you do, you're hurting your cake. What kind of monster would want to hurt a cake?

So, using AP flour in a recipe that calls for cake flour is just fine. However, if you don't trust yourself not to overbeat your cake batter and would prefer to use a flour with a failsafe, you can approximate cake flour in your recipe. Simply replace one-sixth of the weight of all-purpose flour with a non-glutenous starch, like potato, tapioca, or cornstarch. For example:

1 cup all-purpose flour = 150 grams (approximately)

150 grams / 6 = 25 grams

125 grams all-purpose flour + 25 grams starch = 1 cup cake flour

(If you prefer to bake using volume measurements instead of weights: for every 1 cup of all-purpose flour, replace 2 tablespoons of it with starch.)