You require both a good recipe and a certain amount of skill for your scratch-baked cake to turn out “just so.” If the ingredients aren’t mixed deftly, with a light hand, the cake’s airy delicacy can easily be spoiled, and end up as a rubbery, chewy texture. For novices, it’s easiest to get good results with cake flour, a specialized wheat flour created for cake-baking. If you don’t have cake flour on hand, you can adjust your all-purpose flour instead.
Cake flour has a number of distinctive characteristics, but the most important is its low gluten content. Cake flour is typically just 7 to 8 percent gluten-forming proteins by weight, while all-purpose flour can be 11 to 12 percent gluten. When cake flour is called for in the recipe, measure out 3/4 cup of all-purpose flour for every cup of cake flour. Whisk in 2 tablespoons of cornstarch, which is naturally gluten-free and therefore reduces the percentage of proteins. The end result is slightly less flour when measured by volume, but nearly identical by weight. When you mix up your cake, that lower gluten makes it easier to achieve a tender crumb.
This works because cake flour is much lighter and airier than all-purpose flour, thanks to its unusual milling process. The flour is milled solely from blends of “soft,” or low-gluten, wheat, and then bleached by a relatively harsh chlorine process. This gives the granules of cake flour a very porous structure, quick to absorb either fats or liquids in your batter. It even acts as a mild emulsifier, helping bind those two dissimilar ingredients together. Cake flour is also whiter than your all-purpose flour, giving angel-food and other white cakes a pristine, snowy appearance. The cornstarch substitution can’t replicate these characteristics, but is nonetheless a distinct improvement over plain all-purpose flour.
References and ResourcesJoy of Baking: Baking Ingredient Substitution Table
On Food and Cooking: The Science and Lore of the Kitchen; Harold McGee