Where indicated in the recipe, bleached cake and bleached all-purpose flour can be used interchangeably as long as the weight is the same. If you are measuring by volume, refer to Ingredient Equivalences and Substitutions. Cake flour results in a more tender crumb that is ideal for most cakes. If you desire extra tenderness and have only bleached all-purpose flour on hand, you will need to use the suggested amount of potato starch or cornstarch in place of some of the all-purpose flour. (Potato starch gelatinizes at a lower temperature so it will set the cake’s structure faster.) However, in some instances, I like the higher protein of all-purpose flour so that the cake has enough structure and slices without falling apart.
If you are using a national brand of bleached all-purpose flour, the volume given in a recipe will be less than that of cake flour for the same weight. This is because it has more protein and is heavier, so you need less of it. If, however, you are using regional brands, especially from the South, such as White Lily, the protein will be very similar to cake flour and you can use the same volume.
Sponge cakes baked with Wondra in 9 by 2-inch cake pans have a slight decrease of ⅛ to ¼ inch in height, but that loss is more than compensated for by the exquisite tenderness, airiness of texture, and superior flavor. Cakes made with Wondra and baked in sheet pans also lose about ⅛ inch in height, so I have stayed with cake flour or a combination of cake flour and cornstarch, especially where the cake will be rolled, because it gives the rolled cake a much more attractive appearance. If you want to replace cake flour or all-purpose with Wondra, substitute an equal amount in weight. For volume, see Ingredient Equivalences and Substitutions.
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