How does this work?
The primary difference between cake flour and all-purpose (AP) flour is the protein content (which becomes gluten). The protein content of cake flour is about 8%, while the protein content of AP flour is about 10-11%. When you're making cakes, you need a little gluten for structure but not so much that it gets tough and chewy, so you use a flour that's lower in protein. When added to AP flour, cornstarch will inhibit the formation of gluten while also giving structure and 'sponginess' to your cake.
Other starches like arrow root and potato starch can also be used, but our food science guru Harold McGee warns that cakes with these starches will cook more quickly and will often be more moist than those with corn starch.
(Photo and cake decorating by Erika Larson, with permission. Swans Down photo by Faith Hopler for The Kitchn.)
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