Bread flour comes from a type of wheat that contains a high amount of protein. The flour typically has a protein content between 12 and 13 percent while all-purpose flour has a protein content between 7.5 and 12 percent, depending on the brand and where you purchase the flour. The higher protein content means that more gluten develops when you mix bread flour into a dough. The finished product, usually a type of bread, has a chewy texture.

Cake flour has a very low protein content, usually between seven and eight percent. Cake flour is also usually bleached with chlorine, though you can buy unbleached versions. The presence of chlorine affects the starch in batter. Thanks to the bleach, the starch is able to bind with the fat in a cake recipe more effectively. The chlorine also helps the cake rise, since the starch in the flour absorbs water and sugar better. The result is a light texture, even when there is a higher ratio of sugar to flour in the recipe. In a traditional sponge cake, the ratio of sugar to flour is 225 to 100.

Cake flour is the preferred flour for sponge cake. Bread flour is not a suitable substitute for the cake flour since the two types of flour are so profoundly different. The high gluten content in bread flour will give you a cake that is tough rather than its optimal light and crumbly texture.

If you are attempting to make a sponge cake and find that you don't have any cake flour, you still have some options. If a recipe calls for cake flour, use one tablespoon less of all-purpose flour per cup called for. Another way to replicate the action of bleached cake flour using all-purpose is place up to four tablespoons of cornstarch in a one-cup measuring cup, then fill the rest of the way with all-purpose flour. The starch reduces the protein content and makes the mix finer.