Many cake recipes call for special cake flour, rather than the more common all-purpose flour. Although those recipes may also include baking soda and salt, those are not ingredients in cake flour. Using cake flour results in a finer texture and moister crumb in the finished cake than can be achieved using all-purpose flour.
Though cakes themselves often contain baking soda and salt, cake flour itself doesn't.
Cake Flour Ingredients
Cake flour is made from finely milled low protein varieties of wheat. Typically, cake flour contains about 8 percent protein, which is much less than the 11 percent found in all-purpose flour. Usually cornstarch is added to the low protein wheat in cake flours. Commercial cake flour is often treated with bleach, or bleached with chlorine. This increases the acidity of the flour, which enables cakes to set more quickly in the oven.
Cake Flour vs. Self-Rising Flour
Sometimes confused with cake flour, self-rising flour is also usually made with low protein wheat, and commercial varieties are sometimes bleached just as commercial cake flour often is. However, self-rising flour does not contain cornstarch. While it does not contain baking soda, it does include baking powder and salt. Self-rising flour is a popular ingredient in the South, where it is used in many biscuit recipes.
Substituting Cake Flour for All-Purpose
Cake flour can be used instead of all-purpose flour in some recipes but not others. It is suitable for some sweet baked goods such as soft cookies, brownies, and quick breads. However it cannot be used interchangeably with all-purpose flour to make yeast breads because of its relatively low protein content. The higher protein content in all-purpose and bread flours is what enables yeast bread dough to rise.
Making a Cake Flour Substitute
A substitute for cake flour can be made at home. Basically, you would do this by removing a small amount of all-purpose flour and then adding an equal proportion of cornstarch, and then sifting the flour to ensure that the flour is smooth and without lumps.