There are differences between all-purpose flour and cake flour, says Dr. Barbara Streumpler, a nutritionist with the Alabama Cooperative Extension System. The first difference is related to the type of wheat used to produce them.
Cake flour is made from soft wheat, while all-purpose flour is a blend of soft and hard wheat.
The second difference between cake and all-purpose flour is the amount of the starchy portion of the wheat kernel or endosperm that is used to make the flour. The third difference is the extent to which the flour particles are ground.
The term patent refers to the amount of endosperm used to make flour. Long-patent flour contains a higher percentage, including those portions that are more resistant to crushing and contain slightly more protein. Short-patent flour contains the smallest portion of endosperm and a lesser amount of protein.
All-purpose flour, which is commonly used at home for making bread, lies between the long patent or bread flour and cake flour.
Differences in protein content, while not nutritionally important, influence the structure of the finished baked product, says Struempler. In bread flour, gluten -- protein component of the grain -- is stronger and more elastic than in all-purpose flour. Cake flour, has a weak gluten, which is desirable for the delicate texture of cakes.
Should you run out of cake flour and want to bake a cake, you can substitute all-purpose flour, using 2 tablespoons less per cup. The reverse is not true. Desired
results cannot be achieved by substituting cake flour for all-purpose flour.
Source: Dr. Barbara Struempler, Nutritionist, Alabama Cooperative Extension System, (334) 844-2217