Cake flour is made from finely milled soft winter wheat which is high in starch and low in gluten-forming proteins.  Because of its finer granulation, it absorbs fat and moisture more quickly.  Due to bleaching by cholorination, cake flour has a lower pH (more acid) than other flours.  This produces a sweeter flavour and a finer , more velvety crumb because the greater the acidity, the lower the temperature at which the protein coagulate.  This makes it possible for the cake structure to support more sugar, butter and heavier particles such as chopped nuts or chocolate.  Chorination also serves to inhibit gluten formation.  Fat adheres to the surface of chlorinated starch particles, resulting in better aeration (more even uniform distribution of air)
 
It is possible to substitute equal weights of bleached all-purpose for cake flour by adding a small percentage of cornstarch.  But the result will not be the same because the flour is coarser and the pH higher. 
 
While we are in the business of flour, self-rising cake flour cannot be used interchangeable with cake flour because it contains approximately 1.5 teaspoons baking powder and 1/2 teaspoon salt per cup of flour.  This will coarsen d weaken the texture of cakes requiring only 1.25 teaspoons or less baking powder per cup.
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