Billi in Berkeley asks:

How is cake flour different from pastry flour? Can they be interchanged? Is there any other cake flour other than the Pillsbury box? I generally like to buy unbleached flour.

Contributing Editor Susan Nocella, Pastry Chef Instructor at a Bay Area culinary school, replies:

Part of the difference between cake and pastry flours has to do with the protein content. This protein is called Gluten and gives flour strength to support other ingredients or excessive manipulation. Cake flour can have 7-9% Gluten, while pastry flour can have 8-9% Gluten. We use lower Gluten flour for items that we want to be tender.

Cake flour is always bleached. This allows the starch in the flour to absorb more liquids, and therefore a higher percentage of fats and sugars. Bleaching also introduces acids into the flour, which cause the starch to gelatinize (or set) quicker, allowing for a lighter finished product.

All wheat flours have to be aged to improve digestibility and develop gluten. This can be done chemically using dry peroxides and chlorine gas. This flour is labeled “bleached.” Or flour can be aged naturally, and is then labeled “unbleached.” Using chemicals both whitens the flour and removes Vitamin E. Many people do not like to use it because of the reduced vitamin content. Bleached and unbleached flour perform the same way in baking.

I don’t recommend interchanging cake and pastry flour, since they will have a different effect on your finished product. If necessary, you can make pseudo cake flour from pastry flour by cutting it with a little cornstarch.

In addition to Pillsbury, some local supermarkets have started to carry Soft as Silk cake flour, which is popular in the South. Small quantities of Queen Genevieve cake flour are available by mail order from King Arthur Flours. Stores such as Whole Foods and Rainbow Grocery may also carry bulk cake flour.