cake flour vs all purpose

according to Alton Brown, it has to do with chlorination of the flour

cake flour is chlorinated all purpose flour and has a tendency to *set* (please correct me o baker!) faster. this would result in a cake that would cook faster (?) and / or remain moist during cooking

exact quotes I don't remember but I do remember him pointing out the difference

A.P. vs. Cake & Pastry Flours

Actually, all-purpose flour is a blend of hard and soft wheat flours. Soft wheat flour is generally the better choice for baked goods that do not need to rely upon a high gluten content -- most biscuits, pastries, and cakes, for example. On the other hand, when you use a.p. flour for most cakes & other delicate baked goods, their texture can be toughened because of too much gluten. If something is best made with a blend of the two flours, as is the case with popovers, then you can create a suitable mixture.

Cake flour is milled from the soft winter wheat. The gluten content is low, which makes it generally unsuitable for yeast breads. However, a small proportion of soft wheat flour can be added to the bread flour in order to improve the flavor of certain water breads. (I like to make French bread using a mixture of ¾ bread flour to ¼ pastry flour; it’s not strictly necessary to use the pastry flour for good Fr. bread, but I’m convinced that the flavor is closer to the real thing when it is included.)

Nancy Silverton, in the prefatory notes to her book, Desserts, wrote: "[My recipes] are made with unbleached pastry flour, unless otherwise specified. Cake flour is an acceptable substitute. Both these flours are made of 'soft' wheat, which contains little gluten, the protein that can make a pastry tough. Cake flour contains even less gluten than pastry flour. Products made with soft flour will be more tender than if they are made from all-purpose flour....If you aren't sure what kind of flour is in an unmarked bag or jar, just squeeze a handful -- cake flour and pastry flour will hold their shape." (p. 23)

Addendum: A good recipe for an 8-inch Red Velvet cake is included in "food archaeologist" Susan Waggoner's aptly titled Little Cakes -- a slim (fewer than 100pp.) volume published last year. I also drew out my copy of The Magnolia Bakery Cookbook, suspecting that a recipe for this cake would have been listed...but no!

As a curiosity: Could red crystalline decorating sugar be used effectively in Red Velvet cake? It provides a stiking contrasting effect in pound cake.

I got in trouble here once talking about all purpose flour so I have to be careful...

Cake flour is made from soft wheat flours and has a "protein" content of about 7 to 8%. Cake batters are not "developed" like a yeast dough because you want them to be tender. It is the starch in the flour that sets the structure of chemically leavened cakes. The proteins present at higher levels in bread flour (11 to 13%), when combined with water and mixing, result in gluten and the setting matrix for breads, bagels, etc. Both pastry flour (8-9%) and all purpose flour (10%) are in the middle zone of protein content.

You have to understand that flour is only part of the formula. If a formula wants for all purpose flour or cake flour or bread flour for that matter, I would assume that the rest of the formula has been fine-tuned to work in conjunction with the flour of choice. You can switch but the result will be different without a doubt.

Bleached or chlorinated cake flour receives about 1 to 5 ounces of chlorine gas per 100 pounds of flour in the bleaching process. It lowers the pH and greatly improves baking performance as far as volume, grain and texture. Other additives may be added to further whiten the flour.

Chlorinated cake flour along with shortening improvements allowed what is known as "high ratio" cakes. These formulas allow higher levels of sugar and liquids than flour. Cake made with ap or pastry flour are considered "low ratio" and would be characterized by lower volume, less sweet, tunneling or holes and typically less moist.