And here's the answer:
Like baking soda and baking powder, the two are very easily confused but most definitely not interchangeable. We use both extensively in the Muddy's Kitchen, and if you bake much cake in your house, I am sure you have both flours around as well!
I hear from my home-based baker friends that most have attempted to replace cake flour in a cake recipe with all-purpose, just because that is what they happen to have at home. That name is deceiving, isn't it? Most bakers discover that that flour really isn't all-purpose at all when they attempt this substitution! When our Muddy's cupcakes somehow end up being accidentally made with all-purpose flour rather than cake, they tend to turn out something like cornbread: dense, tough, and with an unpleasant almost bread-like crumb.
So why do cake flour and all-purpose flour yield such different results? Much of that answer lies in the protein content.
Just about everyone has heard about gluten these days. You either know someone with a gluten intolerance trying to avoid it, or you know a bread baker who relies on it! Gluten is the combination of the two wheat proteins glutenin and gliadin, and it aptly shares its name with the Latin word for glue. It is primarily found in wheat, but it shows up here and there in other grains, as well. Gluten is what gives baked goods made with wheat flour structure. It holds together all of the other deliciously necessary ingredients in cakes, cookies, pie crust, and breads.
Gluten, as another protein, acts very similarly to albumin. Like albumin, the molecules uncurl themselves and latch onto one another to form the yummy structure of baked goods. Unlike albumin, however, gluten has the starches of wheat flour to act as their friends rather than the water that makes up 90% of egg whites. Starch generally makes up 70-something% of a flour, depending on its type. The simplest explanation behind the strength of wheat gluten is that its buddy, starch, is an ultra-absorbent network of sugar molecules that holds all the water that gluten lets slip through its grasp when baking.
Now for the difference behind all-purpose and cake flours! Both gluten and starch are responsible for these differences. All-purpose flours contain approximately 12-15% wheat gluten, while cake flours typically contain a mere 7-8% gluten. I am sure you are beginning to see the value in this property: the ideal cake is typically soft, tender, and moist with an ultra-fine crumb.
You only need so much gluten to make up such a product. More would make it sturdier, like bread, but not so tasty!
Another important difference between all-purpose and cake flours is that many cake flours are bleached in a special process not ever used in making all-purpose flour. This bleaching, or chlorination, damages starches (more of which are present in cake flour than all-purpose flour anyway), causing them to behave differently. This different behavior is especially welcome in cakes: such damaged starch allows the cake to increase in volume and thus be able to hold more water (e.g., moisture), sugar, and butter while still maintaining some fluff. So starch damage allows a higher sugar-and-moisture-to-flour ratio: sounds like a win-win situation to me!
If you mix up your two containers of cake flour and all-purpose, just take a look at the color and the texture. Thanks to the chlorination, cake flour will be whiter. You will also find that the cake flour has a silkier texture. This texture is thanks to the multitude of these damaged starches compared to the less starchy all-purpose flour as well as the fact that it is more finely milled than all-purpose flour. Anything for a finer crumb, am I right? Our Here Comes Trouble Cake (vanilla cake + caramel icing) is one of the many, many proofs that Muddy's is a firm believer in tender, fine-crumbed cake!