For the most part it should have a smaller percent of protein than all purpose. Higher protein flours are generally used in bread baking and not prefered for cakes, biscuits, pie crust, pastries, and such.

Having said that I have a few recipes that call for cake flour and have used all purpose with no problems or ill affect on the final product. Reading posts here over the years I learned that the protein in flours can vary from different brands and different parts of the country. NOt to forget the different grains it is made with. I seem to remember AnnT mentioning that flour from Canada has higher protein from that in the states.

Hopefully Grainlady and some others will chime in as I am no expert on the subject by any means. What recipe are you making that is calling for cake flour? That could be a factor for sure.

David

Not sure I like the part about bleaching. Bleach is hard on the environment. I think I read somewhere else once about some other kind of chemical processing.
From wisegeek:
When baking a cake, most cooks aim to create a light, fluffy cake with a tender crumb. This requires a flour with a low protein content, as protein promotes the production of gluten, which can make baked goods more tough. It also means that the flour must be very finely milled, to keep baked goods from getting heavy. Finally, a flour which is starchy and able to hold large amounts of fat and sugar without collapsing is required.

All of these needs are addressed with cake flour, which is made from the endosperm of soft wheat. The endosperm is the softest part of the wheat kernel, making cake flour the finest flour available. As cake flour is milled, it is heavily bleached, not only to make it white but to break down the protein in the flour. Typically, cake flour is around seven percent protein, much lower than other flours; bread flour, for example, has twice that amount of protein.

The delicate, fine texture of cake flour is accomplished by heavy milling. The fine grain absorbs fat readily, ensuring that butter and other fats in cakes are well distributed throughout the batter. Cake flour can also carry a high volume of sugar when compared to higher protein flours. Since cake flour is a high-starch flour, it is extremely well suited for certain baking tasks. Cake flour is also lighter than conventional flour, which is why the substitution above falls short of a full cup.

Cookie and cake recipes which call for cake flour should be made using cake flour, if possible. In the production of certain other baked goods, cake flour can replace ordinary flour for a lighter end product, using one cup and two tablespoons of cake flour for every cup of flour called for in the recipe. Cake flour should not be used to make breads and other leavened products, as it is not strong enough. Also, as a general rule, a recipe which calls for "sifted flour" requires the cook to sift the flour before measuring, while "flour, sifted" is flour which is measured and then sifted. Since sifting changes the volume of flour, this seemingly petty distinction is actually very important.

Thank you both, that really enlightened me.I was just wondering if i could make a lighter cake with cake flour. i always use mixes because when i make them from scratch the cakes are too heavy.

AH, the art of cake making...

For a tender cake you'll always benefit from choosing a low-protein flour like cake flour, pastry flour or Southern all-purpose. Not only the low-gluten (protein) level, but it's also a much smaller particle. The fine-grind helps produce better texture. Cake flour is typically bleached (something I avoid if at all possible), but King Arthur Flour has unbleached cake flour (see link below).

But saying that, there ARE some benefits from using bleached cake flour... (source: CookWise by Shirley O. Corriher)

-Bleached flour is slightly acid.

-The acidity causes the cake to set slightly sooner in the oven, which produces cakes with a finer texture.

-Rose Levy Beranbaum points out that lower-pH (more acid) batters produce a sweeter, more aromatic quality in cakes.

-Clorination (bleaching) enhances the starches ability to absorb water.

-Fat sticks to chlorinated starch but not to starch from the same wheat that has not been chlorinated. Since all the air bubbles are in the fat, this leads to a more even distribution of the bubbles, which produces a finer texture.

Cake flour is milled from soft wheat varieties (low-protein varieties), while all-purpose flour is milled from a mixture of hard and soft wheat. Therefore, "all-purpose" flour is designed for all purposes. Just enough hard wheat flour to make a fairly good loaf of yeast bread, and enough soft flour to use for pastry, quick breads, etc. Unfortunately, it's actually not optimal for most things it's used for. Bread flour is milled from hard wheat varieties which have higher gluten/protein levels needed for high-rising yeast breads.

I mill my own flour and use a 3:1 mixture of soft white wheat and oat groats or spelt and mill it very fine for a low gluten, wholegrain cake flour that will work for light cakes like chiffon and angel food. I compensate for using unbleached flour by increasing the sugar slightly.

You will also find using a Southern All-Purpose flour like Martha White, White Lily, Gladiola, Red Band, will work better in recipes where you don't want a lot of gluten-development (cakes, pastry, biscuits, quick breads, etc...) than using National brands of all-purpose (Gold Medal, Pillsbury). If you normally use King Arthur or Robin Hood all-purpose, they have even more protein than National Brands of all-purpose and I'd avoid using them for baked goods you don't want a lot of gluten-development. If you MUST use these high-protein flours for things like pastry or biscuits, you can compensate by adding more fat, which coats the flour and prevents it from developing when the liquid is added.

It's also beneficial to keep mixing to a minimum when making a cake to avoid over-developing the gluten. Mixing methods produce different kinds of cakes. If lightness is what you are looking for, take extreme care on the creaming. You want your butter to be between 65-70°F (take the temperature with an instant read thermometer). You may even want to chill the bowl and the beaters before starting the creaming to keep the temperature on the cold side. While one "expert" recommends creaming for 4 to 5 minutes, another for 6 to 10 minutes, it's a very important step. If you use a stand mixer it will take less time than if you use a hand-held mixer.

Make sure your eggs are room temperature. The volume of the cake can be reduced just by adding cold eggs.

It may sound old-fashioned and extra work, but sifting the dry ingredients together several times will help to evenly distribute the ingredients and improve the finished product.

Don't forget to add the dry and liquid ingredients alternately. I like to add a lot of the dry ingredients in the first addition to the creamed ingredients because the fat will coat the flour to prevent gluten formation. Add about 1/3 of the liquid and mix, more flour, etc.

Keep mixing to a bare minimum to prevent gluten-development. I use a Danish Dough Whisk for mixing batters of any kind to avoid over-developing the gluten.

-Grainlady