Cake flour is made from a type of wheat that has lower protein, so it produces less gluten when it's mixed with liquid. Less gluten=softer end result.
The softest flour is cake flour, then it goes up, pastry flour, all purpose flour, bread flour. If you look at a pile of each type it's easy to tell them apart because the cake flour is softer and finer, and the bread flour is rougher and grittier when you rub it between your fingers. It's a pretty obvious difference. You want more gluten when you make bread because you want your end product to have more structure.
Having said that, use whatever flour it calls for in the recipe. A good balanced recipe will require a certain kind of flour to work right, so if you use AP flour when it calls for cake flour, you'll have to adjust the amount of flour by removing a little or the batter will be too tough and your cake's texture won't be right. Some recipes call for AP flour, some call for cake flour. I use whatever the recipe calls for, so I use both depending.
I don't know if you can get cake flour in bulk, but I'm sure that sombody does make it. Maybe someone else can answer that part of your question, sorry I don't know.
[quote="costumeczar"]
A good balanced recipe will require a certain kind of flour to work right, so if you use AP flour when it calls for cake flour, you'll have to adjust the amount of flour by removing a little or the batter will be too tough and your cake's texture won't be right.
Do you replace the flour that you take out with anything?
No, you just take out something like 1 Tbsp of AP flour per cup, or something like that. I can't remember the exact amount, so don't quote me, but if you do a search for ingredient substitution charts you'll probably find it. You don't need to replace it with anything, but it's always best to use what the recipe calls for!
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