Believe it or not, I've been composing this post in my head for a while. You may be wondering why I'm blogging about flour, but people used to ask me when I worked at Whole Foods: what's the difference in pastry/cake/bread/all-purpose flour and can they be substituted?
Flour basically comes from the wheat plant. The type of flour depends on the amount of protein it contains. That's why cakes baked with cake flour are so light and airy, whereas breads baked with bread flour are dense and chewy. If you were to try to bake a cake using bread flour, it would be a tough mass of dough. All-purpose flour was created for home cooks so that they could bake a variety of things without needing the different types of flours that are used in the industry. We never used all-purpose flour in school and you really don't find it in a pastry shop. There are some instances if the pastry kitchen does not specialize in pastries, they use all-purpose flour to keep overall costs down.
I found the following information from Wikipedia and Professional Baking by Wayne Gisslen:
High gluten flour has an especially high protein content and is used in hard-crusted breads and pizza dough and bagels.
Bread flour is a strong flour, such as patent flour, and is used in making breads. Patent flour is milled from the inner part of the endosperm.
All-purpose flour is a blended wheat flour with an intermediate gluten level which is marketed as an acceptable compromise for most household baking needs.
Pastry flour (also called cookie flour or cracker flour) is flour with gluten content slightly higher than cake flour, but lower than all-purpose flour. It is suitable for fine, light-textured pastries.
Cake flour is a finely milled flour made from soft wheat. It has very low gluten content, making it suitable for soft-textured cakes and cookies. Higher gluten content of other flours would make the cakes tough.
I have found "pastry flour" at Whole Foods Market, but it's really whole wheat pastry flour and does not work as a white pastry flour. You can make your own if you come across a recipe that calls for it. Just use equal parts cake and all-purpose flours. You can substitute cake for all-purpose flour in a cake recipe. There is a guideline on the cake flour box. But I wouldn't just substitute flour in other recipes, because substituting up or down the gluten strength, your end product will be either tougher, or the structure will not be a s strong.