Flour is an ingredient that we often take for granted. Have you wondered what the difference is between conventional and stone ground flour? Conventional flour has the bran and germ removed as these will become rancid within just a few weeks. Stone ground crushes the entire grain, so the bran and germ remain in the flour, adding flavor but shortening shelf life of the flour. Store these types of flour in the freezer to keep them fresh.
Have you ever wondered why flour is bleached? Freshly milled flour will make a very dense loaf of bread. After the flour ages, it will produce a much nicer loaf. A chemical reaction occurs at the end of the glutenin (one of the two proteins that combine to make gluten) protein molecule; this change makes the gluten longer and more elastic. Starting early in the 1900s millers began to bleach flour which caused the chemical reactions to happen immediately instead of occurring over the course of a few weeks.
All wheat flour contains gluten; it is the protein in flour that provides structure in baked foods. Some flours contain more protein than others; bread flour can be 12-13% protein while all-purpose flour has only 11-12% protein. Pastry flour can be as low as 7-8% protein.
Whole wheat flour may be high in protein, but much of that protein comes from the germ and aleurone layer of the wheat. These proteins will not combine to form gluten; they do interfere with gluten formation. This causes a much more dense loaf of bread. Bread flour has very strong proteins that make a light, high, and chewy loaf. Pastry flour has weak gluten that makes a crisp and tender pie crust.
Much of the information in this post is taken from “On Food and Cooking, the Science and Lore of the Kitchen”, Harold McGee, 2004 chapter 10.