Peanut, our resident molecular
biologist, has presented a succinct and interesting account of the differences between bleached and unbleached white
flour. She equates bread flour with unbleached white flour, and in the swirling world of baking semantics, no doubt such terminology is used. Still, as the
local Luddite who grinds his own flour and uses it as soon as possible, let me elaborate on peanut's fine write-up.
Part of the confusion stems from the word "white flour." White flour has had the bran and wheat germ removed, leaving the wheat's endosperm, which is milled into flour. Today, rollers crack the wheat berries and remove this before milling the remaining wheat. In the olden days, the process was a bit different.
In medieval days, the wheat berries were crushed intact. The resulting flour was then sifted (or bolted) through cloths, acting as sieves, separating the finer white flour from the coarser bran. The remains from the first pass were the coarsest. The best flour came from the second bolting. As peanut notes above, this white flour's baking properties improved with aging.
As the flour aged, exposure to the air oxidized the yellow carotenoid pigments in the flour, making it yet lighter. The best flour, passed through two boltings and aged for several weeks, also happened to be the whitest.
Flour will spoil over time; the wheat germ contains oils, and these oils turn rancid. Before 1900, much flour was produced locally. (To be fair, the railroads in the States allowed for long distance travel in short periods, and cooperative grain mills flourished.) Since flour did not keep, local mills provided fresh ground flour to be used immediately.
In 1876, the governor of Minnesota attended the World's Fair in Paris, and marveled at the white French bread rolls. When he returned to the United States, he developed a process using steel rollers that allowed the quick separation of the endospem from the more nutritious parts of the wheat berry.
By the 1920's, the new milling process made white flour available to the masses. The wheat germ and bran were sold as high protein feed to a growing cattle industry. People at home could buy cheap white flour that would last indefinitely on the shelves.
About the same time, however, folks in the States started developing beri-beri and pellagra, diseases now known to be from vitamin B deficiencies. Part of this increase was because of the new method of milling flour. (Part of this also resulted from our methods of processing corn. Native cultures knew to soak corn in alkali prior to cooking, preserving the vitamins--the South suffered from epidemic pellagra after the War Between the States, but it was not addressed as an urgent national issue until the rest of the country shared the epidemic.)
Joseph Goldberger (United States) and Christiaan Eijkman, a Dutch physician, connected pellagra to niacin deficiency. In 1941, South Carolina required the enrichment of bread with vitamins and iron. In 1943. the United States Federal Government did the same.
Americans hardly know their wheat flour anymore. If you wander into a supermarket, most flour is labeled "all-purpose," though this is a bit of a misnomer. In order for bread to rise, the dough needs gluten, a complex formed from water and certain proteins in flour, kneaded together to form a complex webbing that traps the carbon dioxide made by the yeast playing in the dough. "All purpose" flour combine high and low protein wheat flours--the resulting mix does not have enough protein in it to make bread rise.
Bread flour is any flour that has enough protein in it to make a successful airy loaf of bread. Bread flour can be bleached or unbleached--the oxidation associated with bleaching may add some kneadability to the dough, but is not essential for a good rise. Bleaching does not do anything for the flavor of bread, which is not to say that bleached flour cannot make a heavenly tasting bread. Some folks prefer Miller Lite to a double bock.
If you want truly tasty bread, however, you need good tasting flour. The only way you are going to do that is get it fresh ground, bran, wheat germ, and all. If you want a fine crumb, sift your flour, or grind it twice. If you want a good rise, buy a fine high protein hard winter wheat. (The stuff is amazingly cheap here in the States--costs less than 7 cents a pound when bought in bulk.)
If you do not separate the endosperm from the wheat germ, the flour will turn rancid quickly. One of the big mistakes health foodies make is buying whole wheat flour, then pretending the rancid stuff is good. I guess I would convince myself of the same thing if I paid twice as much for the rancid stuff as the white flour runs in the supermarket.