The main difference between whole wheat and wheat bran is that whole wheat contains wheat bran. Whole wheat flour contains all the components of the whole grain: bran, endosperm, and germ. All components can be separated and made into individual flour substances, or can be ground as a whole, the product being whole wheat flour.
Bran is the fibrous outer shell of the wheat grain and is relatively tough when ground into a flour compared to a refined wheat flour. Wheat bran, however, is a critical source of dietary fiber as well as cancer-fighting antioxidants, starch, protein, vitamins E and B, and minerals. Due to its high-fiber content, wheat bran is a brutal opponent to wheat gluten, the agent that allows bread and cakes to rise with an airy texture. Bran is sharp, though not at a noticeable level to humans, and cuts into the gluten, disallowing it to rise properly. As such, most wheat flours that contain bran produce denser baked products than those made with refined flours in which the bran has been removed.
Whole Grain Wheat
Whole grain wheat flour is the product of grinding the entire grain of wheat--the bran, endosperm, and germ. Since whole wheat contains the bran, whole wheat flour maintains all of the benefits of the bran (listed above). The germ is the embryonic sprouting section of the grain, and therefore contains the majority of the grain's fat. By weight, the germ contains 10 percent fat (Wheat Foods Council) as well as B vitamins and trace minerals. Because of the high fat content, the shelf life of whole wheat flour is significantly lower than that of refined flour. The endosperm makes up the largest part of the grain by weight; over 80 percent, and contains a significant amount of protein, riboflavin, niacin, thiamin, folic acid, and soluble fiber.
Deconstructed Wheat Grain
Bleached white flour is the product of deconstructing the whole grain and removing both the bran and the germ; therefore it contains only the nutrients present in the endosperm. However, as the flour is bleached, it passes through a series of chemical cleaners including bromate, benzoyl peroxide, and chlorine dioxide gas. During this process, more of the nutrients are lost, however, no chemical residues are left in the flour and it is safe for human consumption.
Wheat bran, wheat germ, and wheat endosperm (white refined flour) can all be purchased separately and combined to create a whole wheat flour. However, whole wheat flour, a combination of all three, comes already mixed and contains the nutritional benefits of all the separate components.