wheat is milled to remove the bran and germ, leaving the endosperm. The endosperm is the whitest part of the wheat grain, because it is 3/4 starch. From 100 pounds of wheat, 72 pounds of straight flour can be got (the other 28 pounds that is milled away is used for animal feed.) Still, it isn't purely endosperm, as it will contain some particles of the bran and germ which cling so tightly to the endosperm that they weren't removed in the milling. French bakers will use straight flour in their bread making; North Americans and British generally don't.