Wheat flour comes in many different forms. First, let’s talk a bit about the wheat from which it originates.
The wheat berry has three basic parts: the endosperm, the germ and the bran. The germ is the embryonic plant, but is only about 2-3% of the total berry. The endosperm, which is about 75% of the berry, serves as food for the germ as it sprouts. The rest is the bran, which protects the tiny germ.
White flour is almost pure endosperm, whereas whole-wheat flour retains all three parts. As a result, white flour will keep for a long time at room temperature – probably a year or more. Whole-wheat flour, on the other hand, contains the oily germ, and that oil goes rancid at room temperature in just a couple of months. This is why so many people think that whole wheat bread tastes bitter; mostly likely, the bread they ate was made from rancid flour.