Starch is glucose sugar molecules strung together. Starch is enclosed in granules in the endosperm. These granules are embedded in a protein matrix. The protein matrix found in hard wheats tends to be tough and to hold starch granules tightly. These tightly held granules tend to be sheared or broken in the milling process and release their starch contents.
Free starch can absorb large amounts of water.
Hard wheats generally have a high water-absorbing capacity. Hard wheat flours tend to be chunky and easily sieved. Soft wheats, on the other hand, tend to have a weak, crumbly protein matrix. This matrix holds starch granules loosely and tends to release whole granules when milled. This results in a flour composed primarily of whole starch granules.