This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Bakers have known for a long time that flour ground too finely does not produce so good a quality of bread as flour that has not been over-ground. Alsberg and Griffing have reported the following for over-ground flour. Before over-grinding, the starch granules clumped together under the microscope; in the over-ground flour they were scattered. Over-grinding injured the starch, and as previously stated the greater the degree of injury the more the starch was dispersed. Diastase acted on the whole granule rather slowly, but with many injured granules and the increase in cold water extract the action of diastatic enzymes in converting starch to maltose was more rapid; as a result, the rate of fermentation in bread making was increased. Alsberg has suggested adding some over-ground flour to flour reported low in diastase. Since the injured granules swell in cold water they absorb a larger proportion of water. Alsberg states that in the very severely over-ground flours the swelling may resemble heat gelatiniza-tion. Thus in severe over-grinding the baking quality is reduced although the hydration capacity of the flour is increased. Usually increased hydration capacity means increased baking quality. Moderate over-grinding did not affect the gluten but evidence was found that severe over-grinding injured the gluten.