I don't understand the purpose of sifting. I have been told that sifting started because in days of olde you could not be sure all foreign materials had been removed at the mill. It continues today because sifting aerates the flour. But once you mix it with water wouldn't the aeration be gone? Anyone got a minute to 'splain? Thanks.
Flour from a miller is already sifted but during transportation and storage there can be settling and compacting of the flour. Also, since flour has moisture in it (starting at around 14% at the miller's facilities), there can also be clumping of some of the flour particles. Sifting separates the particles and improves the hydration of the flour. It perhaps also helps to more uniformly disperse the remaining ingredients, including water, throughout the dough when it is made. Some members, like November, always sift their flour before using. Most don't. In the few tests I have conducted, I have found that I can "squeeze" more water into a flour that has been sifted, by at least a few percent.