This section is from the book "Elements Of The Theory And Practice Of Cookery", by Mary E. Williams. Also available from Amazon: Elements Of The Theory And Practice Of Cookery; A Textbook Of Domestic Science For Use In Schools.
If you ever visit a flour-mill you will be shown the wheat as it is shovelled from cars into bins, mixed with other seeds, and with dirt, sticks, and nails. It is freed from these by being run through sieves. Next it is either washed or scoured between brushes. Then it is heated, and if dry, moistened to toughen it. It is now ready to be rolled.
Each pair of rollers turns in the same direction, but one moves two or three times as fast as the other; both are grooved, so that they cut or break rather than crush the grain. Each passage between a pair of rollers is accordingly called a break. From the first break the wheat comes out warm from the friction of the roller and looking and feeling something like coarse, damp sawdust. Next it passes to a machine called a scalper, where it is shaken on a wire tray to separate the bran as far as possible from the middlings, or bits of the white middle part of the grain - the part to be made into flour. Then back go the branny parts to be ground over by a second set of grooved rollers, and again "scalped," while the middlings are crushed between smooth rollers, sifted, mixed with other middlings from other breaks of the wheat, and ground over and sifted till completely reduced to flour. The number of breaks varies from four to ten.
Bran and other impurities are removed from both middlings and flour at each stage of the milling process by means of sieves to remove coarse particles and air-blasts to blow away worthless flour-dust. The final purification of the flour after the last grinding, or reduction, is by air-blast and by sifting, or bolting, through silk gauze stretched over cylindrical frames called reels.