Wheat Flour Milling, Second Edition
The term sieving commonly refers to the separation of a ground mass of material into various particle-size classifications. In the flour mill, sieving after each grinding operation classifies the material for the subsequent step (further grinding, purification, etc.) and removes the flour produced in the grinding operation. Sieving machines separate particles by size. The most common sieving machines in the flour mill are 1) a gyrating box sifter containing as many as 30 stacked sieves and capable of making a maximum of eight separations, 2) a reciprocating sieve capable of making two to four separations, and 3) a reel-type machine consisting of a screen cylinder that rotates or moves the material along the surface of the screen and usually makes two to three separations.
Separation by a sieve is normally accomplished by the movement of the material on the sieve screen, which causes the particles smaller than the mesh opening to pass through by the force of gravity. Passage through the apertures, or openings, also can be forced by rotating beaters on the material or by air currents.