28 BIOLOGICAL PROBLEMS 



last by mechanical means. It enters the grinding 

 machinery proper as a particularly clean product. 

 It is ground between rotating steel cylinders, which, 

 according to the special work required of them, have 

 their surfaces either smooth or fluted. A pair of these is 

 so mounted that the distance between the two can be 

 nicely varied and adjusted. Both are made to revolve 

 so that the adjacent surfaces, between which the 

 grains are nipped, travel in one and the same direc- 

 tion, though with different velocities. The corre- 

 spondence in direction diminishes friction, while the 

 difference in velocity of rotation secures that the grain 

 shall be lorn apart as well as crushed. It suffers 

 shear as well as pressure, though the relative degrees 

 of shear and pressure vary in different stages of the 

 milling process, and may be varied if necessary at 

 any one stage. 



The wheat is first passed between a pair of fluted 

 rollers, of which one revolves appreciably faster than 

 the other, so that the grains are torn asunder without 

 suffering very much actual grinding. This process 

 is technically known as a ''break". The products ob- 

 tained at this first stage are immediately "separated ' 

 or "purified". They are sifted, that is, through silk 

 of properly chosen mesh, and the sifting is assisted 

 by the application of a current of air from a fan, which 

 blows particles of husk away from the heavier particles 

 of broken kernel. A certain amount of actual flour, 

 but usually not much, appears at this stage. 



The ruptured kernels are now passed between a 

 pair of smooth rollers rotating at nearly identical 

 velocity. At this stage there is more crushing than 

 tearing, and the process is known as "reduction". 

 Again the products are separated, and this time a 

 large proportion of flour is obtained with a small 

 quantity of well-ground offals. Part of the grain, 



