The third and final step in the development of milling processes was taken when rollers were substituted for stones to perform the grinding process. Iron roller. were generally used it Pest to grind wheat before 1S-10. and, under the name of the Hungarian system, rapidly spread throughout Europe. As early as 1510 Pa ur of Austria invented a middlings purifier which is the subititution for a single grinding between millstones of a succession of grindings between several sets of iron or porcelain rollers. The wheat is gradually reduced by running it through six or seven different sets of rollers, a thorough process of winnowing or sifting intervening be tween the grindings. As a musty odor and dark color are given to Hour if it is heated dur ing the process of manufacture, the rollers are kept cool.
Wheat is made up of a central port ion of starchy cells, A in Fig. 3, surrounded by gluten cell,. 13, containing nitrogenous or proteid matter. which builds up tissue. The germinal portion shown at C' contains phosphates, which possess great food value, as do the inner layers of the husk. D. The exterior coatings. which are of a tibrou, or woody nature, are., on the other hand. (mite in digestible while passing through the alum-Mary canal of a human being. That portion the wheat between the starchy n and the husk is called the iniiiillinos. This is the most valuable portion of the wheat for making not only a nutritious but also a light bread.
Two grades of wheat are known to the n Hier: ll'infrr wheat, or that sowed in the fall and com ing up in the early ;spring: and spring wheat, which °rows during the summer. and is har vested in the fall. The winter w•eat. whose
kernels are softer and more starchy, is more easily separated from the husk than the spring wheat, whose kernel is much harder and much richer in gluten. The richer the wheat, the more difficult becomes the process of separating it from the husk, because the gluten is itself the cause of the toughness. Hence winter wheat pro duces a much whiter flour than spring wheat when both are ground by millstones. But in or der to produce a white flour from either kind of wheat a large portion of the being inseparable from the bran, is lost. Hence the problem how to save and purify the middlings is a vital one with millers. While the old process aimed to avoid middlings as entailing loss of flour, the new process seeks to produce middlings. because out of the middlings comes the high-grade flour and the gluten which gives flour its rising power is saved.
The machines called 'middlings purifiers,' for winnowing the wheat between the successive grindings, are of two general types, the gravity and sieve purifiers. In the gravity purifiers, which are used for the larger-sized grains, the middlings pass in a thin stream through a cur rent of air produced by a revolving fan, and so regulated that the bran is blown away while the wheat, being heavier, drops down into a recep tacle. The sieve is an oscillating strainer of gauze, through which a current of air passes out ward, carrying off the bran, while the wheat passes through the sieve. Many of the middlings purifiers in use are quite complicated in their structure. but the general principles upon which they operate are as described above.