THE MILLING OF WHEAT 275 



nor to change lower grades into higher ones. 1 Of the many 

 processes for bleaching, the only ones having any industrial 

 value seem to be " based on the use of peroxide of nitrogen, pre- 

 pared either by chemical action or by the action of a flaming 

 arc upon atmospheric air."" Flour naturally grows whiter as it 

 grows older. 



The Dust Collector. The milling of wheat always produces 

 flour dust. The ignition of these particles suspended in the air 

 caused disastrous explosions in the Minneapolis mills during 

 1877-78. This led to the development of the dust collector, the 

 first form of which was a filtering diaphragm. Its essential 

 principle is now the vortical or rotary air current, which masses 

 and precipitates smaller particles than- the finest filter could 

 arrest. 



The Grades of Flour most usually made are four in number: 

 (1) Patent; (2) first class; (3) second class; and (4) red dog. 

 The feeds comprise the remainder of the milled product. The 

 basis of flour grading is mainly its purity, that is, its freedom 

 from the bran and germ portions of the wheat kernel. The 

 best flour comes from the center of the grain. The strongest 

 gluten is nearest to the outside of the kernel, but the outside 

 can never be perfectly separated from the bran. The degree 

 of purity varies with the different processes of milling. The 

 equipment of the miller, his special process of milling, and the 

 market to which his products go determine the number of 

 grades of flour that he makes. He may omit some of these four 

 grades, or he may further separate them and make a larger 

 number of grades. Patent flour, for example, may be separated 

 into first and second patent. The requirements of some buyers 

 are for a flour that is sharper or more granular, while those of 

 other buyers are for a flour with fine soft granulations and very 

 white color. After all of the flours have been collected from 

 the various machines into their respective grades, they are con- 

 veyed to the packing bins, from the bottom of which they are 

 drawn by automatic power packers into packages varying in 

 weight from 2 to 280 pounds. 



1 Letters, Frank W. Emmons, Washburn-Crosby Co., and John 

 E. Mitchell, Alsop Process Co. 



2 Sci. Am., S. 61:25263 (1906). 



