Wheat Flour Milling, Second Edition
Pages 259-268
DOI: https://doi.org/10.1094/1891127403.007
ISBN: 1-891127-40-3
Abstract
In a general sense, purification is a term applicable to almost every process in the milling system. Purification is any process that separates bran particles and germ from endosperm. In particular, it is the gradual reduction of middlings to reduce endosperm and flatten the germ and bran particles so that they can be removed in the sifter. The residual tailing systems also separate bran particles from endosperm by reduction of the endosperm and sieving to remove the flattened bran. This chapter explains the process to remove bran from middlings that are produced in the break system.
Although the objective of the break system is to remove the endosperm from the bran without breaking the bran into small particles, some shattering of the bran occurs and results in a mixture of endosperm and bran in the released middlings. The purpose of the purifier is to separate the middlings into three fractions: pure endosperm, a mixture of endosperm with attached bran, and bran particles. The purifier also grades the endosperm into particle size ranges, which can be more efficiently ground separately in the reduction system. The clean separations from the purifier are sent to the head end of the reduction system and the branny materials to the tailings recovery system.
Purifiers can remove bran fiber and classify middlings by size and purity for the reduction system. This is not possible with sifters alone. Purifiers are versatile and, because they make the mill flow diagram more flexible, are excellent tools for maintaining mill balance. The miller can change the flaps on the collection trays below the sieves to direct the middlings to reduction rolls. Purifiers aid greatly in the production of patent flour with low ash and good color. They are essential in producing granular products, like farina and semolina.
However, in recent years, the role of purifiers in the mill flow diagram has diminished as millers have evaluated their cost effectiveness. The trend has been toward higher ash in bakers’ flour grades and less demand for lowash patent flour, which decreases the need for extensive purification. Purifiers require considerable air to operate. When purifiers are used in the reduction and tailings systems, the air dries the material, which increases bran breakage, thus increasing the ash content of the flour. Although in mills that control the relative humidity such drying is minimized (see Chapters 10 and 12), modern flow diagrams have eliminated the use of purifiers in the reduction and tailings system. However, the moisture content is high in the break system middlings, and purifiers can be used to clean the middlings with little loss of moisture. Purifiers are used to clean the middlings from the break system in the flow diagram today. The skill of the operator plays an important part in the successful operation of purifiers, and with fewer personnel in the mills, monitoring and proper adjustment of the purifiers is difficult. Poor milling results often are caused by purifiers that are out of adjustment.
