Wheat Flour Milling, Second Edition
Wheat arriving in the mill usually contains foreign matter, such as strings, straw, parts of bags, wood, stones, or metal, that must be removed before processing. The contaminants usually become mixed with the wheat in the field and during its transfer through the various stages of the grain-handling system. In addition, wheat always contains some seeds of other plants, even if grown under the best conditions. If farming or environmental conditions are poor, the amount of screenings is likely to increase. Although part of the foreign material (dockage), or all of it, is discounted in the trade, the cleaning system in the mill should be designed to allow the miller enough flexibility to properly clean any wheat arriving at the mill.
Upon arrival, the wheat is precleaned with a high-capacity system to separate coarse foreign material, ferrous particles, and dust and to prepare the wheat for storage and safe handling (see Chapter 3). This chapter describes the intensive cleaning of wheat and its preparation for milling to flour for human consumption.
The heterogeneous physical characteristics of wheat (namely, different dimensions, specific weight, and shape even in one load) contribute to the difficulties of separating unmillable materials. The miller must make compromises in the selection of cleaning methods, machine adjustments, and sieves because wheat is not a homogeneous material.