Wheat Flour Milling, Second Edition
The grinding process is the most important step in the milling system. The manner in which the kernel is broken affects the subsequent sifting and purifying operations, both in granulation (particle size distribution) and in the amount of fine bran present in the ground material in the reduction and sizings systems. The grinding process can be classified into four systems: 1) the break system, which separates the endosperm from the bran and germ; 2) the sizing system, which separates the small bran pieces attached to the large pieces of endosperm; 3) the reduction system, which reduces the endosperm to flour; and 4) the tailings system, which separates the fiber from the endosperm recovered from the other three systems. The quality characteristics of the flour particles are influenced greatly by the grinding action used in the reduction system. The most obvious effects are the amount of starch damage in the flour and variations in flour color from bran contamination.
In the grinding operation, energy is expended to break apart the bran and endosperm and reduce the endosperm to flour. This uses about 50% of the power connected with the milling system and results in heat generation and moisture loss in the ground material. Early primitive mills consisted only of grinding to reduce the wheat kernel into flour to make a bread product. As the taste for refined white flour grew, so did the complexity of the milling system. It is during the grinding process that the basic work is done. All other operations—conveying, classifying, purifying, and other blending operations—use less energy and could be considered secondary requirements for the milling system.
The principal forces of grinding are 1) compression, 2) shear, 3) friction/abrasion, and 4) impact. Most grinders operate on a combination of these principles. Some of the most common grinding machines are roller mills, attrition mills, impact mills, ball mills, cutters, disc mills, bran dusters, and pearlers. Each of these machines is discussed in some detail in this chapter.