Wheat Milling Part 4 (Gristing and Grinding)

By : Novozymes and Buhler

7. Gristing

The key step in the quality process is the blending of wheat known to the miller as the “wheat grist”. Based on the type of flour and characteristics, the miller decides which wheat and in what proportions should be used in the grist.

The grist for a flour is chosen by a variety of methods. But it is generally based on the miller’s experience, wheat quality characteristics and knowledge of the customer’s process and needs.

Blending wheat prior to milling is a very popular way of producing flours but it does have some disadvantages. Each wheat grain in a grist has its own irregular characteristics, depending on its variety or source, such as grain size, shape, hardness, moisture content, protein quality, and quantity.

These characteristics may affect the way the grain performs through the mill. So if the grains are blended before milling, the mill settings have to be a compromise between the various milling characteristics. In case the wheat varieties to be mixed are quite different in hardness, it is advisable they are cleaned and conditioned separately to achieve the same softness at 1st break. If different kinds of wheat are milled individually, the produced flour batches are blended.

8. Grinding

Wheat grinding involves a series of breaking rolls, graders (plansifters), purifiers and reduction rolls.

Usually the break system consists of 4 to 5 passages, supported by additional bran finishers. The target of the break system is to separate the bran from the endosperm as efficiently as possible, producing clean intermediate products and minimizing the production of flour at the same time :
  • Four break passages are required when grinding hard wheat varieties or for achieving extraction rates of up to 76%
  • Four and half to five breaks are suitable for soft wheat varieties or extractions between 76-80%. Should the extraction have to be higher than 80%, an extra passage or the so called “bran grinding” passage, has to be foreseen as well

To obtain a better grinding work at the last two break passages, they are split into coarse and fine in medium to large size mills. Depending on the type and extraction of flour to be produced and the type of wheat to be milled, in terms of hardness, every bran stream (coarse and fine) has to be passed over bran finishers at least 1-3 times.

The breaking of wheat is done with rolls that crush the kernels spinning toward each other at a high speed rate.
Cross section of a roller mill (Picture is a courtesy of Buhler AG, Uzwil, Switzerland)
The break stock has to be graded according to size. This process is already started in the break sifters. Most commonly additional sifter sections, or the so-called “graders” or most commonly “plansifters” are required to complete this process. The plansifter is a stack of sieves of decreasing mesh size that separate particles by size.
 
Cross section of a plansifter (Picture is courtesy of Buhler AG, Uzwil, Switzerland)
The number of plansifters is determined by the size of the mill, maximum number of sieves per sifter section, and number of granulations (groups of products of the break stock) applied.

The graded product streams – leaving the break sifters and graders – still contain bran particles. These bran particles cannot be separated from the endosperm particles due to their similarity in size. With the help of air, purifiers can further group these different granulations into clean endosperm particles; endosperm with bran particles still adhering (composite stock); and bran pieces.

For high yields of low ash flour, it is necessary to purify the semolina and middling before they are further reduced. If semolina and middling are the desired finished products, then the use of purifiers is absolutely necessary. The number of purifier passages depends on the amount of low ash flour to be produced – whether semolina or middling is to be sold or not – the type of wheat to be milled (the harder the wheat, the more important  is purification).
Cross section of Purifier (Picture is a courtesy of Buhler AG, Uzwil, Switzerland)

The composite particles obtained from different purifiers, have to be treated by rolls again. The scratch rolls (smoother than the breaking rolls) treat the composite stock in such a manner, that the adhering pieces of bran can be efficiently separated from the endosperm. Depending on the importance of producing a high amount of low ash flour or also on the amount of semolina required, this process can be completed with more or less pressure or care.

The number of scratch passages can vary between one and two and depends on the importance paid on the process (finished products) as well as on the size of the mill.

The purpose of the reduction system (series of even smoother rollers) is a more or less careful reduction of semolina and middling into flour. This reduction has to be done in several steps to achieve a finished product whose baking characteristics do not get worse by overheating and also to be able to produce products with different ash values and granulations. The number of reduction passages is determined by the hardness of the wheat to be milled, granulation of the flour to be produced, and extraction of flour.

The bran finisher removes adhering flour particles from the bran, thus increasing flour yield at the mill. This is achieved using slanted beaters that give the tangentially incoming material stream an additional axial motion improving the yield of flours.




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