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Right and steady sifting in flour production

06 February 20156 min reading
“The sieving efficiency depends on the screen acceleration, the rate of movement of the material relative to the sieve surface, screen opening size, percent open area in the sieve, amount of sieve surface allocated for separation process, and material load on the sieve. With all these factors playing a role during sieving, if not optimized, the sifting process could lead to inconsistency in the milling process.” eleme2_kapakDr. Kingsly Ambrose is an Assistant Professor at Department of Grain Science and Industry, Kansas State University. Ambrose who obtained his Ph.D. from Purdue University in 2010 has published more than 40 peer reviewed articles. Dr. Ambrose who is also co-inventor of one patent is a member of American Association of Cereal Chemists (AACC) International and American Society of Agricultural and Biological Engineers (ASABE). Considering his knowledge on flour sifting process in milling, we interviewed with Dr. Ambrose whose research, teaching and extension program at K-State focuses on application of particle technology in milling and grain processing industries. In Dr. Ambrose’s answers, you can find the details about the importance of flour sifting process, things to take into consideration in sifting systems and more. On the basis of your former experiences, what could you tell us about the place and importance of sieving (flour sieving) in production process? Sieving, in a grain processing facility, refers to the separation of ground material into various particle sizes. Sieving is carried out with the objective of producing finest quality flour products with minimum bran contamination and in a narrow particle size range. A flour mill utilizes different break and reduction systems to grind wheat. The streams from these grinding systems are classified by sieving to separate the flour produced at each grinding step. So, sieving play an important role in the mill output, production efficiency, and quality. With increased consumer demand for hygienically safe and pure products, sieving is vital for plant reliability. How do the sieving and grading processes work? What is the effect of sieving on the quality of flour? The sifters (sieving machines) separate the flour particles based on size. During sifting, the particles comprising the ground mass will be constantly moving on the sieve screen. If the particle size is smaller than the sieve opening, the particles fall through the opening. Depending on the sieving stage and their size, these particles will be collected through the outlet or sifted again on a screen with smaller opening. The separation process can be improved by the use of sieve cleaners. But, the sieving efficiency depends on the screen acceleration, the rate of movement of the material relative to the sieve surface, screen opening size, percent open area in the sieve, amount of sieve surface allocated for separation process, and material load on the sieve. With all these factors playing a role during sieving, if not optimized, the sifting process could lead to inconsistency in the milling process. Including variability in the milling process, quality of flour will also depend on the sieving process. Improper sifter cloth tension and other factors mentioned above might lead to non-uniform granulation, bran contamination in flour, and results in poor flour yield. Could you give us some information about sieving systems and technologies that are used currently? Do the materials used in sifters have an effect on sieving process? Common sifters that are being currently used by the flour industry includes gyrating sifters, reciprocating sifters, combination of reciprocating and gyrating sifters, and reel type sifters. Gyrating sifters, which gyrates in a horizontal plane imparts motion to the ground mass to improve the separation process. Through engineering advances in design of sifters, the gyrating action is self-balancing with low power consumption. For limited capacity operations, reciprocating sifters are normally used. For break scalpers, a combination mechanism of gyrating and reciprocating is being used. In flour milling, smooth materials are used as screen cloth materials primarily to reduce the friction drag effect on the ground mass particles. In addition, it has been found that smooth sieve screen materials increase the throughput of sieves. Smooth wires, made of tinned mill or stainless steel mill, are used in the break material streams (for e.g. first break stream of wheat) to support heavy loads. Lighter screen materials such as bolting cloth are used in screen with smaller opening. In addition, nylon GG and polyester GG screens are used on coarse middlings and in purifier screens, respectively. The screen material and the weave style affects the percent open area available for particle separation, ability to withstand load, particle movement on the surface, and the sifting efficiency. What kinds of criterions are taken into consideration while choosing the right technology in sieving process? The important factors that affect the sifter operation are the revolutions per minute and the throw (diameter of the inscribed circle of gyration) which imparts movement to the ground mass particles. The sifter speed should be higher than the critical speed of the particles to have an effective separation. Similarly, the gyration should provide a large throw. A larger throw results in better movement for coarse ground products while a short throw will be better for flour. In addition, the screen acceleration should be lower for break streams than for the fine material separation. Along with the sifter motion, the aperture size and percent open area are the other two important factors that affect the sieving process. Aperture depend on the diameter of the threads used in the screen. So, these factors should be considered while choosing the right technology and screens for sifters. What is the role of technology in a good sieving process? ıs it true that only technology is sufficient for a good sieving process? What should miller consider for qualified sieving process while choosing and applying the technology for it? With the advancement in technology, flour milling industry is being able to use better drive systems, FDA approved lubrication materials, etc that has resulted in better separation process. Other than technology, the mill operator’s judgment also plays an important role in maintaining the throughput without compromising flour quality. Factors such as load (amount of material on the sieve) and sifting time greatly influence the process efficiency. On a screen surface, there should be enough material to cover the screen completely. At the same time, care should be taken that the screens are not overloaded. In addition, time taken by a particle during the separation process affects the sifting efficiency. Sieve cleaners are an integral part of sifters in flour mills. Sieve cleaners, such as ball cleaners or cube cleaners, help keep the sieve apertures open by preventing blocking or blinding. Appropriate sieve cleaners could be used to increase the efficiency of separation process. So, these factors should be optimized by the mill operator to have good sifting efficiency.
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