Some factors affecting sieving performance and efficiency KeShun Liu ⁎ Grain Chemistry and Utilization Laboratory, National Small Grains and Potato Germplasm Research Unit, USDA-ARS,1691 S. 2700 West, Aberdeen, ID 83210, USA abstract article info Article history: Received 6 August 2008 Received in revised form 9 March 2009 Accepted 17 March 2009 Available online 25 March 2009 Keywords: Size separation Flour Powder Reverse sieving Screening Sieve blinding Sieving or screening has been the oldest yet most important unit operation for industrial separation of solid particles or as a laboratory method in size analysis. A stack of sieves with decreasing mesh size is usually used. Alternatively, particles can be sifted in a fine to coarse order by multiple sieving steps with each step using a single sieve. The latter is referred to as reverse sieve method. This study compared the two methods for sieving performance and efficiency using flours made from soft white and hard white wheat, hulless barley and medium grain rice. Additional factors, including milling method (impact vs. abrasive), flour moisture (7% vs. 11%), duration of sieving (60 vs. 120 min), and tapping (percussion during sieving), were also investigated. Mass frequency and protein content of oversize fractions were measured. Results show that all the variables and their interactions had significant effects on sieving performance and efficiency. Among them, tapping was most important, followed by sieving duration, sieving method, milling method, flour type, and flour moisture. When other conditions were equal, the reverse sieve method always gave improved sieving efficiency over the stacked sieve method. The observation can be attributed to the beneficial effect of oversized particles on reducing sieve blinding by near or sub-sieve sized particles. Furthermore, the reverse sieve method also expanded the difference in protein content among sieved fractions. Because of its practical significance, this so far unreported effect would bear further confirmation of other sieving and screening conditions. Published by Elsevier B.V. 1. Introduction The size distribution of particulate matter is very important in determining its physicochemical properties in a large number of processes of various industries (e.g. production of food powders, chemicals, colorants, paints, and pharmaceuticals). The sieves/ screens are the oldest and most widely used working elements for the separation of solid particles by size. They are used both industrially and in laboratories for the classification of particulate material. Often the term screening is used to refer to a continuous sizing operation as distinct from sieving, which usually means a batch process. Although sieving/screening has played an important role in studying and processing particulate materials, it has not received enough scientific attention [1]. Simplicity and familiarity of the process may explain this curious situation. In reality, the sieving process is governed by multidisciplinary principles, ranging from physics to applied fluid mechanics. Many factors have been identified to affect this unit operation, including the size and shape of particles relative to the aperture of the sieve, the mesh size of the sieve itself, the amount of material on the sieve surface, the direction of movement of the sieve, the rate of movement of the material relative to the sieve surface, etc. [1–6]. Furthermore, the interactions among variables are so complex that no satisfactory method of evaluating and predicting the sieving process has yet been developed [5,7]. This has led to the inefficient operation of industrial sieving equipment as well as misleading and erroneous results of laboratory sieve analysis [1]. Among all the elements of the sieving operation, sieve blinding is considered as the most important and direct controlling factor. Sieve blinding occurs when particles block up and lodge in the sieving mesh. It reduces the effective transfer area on the surface, resulting in reduction of sieving rates (sieving performance or capacity) and the degree of sharpness of particle separation (sieving efficiency) [1,2,4,8]. In cereal science, the subject of flour particle size has intrigued many investigators, mostly for its effect on flour quality [9]. Flour is a blend of particles. Flours of different particle sizes differ in physical properties and chemical composition [10,11]. These properties in turn affect flour performance in final products [11,12]. Although flour particle size can be reduced by regrinding a sample, further reduction of flour particle size by grinding is accompanied by an increased level of starch damage, which negatively affects flour performance in many final products [13]. An alternative method is to separate flours according to particle size through sieving or air classification. The fractioned flours are characterized by not only the difference in chemical composition and physical properties [10,11,14] but also minimal starch damage [12]. However, fractionating flour by sieving, although relatively simple, is limited by sieve blinding. With regard to the sieving process, either for industrial separation of solid particles or as a laboratory method in size analysis, a stack of Powder Technology 193 (2009) 208–213 ⁎ Tel.: +1 208 397 4162; fax: +1 208 397 4165. E-mail address: Keshun.Liu@ars.usda.gov. 0032-5910/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.powtec.2009.03.027 Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec