Available online at www.sciencedirect.com Separation and Purification Technology 61 (2008) 461–468 Physical properties that govern fiber separation from distillers dried grains with solubles (DDGS) using sieving and air classification Radhakrishnan Srinivasan ∗ , Vijay Singh 1 Department of Agricultural and Biological Engineering, University of Illinois at Urbana-Champaign, 360G, 1304 West Pennsylvania Avenue, Urbana, IL 61801, USA Received 22 June 2007; received in revised form 30 December 2007; accepted 3 January 2008 Abstract Distillers dried grains with solubles (DDGS) is a coproduct of the dry grind corn to fuel ethanol process. Recently, we showed that Elusieve process, the combination of sieving and elutriation (air flow), was effective in separating fiber from DDGS. DDGS was sieved into different fractions and the sieved fractions were air classified to separate fiber. In this study, we determine relevant physical properties and terminal velocities of fiber and nonfiber that govern fiber separation from DDGS. Particle densities were determined using a pycnometer and equivalent spherical diameters were calculated from mean particle volume, obtained as mean particle mass divided by particle density. Sphericity of fiber was calculated as the ratio of surface area of sphere having the same volume as fiber to the actual surface area of fiber. Sphericity of nonfiber was calculated from experimentally determined terminal velocities of nonfiber and using the correlation between terminal velocity and sphericity. Terminal velocities of fiber and nonfiber are the lowest air velocities at which fiber and nonfiber, respectively, were carried by air. Particle density of fiber was higher than particle density of nonfiber. It was visually observed that fiber was flat and nonfiber was not flat. Sphericity of fiber and nonfiber ranged from 0.19 to 0.47 and 0.72 to 0.92, respectively. Within each sieved fraction, fiber particles had lower or comparable equivalent spherical diameters relative to nonfiber particles. Fiber particles experienced higher drag force and aspirated at low air velocities because: (1) they had lower or comparable mass relative to nonfiber particles and (2) their shape was flatter than nonfiber particles. The difference in shapes and equivalent spherical diameters of fiber and nonfiber within each sieved fraction resulted in difference in their terminal velocities within sieved fractions. This difference in terminal velocities allows separation of fiber from DDGS. Sieving of DDGS into fractions and then blowing air through sieved fractions was effective in separating fiber. © 2008 Elsevier B.V. All rights reserved. Keywords: Distillers dried grains; DDGS; Fiber; Terminal velocity; Air classification 1. Introduction Fuel ethanol production from cereal grains, like corn, is increasing rapidly due to the need for alternate energy sources [1]. In the dry grind process, starch in the corn is converted to ethanol and the remaining corn components (protein, fat, fiber and ash) form the coproduct, distillers dried grains with solubles (DDGS). Fiber separated from cereal grains would be a valuable coproduct, which can be used to produce additional “cellulosic” ethanol [2]. Fiber can be used as combustion fuel, cattle feed and ∗ Corresponding author at: Department of Agricultural and Biological Engi- neering, Mississippi State University, Box 9632, MS State, MS 39762, USA. Tel.: +1 662 325 8536; fax: +1 662 325 3853. E-mail addresses: radha@abe.msstate.edu (R. Srinivasan), vsingh@uiuc.edu (V. Singh). 1 Tel.: +1 217 333 9510; fax: +1 217 244 0323. to produce products such as corn fiber gum, oligosaccharides, phytosterols and polyols [3–6]. Recently, we showed that Elusieve process, the combination of sieving and elutriation (air flow), was effective in separating fiber from two commercial DDGS samples [7]. Though siev- ing and air classification had been investigated individually for producing fiber rich fractions from DDGS [8,9], no previous work had attempted the combination of sieving and air classi- fication for separating fiber from DDGS. Air classification of whole DDGS did not result in effective fiber separation [9]. Sieving DDGS into different sieved fractions and then air clas- sifying the sieved fractions resulted in effective fiber separation [7]. Recently, we also showed that Elusieve process was effective in separating fiber from corn meal flour [10]. The combination of sieving and air classification has been utilized, though inefficiently, in wheat flour milling. Wheat middlings, a process stream, is subjected to simultaneous air 1383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2008.01.002