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 (airflow), 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. (c) 2008 Elsevier B.V. All rights reserved.