Packing characteristics of starch granules

The rheological properties of granular materials and dispersions of solid particles in fluids are dependent on the packing characteristics of the particles. Maximum packing fractions (phi (m)) have been measured for corn, wheat, rice, potato, and amaranth starches, in the dry state and dispersed in either ethanol or hexane, using a tapping method. The observed maximum packing fraction increases with tapping time to a constant value. Values measured for dry starches were lower than those measured in liquids and reflect the effects of granule shape and intergranular friction. Values measured in fluids for potato, corn, and wheat starches were all similar in magnitude, and in the range of values (0.58-0.63) for random loose packing and random close packing of monodisperse spheres. Values for amaranth and rice starches were significantly lower due to agglomeration and clumping of individual granules. Blends of corn and potato starches show a slight enhancement of packing. with some phi (m) values greater than potato starch, consistent with data for bimodal blends of spheres. Blends of rice and potato starches displayed enhanced packing above ideal mixing but did not exceed the packing fraction of the potato starch. Knowledge of starch pricking fractions is required for fundamental understanding of the rheological properties of granular starch-filled materials and important for predicting processing characteristics.