(717d) Volume Fraction Dependence of Linear Viscoelasticity of Starch Suspensions

Starch is unique among carbohydrates because it occurs naturally as discrete particles, called granules which consist of a linear polysaccharide (amylose) and a branched polysaccharide (amylopectin). The material properties of starch vary widely depending on the physical and chemical properties the granules, which allows starch to find use in a wide range of applications including food stabilizers, gelling agents, liquid creams, fabric stiffeners, and binders for pharmaceuticals and paper products. Motivated by a need to develop rational guidelines for processing such products, we investigate the rheology of maize and rice-based starch dispersions in this study. The rheology of these dispersions is interesting from a scientific standpoint because the size and deformability of the granules change in time due to water uptake, which gives rise to thixotropy. The evolution of volume fraction φ and linear viscoelasticity of 8% w/w suspension of waxy and normal maize, waxy and normal rice starch when heated at different temperatures in the range of 60 to 90 °C were characterized by particle size distribution and G’, G” in the frequency range of 0.01 to 10 Hz respectively. The pastes exhibited elastic behavior with G’ being much greater than G”. G’ increased with time for waxy maize and rice starch at all times. For normal maize and rice starch, however, G’ reached a maximum and decreased at longer times for temperatures above 80 °C due to softening of granules as evidenced by peak force measurements. Experimental data of G’ vs φ for all starches subjected to different heating times and temperatures fall into a master curve except for conditions under which the granule becomes deformable, i.e. longer time data of normal maize and rice at temperatures above 80 °C. We complement these results with Stokesian dynamics simulations to predict the linear viscoelasticity of starch suspensions as a function of volume fraction. The insight from these simulations allow for better design of starch rheology, which will find use in many of the applications listed above.