Role of Granule Size and Size Distribution in the Viscosity of Cowpea Starch Dispersions Heated in Excess Water

The role of granule size and size distribution, measured by laser diffraction, in affecting the flow behavior and yield stress of cowpea starch dispersions (2.6% w/w) heated for various time intervals at 67, 70, 75 and 80C was studied. Flow data on the dispersions at 20C were fitted to the power law flow equation. The standard deviation of the granules described the transition of flow behavior from shear thickening (dilatant) in the early stages of gelatinization to shear thinning (pseudoplastic) in the latter stages. It was an important variable for also correlating the critical shear rate, λ˙c, for the onset of shear thickening in starch dispersions. The granules swelled to a maximum of about 3.5 X raw starch granule mean diameter and a granule mass fraction of 65%. The consistency index of the dispersions increased with granule mean diameter.