Influence of granule size on the flow behaviour of heated rice starch dispersions in excess water

The rheological properties of both waxy and non-waxy rice starch cooked in excess water have been studied in relation to their swelling behaviour by varying both cooking temperature (50–100 °C) and concentration (1–6%). Despite a completely different swelling behaviour with temperature, a unique linear relationship was obtained between starch weight fraction cQ, determined by means of swelling experiments and granule volume fraction Φ, assessed using particle size analysis. The rheological properties of these starch dispersions was strongly dependant on both shear rate and granule volume fraction and were reminiscent of concentrated dispersed systems with a sharp rise in relative viscosity near the maximum packing fraction. The evolution of this maximum packing fraction with shear indicated some orientation/deformation of the swollen granules. The modelling of the evolution of viscosity with shear at constant volume fraction with one relaxation mechanism allowed for the estimation of the time scale of this relaxation. The dramatic increase of this relaxation time with the cooking temperature proved that this relaxation time corresponded to deformation of the swollen granules under shear.