Impact of ghosts on the viscoelastic response of gelatinized corn starch dispersions subjected to small strain deformations

Corn starch dispersions (5.0% w/w) were gelatinized by heating at 90 °C for 20 min using gentle stirring. Under these conditions, ghosts, which are insoluble material with high amylopectin content, were detected by optical microscopy. Strain sweep tests showed that the gelatinized starch dispersions (GSD) exhibited a loss modulus (G″) overshoot at relatively low strains (∼1%). In order to achieve a greater understanding as to the mechanisms giving rise to this uncharacteristic nonlinear response at low strains, very small constant torques (from 0.05 to 0.5 μN m) were applied in the bulk of the GSD with a rotating biconical disc. This resulted in small deformations exhibiting torque-dependent inertio-elastic damped oscillations which were subjected to phenomenological modelling. Inertial effects played an important role in the starch mechanical response. The model parameters varied with the magnitude of constant small applied torque and could be related to microstructural changes of ghosts and to the viscoelastic response of GSD.