Submitted to: Polymer Preprints
Publication Type: Proceedings
Publication Acceptance Date: August 21, 1999
Publication Date: N/A
Interpretive Summary: The behavior of dissolved starch is important for its use in many applications, both food and non-food. When prepared under certain conditions, starch solutions may show unusual behavior called shear- thickening. We prepared starch solutions by various methods to determine the source of this behavior. It was found that simple stirring led to shear-thickening, while agitating the solutions by shaking eliminated it. Visual examination of the solutions showed that shaking eliminated residual structure in the solution which remained after stirring only. It was shown that this structure was the source of the observed shear-thickening, and once removed, the structure did not reappear. These results are useful to cereal scientists and polymer scientists, as well as suppliers and users of starch materials.
Technical Abstract: When the starches were solubilized in most commonly used solvents (i.e., NaOH solution or Dimethylsulfoxide (DMSO)/H2O mixture), it is found that the solubilization procedure seriously affects the final state of the solutions. When the solution of the starches was stirred, the incompletely solubilized patches formed during the solubilization causes shear-induced aggregation yielding high viscosity and shows shear- thickening behavior (state I). On the other hand, when the solution was prepared by shaking, a clear solution with low viscosity was obtained (state II). Although both of these states are stable, only the transition from state I to II was allowed by shaking while the inverse transition was not induced by stirring. Only the state I solution showed shear-induced pattern, but the formed patterns disappeared after shaking the solutioon. These observations were explained by shear-induced aggregation of starches in solution. Optical micrographs and SEM images of these solutions suppor our interpretation.