Thermomechanical behavior of concentrated starch-water preparations

The theological behavior of concentrated starch preparations from various origins was studied by dynamic mechanical thermal analysis (DMTA). Four types of starch were used: wheat, potato, normal, and waxy corn adjusted to moisture contents in the 42-49% (w/w) range. The thermal treatments of the starch-water mixtures consisted of heating to 85 degrees C and cooling to room temperature, both at a rate of 1 degrees C/min. During heating, the storage modulus (E') appearance was first characterized by an increase with a maximum at approximate to 70 degrees C (or potato starch at 63 degrees C) followed by a decrease to 85 degrees C. During cooling, storage modulus increased steadily down to room temperature. The magnitude of these variations depended on the starch type. Despite some differences, all the loss tangent curves showed a decrease during heating from 60-70 degrees C to 85 degrees C, followed by a plateau during cooling. To propose an interpretation for the DMTA results, we measured, by laser-light diffraction, the influence of heating (up to the maximum E' peak) on the distribution of the granule sizes of the different starches. Moreover, differential scanning calorimetry (DSC) was used to measure the temperature range where the melting of starches ordered regions occurred. Partial melting enthalpies were plotted against temperature. The hypothesis of a relationship between swelling and an increase in rigidity during heating seemed to be confirmed by laser-light diffraction, whereas DSC indicated the decrease in rigidity was caused predominantly by order-disorder transitions. During cooling, amylose gelation plays a major role in the rigidity increase, but a contribution of amylopectin is not excluded.