THERMOMECHANICAL BEHAVIOR OF CONCENTRATED STARCH-WATER PREPARATIONS

The theological behavior of concentrated starch preparations from vari ous 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 t reatments of the starch-water mixtures consisted of heating to 85 degr ees C and cooling to room temperature, both at a rate of 1 degrees C/m in. During heating, the storage modulus (E') appearance was first char acterized 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 degre es 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 d ecrease 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 he ating (up to the maximum E' peak) on the distribution of the granule s izes of the different starches. Moreover, differential scanning calori metry (DSC) was used to measure the temperature range where the meltin g of starches ordered regions occurred. Partial melting enthalpies wer e plotted against temperature. The hypothesis of a relationship betwee n swelling and an increase in rigidity during heating seemed to be con firmed by laser-light diffraction, whereas DSC indicated the decrease in rigidity was caused predominantly by order-disorder transitions. Du ring cooling, amylose gelation plays a major role in the rigidity incr ease, but a contribution of amylopectin is not excluded.