An examination of the composite model for starch gels

Aqueous suspensions of starch granules, heated to above their gelatinization temperature, can cool to give rubbery gels. The current physical model of these systems attributes gelation to the association of amylose (linear polysaccharide), extracted from the granules into the continuous aqueous phase. The granules themselves are pictured as randomly dispersed in the matrix, reinforcing the gel in the manner that carbon black reinforces rubber. The characteristic increase in a composite's rigidity with the volume fraction of the reinforcing particles is studied in the present work, using an artificial starch gel, as a means of testing this physical picture. Granules freed of amylose are combined with purified amylose solution to give gels with particle volume fractions covering a wide range. A new technique is presented for preparing the gels at space filling granule volume fractions. The variation of the reinforcement is in reasonable agreement with that predicted by Nielsen's modification of Kerner's equation. Granules swollen at 85 degrees C are found to be less rigid than those swollen at 70 degrees C and their reinforcing attributes are correspondingly less. The weak dependence of the loss tangent on the granule volume fraction also agrees with prior experience in reinforced systems. The picture which emerges is thus entirely consistent with the current physical model of these systems. (C) 1996 Society of Rheology.