Dynamic rheological measurements and analysis of starch gels

The effect of starch modification and concentration on the dynamic shear properties of starch gels in water was studied. The four types of starches used were native corn starch, medium hydroxypropylated distarch phosphate, highly hydroxypropylated distarch phosphate, and distarch phosphate. Concentrations of 4, 6, 8, and 10% (w/w) in distilled water were used for each starch type. All starches showed viscoelastic solid behavior at higher concentrations (6-10% w/w). Critical gelling concentrations were found to be 4, 4, 6, and 6% for native corn starch, medium hydroxypropylated distarch phosphate, highly hydroxypropylated distarch phosphate, and distarch phosphate, respectively. Dynamic measurements were performed to study the effect of concentration and the extent of modification on starch gel properties. The linear viscoelastic region for each type of starch was determined using dynamic strain sweep. Both the elastic and viscous modulus showed only slight dependence upon frequency. The starch gels were classified as weak gels on the basis of their mechanical spectra. Viscous moduli (G″) showed a higher dependence upon frequency compared to elastic moduli (G′) for all starch types. Equilibrium shear moduli (G_e) were determined by extending the spectra to zero frequency. Increasing concentration increased the values of G_e. Fractional differential model (FDM) was used to model the frequency dependence of the gels. A higher degree of cross-linking resulted in a more shear-resistant gel.