Rheological and thermal properties near the sol-gel transition in gellan gum aqueous solutions and mixed polysaccharides

Rheological and thermal properties of sodium form gellan gum (GELL) solutions with and without a salt have been monitored by oscillatory measurement, steady-shear viscosity measurement and differential scanning calorimetry (DSC). The interaction between GELL and konjac glucomannan (KGM) with and without a salt has been studied by mechanical spectroscopy and DSC. Temperature dependence of the loss shear modulus G " for GELL solutions of low concentrations without a salt showed one step-like change at a certain temperature, but that for concentrated GELL solutions (>2.0%) showed two step-like changes. The higher temperature process was attributed to the helix-coil transition and was found inbetween the exothermic and endothermic peak temperatures, T-s and T-m, observed in cooling and heating DSC curves, while the lower temperature process was attributed to the sol-gel transition. Temperature dependence of G " for GELL solutions of higher concentrations (>3.2%) showed a larger hysteresis, and an endothermic peak for these solutions in a heating DSC curve split into more than two peaks, while the cooling curve showed only one exothermic peak. Divalent cations with lower concentrations could give stronger effects on the rheological properties of GELL than monovalent cations. DSC results indicated that the gelation mechanism in GELL with divalent cations was markedly different from that with monovalent cations. The steady-shear viscosity measurements indicated that as GELL molecules took a coil conformation, GELL solutions showed a wide range of Newtonian plateau, and that the shear-thinning behavior became more conspicuous with conformational change of gellan gum molecules from coil to helix. The rheological results indicated that the synergism in GELL and KGM mixtures occurred at sufficiently low temperatures where individual helices of GELL molecules were aggregated. With progressive addition of monovalent cations, the storage shear modulus G' and loss shear modulus G " for mixtures gradually increased, and not only the helix-coil transition temperature of GELL molecules in mixtures but also the sol-gel transition temperature for mixtures shifted to higher temperatures with increasing concentration of salts. Moreover, in the presence of sufficient monovalent cations, mixtures formed elastic gels with a large thermal hysteresis. In the presence of divalent cations, the synergistic interaction was promoted up to a certain concentration, but the excessive addition of divalent cations suppressed the synergistic effect. It was suggested that mixtures of gellan and kappa-carrageenan formed phase separated gels.