Rheological and thermal studies of gel-sol transition in gellan gum aqueous solutions

Abstract

The rheological and thermal properties of sodium-form gellan gum solutions with and without sodium chloride, potassium chloride, calcium chloride and magnesium chloride were studied by dynamic viscoelastic measurement and differential scanning calorimetry. Temperature dependence of the loss modulus G″ for gellan gum solutions of lower concentrations without salt showed one step-like change at a certain temperature, however, that for concentrated gellan gum solutions (> 2.0%) showed two step-like changes. The higher temperature process was attributed to the helix-coil transition, and was found in between 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 gellan gum solutions of higher concentrations (> 3.2%) showed a large hysteresis, moreover, the temperature at which G″ showed the second step decrease in the heating process shifted to higher temperatures with increasing concentration of gellan gum. The cooling or heating DSC curves for gellan gum solutions of lower concentrations showed a single exothermic or endothermic peak, and both T s and T m shifted to higher temperatures, and both exothermic and endothermic enthalpies increased with increasing concentration of gellan gum. However, for a gellan gum solution with a concentration higher than 3.2%, two endothermic peaks were observed on heating while the cooling curve showed only one exothermic peak. The lower temperature endothermic peak corresponds to the first step decrease of G″ in the heating process, and the higher temperature endothermic peak corresponds to the second step decrease of G″. The viscoelastic behavior of gellan gum solutions was influenced much more strongly by divalent cations than by monovalent cations. DSC heating curves for a 1% gellan gum solution in the presence of sufficient monovalent cations showed multiple endothermic peaks, however, these multiple peaks were observed at significantly higher temperatures than those for a gellan gum solution of high concentration (3.2%) without salts. The endothermic peaks in the presence of sufficient divalent cations in the heating DSC curve were too broad to be resolved from the baseline, however, many small peaks were observed at higher temperatures and the largest endothermic peak was observed at a temperature higher than 100 °C. Therefore, although divalent cations promote the formation of thermally stable junction zones much more strongly than monovalent cations, the gellan gum gels containing divalent cations may also consist of physical junction zones of hydrogen bonds because these junction zones can be unzipped on heating to 120 °C.