EFFECTS OF SALTS ON THE GEL-SOL TRANSITION OF GELLAN GUM BY DIFFERENTIAL SCANNING CALORIMETRY AND THERMAL SCANNING RHEOLOGY

The rheological and thermal properties of sodium form gellan gum solut ions with and without sodium chloride, potassium chloride, calcium chl oride and magnesium chloride were studied by dynamic viscoelastic meas urement and differential scanning calorimetry, Temperature dependence of the loss modulus for gellan gum solutions of lower concentrations w ithout salt showed a one step-like change at a certain temperature, ho wever that for concentrated gellan gum solutions (>2.0%) showed two st ep-like changes. The higher temperature process T-hc may be attributed to the helix-coil transition and found in between the exothermic and endothermic peak temperatures T-s and T-m observed in cooling and heat ing DSC curves, while the lower temperature process T-sg may be attrib uted to the sol-gel transition. Temperature dependence for gellan gum solutions of higher concentrations (>3.2%) showed a large hysteresis, moreover, the temperature at which the loss shear modulus G '' showed the second step decrease shifted to higher temperatures with increasin g concentration of gellan gum. The cooling or heating DSC curves for g ellan gum solutions of lower concentrations showed a single exothermic or endothermic peak, and both exothermic peak temperature T-m and end othermic peak temperature T-m shifted to higher temperatures, and both exothermic and endothermic enthalpies increased with increasing conce ntration of gellan gum. However, for a gellan gum solution of a concen tration higher than 3.2%, the endothermic peak in the heating DSC curv e split into two peaks, while the cooling curve showed only one endoth ermic peak. The lower temperature endothermic peak in the heating DSC curve corresponds with the first step decrease of G '' in the heating process of rheological measurement, and the higher temperature endothe rmic peak corresponds with the second step decrease of G ''. The visco elastic change of gellan gum solutions was more remarkable by the addi tion of K+ than Na+, and by Ca2+ than by Mg2+. The viscoelastic behavi or of gellan gum solutions was influenced much more strongly by divale nt cations than by monovalent cations. DSC cooling curves of gellan gu m solutions showed a single exothermic peak shifting to progressively higher temperatures with increasing concentration of monovalent cation s. At low concentration of monovalent cations, the DSC heating curves showed a single endothermic peak, however, with more progressive addit ion of salt, the endothermic peak gradually developed bimodal characte r and eventually split into multiple peaks. With increasing concentrat ion of divalent cations, the exothermic and endothermic enthalpies est imated for a main peak increased up to a certain concentration and the n decreased. Moreover, the endothermic peaks in the presence of suffic ient salts were too broad to be resolved from the baseline, and many o ther peaks were observed at higher temperatures. Gellan gum solutions with sufficient divalent cations form firm gels on cooling to below th e setting temperature, and then it was difficult to remelt them, which was quite different from the behavior of thermoreversible gels formed in the presence of monovalent cations.