Rheological and thermal properties near the sol-gel transition of gellan gum aqueous solutions

Abstract

The effects of monovalent and divalent cations on the rheological and thermal properties of gellan gum aqueous solutions have been monitored using rheological measurements and differential scan ning calorimetry (DSC). The transition temperatures of coil-helix, T _ch, and sol-gel, T _sg, in gellan gum solutions were detected by both thermal scanning rheology and DSC. The phase diagrams of gellan gum solutions with and without monovalent cations consisting of three regions (sol-I, sol-II and gel) were proposed. The order of effectiveness of the monovalent cations in promoting ordered structures followed the Hofmeister series Cs⁺ > K⁺ > Na⁺ > Li⁺. The experimental results indicated that gelation of gellan gum occurred with the subsequent aggregation of helices, and the gellan gum systems where T _ch and T _sg occurred individually did not show thermal hysteresis in the cooling and heating processes; however, the gellan gum systems where T _sh and T _sg occurred concurrently involved the thermally stable junction zones formed by fairly well aggregated helices, and so these systems showed conspicuous thermal hysteresis. The gelation mechanism of gellan gum in the presence of divalent cations was substantially different from that with and without monovalent cations. On cooling, the divalent cations immediately interacted with gellan gum segments to form the specific ordered structures at temperatures higher than T _ch. With progressive addition of divalent cations, these ordered structures stabilized by divalent cations increased stoichiometrically and became extremely thermally stable. It is suggested from rheological results that the ordered structures involving the specific cation—polyanion interaction could lead to the formation of an elastic gel. Judging from the DSC results, they were gradually melted at various temperatures by the subsequent heating.