Acidified/basified gellan gum gels: The role of the structure in drying/rehydration mechanisms

This work investigates the influence of pH variation on gellan gum gel structure upon drying and rehydration. A comparison of the texture of wet gels prepared in acid and basic conditions showed that decreasing the pH of the gel to the pKa resulted in an increase in the gel strength and the Young's modulus. On further lowering the pH, both properties decrease dramatically. Post-gelation exposure to water and basic solution led to a reduction in mechanical properties, whereas exposure in acidic conditions resulted in gels with increased strength. The effect of the polymer concentration was investigated at pH 3.5 (corresponding to the highest strength and elastic modulus); it was observed that the gels set, even at very low concentrations, and that 0.5% w/w generated a gel as strong as the gel 2% w/w at natural pH. Thermal (DSC) and structural (FT-IR) analyses showed that when decreasing the pH down to the pKa, the polymer chains within the junction zones interact more strongly to each other and the structure has fewer disordered chains; below the pKa, the network became more aggregated and phase separation between the solvent and polymer occurred. For all the gels prepared at different pHs, drying (air drying and freeze drying) and rehydration experiments were performed to investigate the water desorption/absorption rate in relationship to the gel structure. It was observed that the drying kinetics is not pH-sensitive. For both the drying techniques, it was observed that gels prepared at pH 4, recovered their initial shape, leading to higher rehydration ability.