Thermoreversible gelation and scaling behavior of Ca2+-induced κ-carrageenan hydrogels

The effect of calcium cations (Ca2+) on thermoreversible gelation properties, microstructure, and scaling behavior of κ-carrageenan in aqueous solution was studied by micro-differential scanning calorimetry (micro-DSC), field emission scanning electron microscopy (FESEM) and rheology. It was found that κ-carrageenan molecules self-associated into fibrils, and the diameter and number of fibrils increased with increasing Ca2+ ion concentration. The formation and melting of κ-carrageenan hydrogel was thermally reversible and extremely sensitive to Ca2+ ions. The viscoelastic property at the gel point was examined using the Winter-Chambon criterion to obtain the critical relaxation exponent n and the critical gel strength Sg. n and Sg were found to decrease and increase with increasing Ca2+ ion concentration respectively. The normalized gel strength, Sg/Ca2+, was not independent of Ca2+ ion concentration, suggesting a strong effect of Ca2+ ions on both size and density of junctions formed during gelation. A schematic diagram has been proposed to explain how Ca2+ ions could facilitate the coil-helix transition, leading to the formation of more aggregates acting as junctions.