Rheological properties of sodium alginate in an aqueous system during gelation in relation to supermolecular structures and Ca2+ binding

Gelation behavior of sodium alginate was investigated in an aqueous system at various CaCO3 doses using two alginate samples of different mannuronate (M)/guluronate (G) ratios but with comparable molecular masses. Macroscopic rheological properties of the polysaccharide were discussed during gelation in relation to microscopic supermolecular structures imaged by atomic force microscopy and Ca2+ binding. At fixed concentrations of the polysaccharide (0.5 w/v%) and glucono-δ-lactone (15 mM) as an acidic material, the G-rich sample was higher in elasticity with rod-like molecular assemblies at the highest CaCO3 dose; 15 mM, while the M-rich sample was higher in elasticity with network-like molecular assemblies at the lowest CaCO3 dose; 3.75 mM. Gelation behavior near the sol-to-gel transition was generally described by the percolation model, except for the M-rich sample at the lowest CaCO3 dose. A critical CaCO3 dose corresponded to 7.5 mM at which both alginate samples showed essentially the same gelation kinetics. In an equilibrium state, it was estimated that the G-rich sample was bound to larger amount of Ca2+ at the lowest CaCO3 dose, while the M-rich sample was bound to larger amount of Ca2+ at the highest CaCO3 dose. The amount of bound Ca2+ did not directly relate to elasticity of the system. Based on these results, dominant block structures are suggested for gelation.