Association of κ-carrageenan subjected to deep alkaline hydrolysis

κ-carrageenan is a linear sulfated anionic gelling polysaccharide obtained from red seaweed algae by an alkaline hydrolysis. We applied static and dynamic light scattering (DLS), capillary viscometry, FT-IR, and electrophoretic DLS to gain insight into the effect of deep alkaline hydrolysis (95 °C, pH = 10 during T_h = 60, 75, and 90 min) on κ-carrageenan macromolecules in a coil conformation. As DLS of coil-like κ-carrageenans is usually complicated by spurious permanent aggregates, the alkaline hydrolysis of κ-carrageenans has not been studied by DLS. By applying a double-dialysis procedure (first against water, then against 0.1 M NaCl), we succeeded in obtaining dilute solutions of coil-like κ-carrageenans with unimodal DLS distributions. Association that started with increasing concentration was attributed to attractive dipole-dipole interactions between ion-pairs formed by condensed counter-ions in the vicinity of carboxyl groups of the reducing ends. The second type of stickers resulted from the compositional heterogeneity of κ-carrageenan, namely, from the traces of ι-carrageenan blocks which amount increased with hydrolysis. With increasing T_h we observed both the progressive decrease of molecular mass and the growth of association. Lowering the temperature and increasing concentration during lyophilization strengthened the intermolecular junctions, leading to the formation of permanent aggregates.