Monocationic salts of carrageenans: Preparation and physico-chemical properties

Highly pure monocationic forms of κ-, ι-, λ-carrageenans and furcellaran were prepared with the aid of successive dialysis and ion exchange steps. Two approaches were used to obtain the samples: direct exchange of the galactan-bound cations with the aid of ion-exchange resin and generation of H+-forms of the polysaccharides followed by neutralization process. The galactan samples were characterized by 1H-NMR, 13C-NMR, pulsed-field-gradient stimulated echo 1H-NMR, FTIR, FT-Raman spectroscopy, ICP-OES, size exclusion chromatography, viscometry, static and dynamic rheometry methods. The ability of cations to enhance gelation follows the order of Li+ < Na+ ≪ Mg2+, Ca2+ < NH4+ < K+ for κ-type carrageenan and NH4+, Li+ < Na+ < K+ < Ca2+ for ι-carrageenan samples prepared by ion exchange. Pure K+-forms of κ-carrageenan and furcellaran form gels of moderate gel strength at 1.5%, but also Ca2+-ι-carrageenan affords workable gels. Mixed cationic content using both mono- and divalent cations promotes gel formation of κ-carrageenan more effectively compared to furcellaran. κ-Carrageenan forms exceptionally strong gels (G′ > 1 × 105 Pa) in the presence of equal amounts of Rb+ and Sr2+ ions. The rheological properties are notably pH-dependent. Among the monocationic samples, Ba2+-forms of κ-carrageenan and furcellaran yield the strongest gels in alkaline media, whereas Rb+- and Cs+-forms show the highest gelling capacities under acidic conditions. The widely employed method involving excessive dialysis against water as the final purification step causes cation depletion, acidification of the carrageenan sample and significant degradation during the freeze-drying step.