Rheological investigation of alginate chain interactions induced by concentrating calcium cations

Chain interactions of sodium alginate during its gelation were investigated by a new gelation method which was based on a Ca2+-concentrating gelling process (CCGP) produced by water evaporation of an alginate solution containing CaCl2. For two commercially available sodium alginate samples (low viscosity (LA) and medium viscosity (MA)) having different molecular weight distributions but the same G/M blocks, the critical Ca2+ concentrations for their gelation were found to be 4.6 (for LA) and 4.5 (for MA) μmol/mL after evaporating water from 1% of alginate solutions containing 4 μmol/mL of CaCl2. The CCGP gelation method for alginate under the above conditions were confirmed by rheological measurements and the observed highly ordered and uniform mesh structure of the CCGP-formed alginate gels shown in cryo-SEM images. Combinations of LA and MA at different ratios (0:4, 1:3, 2:2, 3:1, 4:0 on weight basis) were studied using the CCGP gelation method to further the understanding of the alginate chain interactions during gelation. Different LA/MA mixtures exhibited different rheological properties in either non-gelled or gelled systems, indicating that the molecular weight distributions of the sodium alginates influence the alginate chain interactions mediated by Ca2+. Thus, an appropriate combination of LA and MA is required for a strong alginate interchain interaction during CCGP, and alginate products with desirable characteristics can be produced by manipulating the mixing ratios of sodium alginates having different molecular weight distributions even at the same total composition and distribution of G/M blocks.