Rheology and microstructure of κ-carrageenan under different conformations induced by several concentrations of potassium ion

Rheology, micro-DSC and confocal microscopy were used to study the effect of potassium ion on the viscoelastic behavior, disorder-order transition and microstructure, respectively, of κ-carrageenan in solution under different conformations at 60, 25 and 9°C. At 60 and 25°C the rheological behavior of 0.5% κ-carrageenan with 0-80mmol/dm³ and 0-5mmol/dm³ KCl, respectively, was typical of viscoelastic solutions of random coiled polymers. At 9°C and below a critical ionic concentration of about 7.0mmol/dm³, κ-carrageenan adopted an ordered conformation in which helical structures did not aggregate and hence did not form self-supporting gels. Changes in polysaccharide stiffness were estimated from intrinsic viscosity variations as a function of ionic content. In the ordered state, the stiffness was higher than in the disordered state, whereas a liquid-like viscoelastic behavior was still exhibited. In 0.5% κ-carrageenan at 25°C, increasing KCl from 0 to 300mmol/dm³ produced gels of increasing rigidity. However, above 100mmol/dm³ such increase was marginal. Confocal images evidenced a three-dimensional network whose continuity depends on polysaccharide and salt concentrations. These observations are consistent with the rheological behavior of the self-supporting gels obtained with κ-carrageenan concentrations in the range of 0.05-1%.