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 0–80 mmol/dm3 and 0–5 mmol/dm3 KCl, respectively, was typical of viscoelastic solutions of random coiled polymers. At 9 °C and below a critical ionic concentration of about 7.0 mmol/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 300 mmol/dm3 produced gels of increasing rigidity. However, above 100 mmol/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%.

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