Dissolution and gelation of κ-carrageenan

The rheology and conformational helix transition of κ-carrageenan in aqueous solutions of glycerol is studied. Carrageenan forms a hydrated gel in water but a weakly flocculated network of partially hydrated particles in glycerol. The melting transitions of carrageenan in glycerol/water solutions show a single peak in the complex moduli at low and high glycerol concentrations, and two peaks at approximately equal concentrations of glycerol and water. These are thought to be due to the breakage of hydrogen bonds (low temperature peak) and ion dissociation (high temperature peak). The helix-coil transition is shifted to higher temperatures in solutions with higher glycerol content. Comparison of the rheology and OR curves reveals that, though the gelation and conformational transition temperatures correspond for samples in water and also in glycerol, there is more hysteresis between the heating and cooling curves at high water concentrations. This suggests that gelation occurs through helix aggregation in water but the lower solubility of carrageenan results in a more random network of helices in glycerol.