Effect of ammonium salts on rheological and thermal properties of kappa-carrageenan gels

In order to clarify the relationship between structure and properties, dynamic viscoelastic measurements and differential scanning calorimetry were carried out for kappa-carrageenan gels containing ammonium salts. Elastic modulus E' of kappa-carrageenan gels as a function of the concentration of NH(4)F, NH(4)Cl, NH(4)Br or NH(4)I increased with increasing concentration of salt and then decreased. In the case of gels containing iodide ions, the peak of E' increased and shifted to higher concentrations. This has been attributed to the rearrangement of kappa-carrageenan molecules towards a more stabilized structure in the presence of iodide ions. The endothermic peak temperatures accompanying gel-to-sol transitions shifted to higher temperature with increasing concentration of salt. This has been attributed to the shielding of the electrostatic repulsion of sulphate groups in kappa-carrageenan molecules by ammonium ions as in the case of the addition of alkali metal or alkali earth metal ions or guanidinium ions. In both thermal and rheological properties, gels containing iodide ions showed different behaviour to those containing bromide, chloride or fluoride ions.