Effects of potassium, sodium and calcium on the microstructure and rheological behaviour of kappa-carrageenan gels

Abstract

The effects of cations and mixtures of cations on kappa-carrageenan gelation was studied by electron microscopy and viscoelastic measurements. On cooling only potassium-kappa-carrageenan formed a transient fine structure at 54-52°C. The structure became unstable at 51°C, whereafter long stiff superstrands with two to three strands in parallel formed a coarse network. The balance between the fine and the coarse gel strands was dependent on the potassium concentration. In contrast to the potassium form, sodium-kappa-carrageenan formed relatively weak gels and showed almost no dependence on the sodium ion concentration. The sodium form had a network structure with flexible superstrands of uniform thickness. The calcium-kappa-carrageenan formed relatively weak gels in a limited calcium concentration range, whereafter salting-out effects were observed. The calcium-induced gels had a very fine network of an entanglement type without any of the coarse superstrands seen in sodium or potassium forms. A transient gel structure was not found for sodium- or calcium-kappa-carrageenan. Strong synergistic effects were found between calcium and potassium. Depending on the ratio of potassium to calcium, it was possible to make 1% gels in the temperature range 20–40°C with storage moduli varying from 70 to 43 000 Pa. Apart from adding KCl to the pure calcium form and CaCl 2 to the pure potassium form of kappa-carrageenan, intermediate ion forms were produced by ion exchange where the potassium to calcium ratio was varied between 0·2 and 0·8. Synergistic effects were also observed when sodium was added to potassium-kappa-carrageenan, but not when potassium was added to sodium-kappa-carrageenan. No synergistic effects were observed between calcium and sodium in any form.