Formation of calcium-mediated junction zones at the onset of the sol-gel transition of commercial kappa-carrageenan solutions.

Gelling temperatures of commercial kappa-carrageenan solutions were measured using dynamic oscillatory shear rheometry, in response to varying polymer (0.4% to 1.6%, w/w) and added Ca(2+) (5 to 20 mM) salts. Gelling temperatures were found to shift to higher temperatures with increased polymer and Ca(2+) concentration. The Tang model was used to estimate the number of Ca(2+)-mediated cross-links formed with kappa-carrageenan at the onset of junction zone formation. Based on this model, 6 kappa-carrageenan double helices were estimated to be cross-linked by 5 Ca(2+) ions through ionic bridging and electrostatic attraction, within a cross-linking region equivalent to one pitch of the double helix.