Rheological properties of gellan solutions: effect of calcium ions and temperature on pre-gel formation

The critical overlap concentration (C∗) for dilute gellan dispersions was studied with respect to the effect of calcium ion concentration (0.25 and 0.5 mM Ca2+) and temperature (5–25 °C), using steady shear rheometry. C∗ was assumed to be associated with the formation of pre-gel structure at gellan concentrations close to the gelation threshold. As temperature decreased or Ca2+ concentration increased, C∗ declined. The temperature dependence of C∗ in the presence and absence of added Ca2+ could be described by the Eldridge–Ferry model, where heat of cross-link formation declined in a nearly linear manner with increasing levels of Ca2+. This was interpreted as the result of a shift from a system dominated by hydrogen bonds to one mediated by ionic interactions. An alternative model, developed by Tang, Tung, and Zeng (1997a) for an ion-mediated polymer gelation mechanism, determined the energy of cross-linking at C∗ to be only slightly less than reported for self-supporting gellan gels. This model also indicated that the number of Ca2+ ions involved in cross-linking two adjacent double helical gellan strands was three, which suggested that the minimum size of a junction zone was equivalent to a single pitch of the helix. In addition, the apparent viscosity of 0.1 and 0.2% (w/w) aqueous gellan dispersions was examined over a range of Ca2+ concentrations and shear rates. Results indicated an increase in apparent viscosity to a maximum, which corresponded to a 0.5:1 ratio of Ca2+ ions to gellan carboxyl groups. Upon further Ca2+ addition, apparent viscosity declined, indicating a weakening or breakdown of pre-gel structure.