Effect of ion-binding on the formation of temporary viscoelastic networks of proanthocyanidin biopolymers

Abstract

The gel formation of the alkali salts of proanthocyanidin polymers from Pinus radiata bark with calcium(II) and aluminum(III) has been investigated. The calcium- and aluminum-induced gelation of the hot water extract (HWE) has been studied by UV spectroscopy as well as through conductivity and rheological measurements. The ability of an HWE to complex was observed to increase significantly as the solution's pH became more alkaline, and the conductivity data are consistent with UV spectra with respect to structural change as metal binding occurs. The addition of the cationic species Ca2+ and Al3+ markedly increased the elasticity of the extracts, even at low extract concentration. Rheological measurements of metal binding in the HWE paralleled the UV spectral and conductivity behavior, thus indicating that the HWE binds more readily with calcium than with aluminum. In the concentrated HWE system, maximum association of the HWE with metal ions was found to appear at the same molar ratio of 3.6. The addition of such cationic species to the extracts is thought to reduce their fluidity primarily by increasing the number of physical electrostatic crosslinks. Such an increase results, in turn, in an increase in the apparent molecular weight of extract polymers and a decrease in fluidity. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:1795–1805, 1997