“Weak Gel”-Type Rheological Properties of Aqueous Dispersions of Nonaggregated κ-Carrageenan Helices

Abstract

The rheological properties of κ-carrageenan helices dispersed in an aqueous medium, which prevents aggregation of helices, were investigated. A dispersion of 1.5% w/w nonaggregated κ-carrageenan helices exhibited gel-like dynamic mechanical spectra at 20 °C; that is, the storage modulus G‘ predominated over the loss modulus G‘‘ in the entire frequency range examined (0.5−100 rad/s). However, the observed slight frequency dependence of the moduli and the relatively large value of tan δ (= G‘‘/G‘ > 0.1) were typical of so-called weak gels. The magnitude of G‘ of the κ-carrageenan weak gels was less than that of conventional gels formed by 0.15% w/w κ-carrageenan in an aggregating condition at 20 °C. Under large deformation, enough for the conventional gels to rupture, the weak gel systems flowed but never ruptured, suggesting that the weak gel-type rheological properties of the κ-carrageenan dispersions were due to a sufficiently long relaxation time of topological entanglements among double-helical conformers but not due to the formation of a three-dimensionally percolated permanent network. Keywords: Weak gel; κ-carrageenan; viscoelasticity; coil−helix transition; sol−gel transition