Rheological properties of the mucilage gum ( Opuntia ficus indica)

Abstract

The rheological properties of aqueous solutions of the mucilage isolated from Opuntia ficus indica have been examined. Steady-shear viscosities in a range of shear rate from 1 to 300 s −1 were observed as a function of mucilage concentration, temperature, pH and ionic strength. A non-Newtonian shear-thinning behavior was observed. A quite important increase in pseudoplasticity (or shear thinning) due to increase in the mucilage concentration, in the range from 1 to 10% (w/w) was showed. In the range of shear rates examined, viscosity values of a mucilage solution at 10% are similar to those shown by a xanthan 3% aqueous solution, both at 25°C. The Ostwald–de Waele or power law model successfully correlated the viscosity–shear rate data. The viscosity was dependent on ionic strength, as in the case of polyelectrolytes, and decreased with ionic strength. This behavior was more pronounced when using divalent cations. A marked dependence of viscosity on pH was also observed, as pH was increased from acidic to alkaline conditions, the viscosity increased. Elastic steady-shear properties were evaluated as the primary normal stress difference. Mucilage aqueous solutions showed high elastic properties, similar to high elastic synthetic polymers like polyisobutylene. At constant shear stress, the normal stresses decreased as the mucilage concentration was increased. The mechanical spectra in the linear viscoelasticity range were observed at a frequency range from 1 to 50 rad/s, in the temperature range from 5 to 35°C. At low mucilage concentration (<3%) the behavior was typical of a dilute solution. However, with increasing concentration, the solution tended toward a weak gel behavior. A cross-over point between G′ and G″ was observed at low frequencies (≈10 rad/s) and 35°C, for a mucilage concentration of 5%, suggesting a change in the conformation of the molecule from entanglements to a random coil configuration. Results from steady-shear measurements also suggested a random coil configuration.