Rheological properties and surface tension of Acacia tortuosa gum exudate aqueous dispersions

Steady shear flow properties of Acacia tortuosa gum aqueous dispersions at different concentrations (15–40% w/v) were studied at 20 C using a controlled shear rate Couette rheometer and Mooney–Ewart sensor systems. Flow curves showed shear thinning non-Newtonian behaviour with a tendency to a Newtonian plateau at high-shear rate. The results obtained fitted the Sisko model very well. The influence of temperature (5–40 C) on the flow curves was studied at given gum concentration (40% w/v). The infinite-shear rate Newtonian viscosity derived from the Sisko model fitted an Arrhenius-type equation. A controlled stress rheometer was used to conduct small amplitude oscillatory shear tests with the double cone geometry on A. tortuosa gum dispersion (40% w/v). It was clearly observed that the gum dispersion exhibited viscoelastic properties in a given temperature range (5–25 C). The elastic component was always higher than the viscous one. The frequency dependence of the dynamic moduli and the failure of the Cox–Merz rule supported the existence of a soft gel-like structure which weakened with temperature. The surface tension of A. tortuosa aqueous solutions was measured in a concentration range (0.01–1.5% w/v). The equilibrium value reached at (0.5% w/v) was (42.6 mN/m), which is significantly lower than the corresponding to A. senegal gum (46.9 mN/m). The rheological behaviour and surface tension of A. tortuosa gum suggest that it may compete with arabic gum in some industrial applications. 2007 Elsevier Ltd. All rights reserved.