Physicochemical and rheological characterization of Prosopis juliflora seed gum aqueous dispersions

The gum from Prosopis juliflora seed was isolated and its protein (0.60%), fat (0.55%) and total carbohydrate (98.46%) percentages, specific rotation (+64.01), intrinsic viscosity (1178 mL/g) and calculated molecular weight were determined. This gum turned out to be a galactomannan with an M/G ratio of 1.74:1. The rheology of concentrated aqueous dispersions of this gum was studied as a function of gum concentration in the (0.6–1.4) %w/v range. The mechanical spectra were consistent with the occurrence of random-coil macromolecular solutions forming entanglement networks. The terminal relaxation time estimated from mechanical spectra increased with gum concentration. The crossover frequency, terminal relaxation time and corresponding viscoelastic moduli of the 1% w/v dispersion were compared to those of commercial and non-traditional galactomannan gums. Steady-shear flow curves showed a low-shear Newtonian region, shear thinning behaviour above a critical shear rate and fitted the 2-parameter empirical model proposed by Morris (1990). The specific viscosity derived from the zero-shear viscosity scaled with C [η] with an exponent of 3.7. An Arrhenius-type equation fitted the temperature dependence of the zero-shear viscosity and of the apparent viscosity at 10 s−1, for the 1% w/v dispersion. The energy of activation for the latter shear rate was much lower than for the former. Decreasing temperatures from 20 °C to 5 °C hardly influence the critical stress amplitude but brought about a small increase in the terminal relaxation time and greater values of G′ and G″. Deviations from the Cox–Merz rule were found to depend on both the gum concentration and temperature.