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 [h] 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 degrees C to 5 degrees 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 CoxeMerz rule were found to depend on both the gum concentration and temperature. (C) 2013 Elsevier Ltd. All rights reserved.