Solution rheology of mesquite gum in comparison with gum arabic

Abstract

Commercial samples of mesquite gum and food-grade gum arabic were purified by filtration, alcohol precipitation, and extensive dialysis, and their Theological properties were characterised over the full range of concentrations at which solutions could be prepared (up to ~50% w/w). Both gave typical solution-like mechanical spectra, with close Cox-Merz superposition of η (⋗g) and η ∗(ω) and only slight shear thinning at the highest accessible concentrations, and (ln η rel) c varied linearly with log c from below 2% w/w to above 50%. The intrinsic viscosity of mesquite gum ([η]≈ 0.11 dl g −1) was appreciably lower than that of gum arabic ([η]≈ 0.19 dl g −1 in 0.1 m NaCl at 20 °C), and was independent of ionic strength above I ≈ 0.05, indicating a compact structure capable of only limited contraction. Departures from dilute-solution behaviour (η ~ c 1.4) occurred at c [η]≈ 1 for both materials, with a progressive increase in concentration dependence at higher space-occupancy, behaviour typical of soft, deformable particles, rather than of interpenetrating macromolecules. The increase in viscosity with increasing concentration was steeper for mesquite, consistent with evidence from size-exclusion chromatography and dynamic light scattering that the larger (and presumably more deformable) ‘wattle blossom’ component of gum arabic was absent from the mesquite gum sample.