Structure conformational and rheological characterisation of alfalfa seed (Medicago sativa L.) galactomannan

Abstract

In the present work a galactomannan extract of low protein residue (< 1.3 % wt dry basis) was isolated from alfalfa (Medicago sativa L.) seed endosperm meal. The alfalfa gum (AAG) comprised primarily mannose and galactose at a ratio of 1.18:1, had a molecular weight of 2 × 10⁶ Da and a radius of gyration of 48.7 nm. The average intrinsic viscosity of the dilute AAG dispersions calculated using the modified Mark-Houwink, Huggins and Kraemer equations was 9.33 dLg^-1 at 25 °C. The critical overlap concentration was estimated at 0.306 % whereas the concentration dependence of specific viscosity for the dilute and semi-dilute regimes was ∝ C^2.3 and C^4.2, respectively. The compliance to the Cox-Merz rule was satisfied at 1% of AAG, whereas a departure from superimposition was observed at higher concentrations. Viscoelasticity measurements demonstrated that AAG dispersions exhibit a predominant viscous character at 1 % wt, whereas a weak gel-like behaviour was reached at AAG concentrations ≥3 %.

Keywords: Flow behaviour; Galactomannan; Intrinsic viscosity; Lucerne; Molecular weight; Oscillatory thermo-rheology; Viscoelasticity.