Influence of age on molecular characteristics and rheological behavior of nopal mucilage

Nopal mucilages are of interest due to a variety of potential applications, which include their use as thickeners and rheological modifiers. In this work, changes in molecular characteristics of nopal mucilages with their age and its influence on the rheological behavior of their solutions were analyzed by light scattering, high performance liquid chromatography, a colorimetric method, and linear viscoelastic rheometrical measurements. For this, mucilages obtained from cladodes from Opuntia ficus-indica with different ages, namely 20, 80, and 600 days, respectively, were extracted using water as solvent and then subjected to freeze-drying. The weight-average molecular weight (M_w) of the mucilages was found to increase along with the age; meanwhile, the concentration of uronic acids increased and the galactose, rhamnose, and xylose contents decreased. Increasing M_w with age resulted in enhanced viscoelastic behavior of solutions, namely, higher viscosity and elasticity at lower mucilage concentrations. Also, along with increasing M_w, decrease in neutral sugar contents and increase of pectic compounds (uronic acids) and Ca⁺ cations in mucilages with age promoted formation of weak gels. Overall, the molecular weights and rheological behaviors reported in this work for nopal mucilages are comparable to those of widely used thickeners such as xanthan and guar gums, which suggest their use in similar applications regarding its age.