Physicochemical and rheological properties of mucilage extracted from Opuntia ficus indica (L. Miller). Comparative study with guar gum and xanthan gum

The physicochemical and rheological properties of aqueous solutions of the mucilage isolated from Opuntia ficus indica (L. Mill) at different concentration (0.5, 1, 1.5 and 4.5% w/v) were examined. The intrinsic viscosity [eta] found for precipitate mucilage (PM) and dialyzed mucilage (DM) were 22.6 and 15.3 dl/g respectively. Electrophoretic measurements showed that the zeta potential of PM and DM was negative in all the pH range studied. PM reduced the surface tension of water and was concentration dependent. The surface activity of PM (57 mN/m) was similar to that of guar gum (55 mN/m) and xanthan gum (52 mN/m) at 1.5% (w/v) concentration. A non-Newtonian shear-thinning behavior was observed. The Ostwald-de Waele model successfully correlated the viscosity-shear rate. At equal hydrocolloid concentration, the consistency coefficients (k) of mucilage solutions were lower than those of guar gum (GG) and xanthan gum (XG). However, the consistency coefficient of a mucilage solution at 4.5% (w/v) (20.9 Pa s) was in the same order as that shown by GG at 1.5% (w/v) (28.8 Pa s) aqueous solution. No effects of pH and ionic strength on the viscosity of PM and DM were observed. The mechanical spectra showed a crossover point between G' and G" at low mucilage concentrations of 0.5 and 1% (w/v), exhibiting higher relaxation time than GG. The more concentrated solutions of PM and DM (1.5 and 4.5% w/v) displayed a predominant elastic behavior and did not meet the Cox-Merz rule, similarly to GX. The more concentrated solutions of PM and DM did not meet the Cox-Merz rule. This behavior would suggest the formation of colloidal aggregates.