Rheology and Aggregation of Cactus (Opuntia ficus-indica) Mucilage in Solution

The mucilage obtained from cactus is commonly described as water-soluble pectin-like polysaccharide. Although few potential uses of this material have been described, it is not an industrial hydrocolloid. The objective of this study was to evaluate the rheological behavior of the polysaccharide isolated from the cladodes of Opuntia ficus-indica. The polymer had a weight average molecular mass (Mw) of 3 × 106 and a number average molecular mass (Mn) of 2.4 × 106, with a polydispersity index (Mw/ Mn) of 1.4, as assessed by SEC-HPLC. Both Mw and Mn values exceeded those previously reported for the polysaccharide from O. ficus-indica. This overestimation of molecular mass may indicate extensive formation of large macromolecular aggregates in solution. The viscoelastic behavior of concentrated mucilage solutions (0.4% to 5.8% w/w; NaCl 0.1 M at 20°C) was investigated under sensitive dynamic mechanical spectroscopy and steady-shear rheological tests. Frequency (ω) dependence of the mechanical storage (Gʹ) and loss (Gʹʹ) mechanical moduli described spectra characteristic of an entangled network system where the polysaccharide is in the ``random coilʹʹ fully disordered conformational state. The viscous (Gʹʹ) response predominating over the elastic (Gʹ) one at low ω, which, at higher ω, behave like an elastic solid (Gʹ > Gʹʹ). However, under steady-shear deformation tests, shearthinning curves deviated from the typical random-coil behavior well documented for many polysaccharides used as industrial thickeners. This behavior is rationalized also as a consequence of the formation of colloidal aggregates in agreement with SEC-HPLC evidence. The formation of these large aggregates may underlie some of the special functional properties already identified for the use of cactus mucilage in food and other products.