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

Abstract

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 (?) dependence of the mechanical
storage (G?) and loss (G??) mechanical moduli described spectra characteristic of an entangled
network system where the polysaccharide is in the ``random coil?? fully disordered
conformational state. The viscous (G??) response predominating over the elastic (G?) one at low
?, which, at higher ?, behave like an elastic solid (G? > G??). However, under steady-shear
deformation tests, shear- thinning 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.