Precipitation solvents effect on the extraction of mucilaginous polysaccharides from Opuntia ficus-indica (Cactaceae): Structural, functional and rheological properties

Abstract

This paper discusses mucilage extracted from Opuntia ficus-indica (Cactaceae) cladodes, also known as Barbary fig. Mucilaginous polysaccharides were extracted in hot water and purified with two distinct organic solvents, ethanol and isopropanol. The mucilaginous extract yield precipitated in isopropanol "MUIS″ (i.e. 23.2%) is higher than that of ethanol "MUET″ (i.e. 19%). The biochemical composition of mucilaginous extracts shows that the change in precipitation solvents has no significant effect. In addition, the microstructure of mucilaginous extracts exhibits amorphous and porous structures, which favor their capacity to retain moisture. The mucilaginous extracts analysis by ion chromatography reveals that after acid hydrolysis, MUIS has a higher total of sugars (46.9%) than MUET (37.6%). Mucilaginous extracts vary in their main content of monomers such as arabinose, galactose, glucose, and xylose as well as in the uronic acid fraction. The contact angle measurements were also established in order to investigate mucilaginous extracts’ hydrophilic or hydrophobic surfaces. Three tested liquids were used namely: distilled water, diiodomethane, and ethylene glycol. The X-ray diffraction analysis displayed that the precipitation solvent does not affect the amorphous structure. The TGA analyses of extracts under an oxidative atmosphere reveal that the oxygen flow accelerates the kinetics of degradation. Under nitrogen flow, mucilaginous extracts have intrinsic thermal resistance in specific temperature ranges. The DSC results show that the polysaccharide purification technique influences the glass temperature (Tg), which is around 53 °C and 61 °C for MUET and MUIS, respectively. In an aqueous solution, the mucilaginous extracts are highly heterogeneous, and the precipitation solvents and temperature affect their particle size. Mucilaginous extracts were concentration-dependent and reduced the surface tension of water to 43.5 mN/m and 30 mN/m for MUET and MUIS at 7 w/v%, respectively. The rheological analyses show that a non-Newtonian shear-thinning behavior was observed. The Ostwald-de Waele model successfully predicted the viscosity-shear rate relationship. The consistency coefficient (k) of MUIS at 7 w/v% (0.117 Pa.s) is higher than MUET at 7 w/v% (0.081 Pa.s). The critical concentrations (C*) of MUET and MUIS are 1.75% and 3.45 w/v%, respectively.