Structure characteristics and rheological properties of acidic polysaccharide from boat-fruited sterculia seeds

An acidic polysaccharide (APS) was extracted from the boat-fruited sterculia seeds (Semen Sterculiae Lychnophorae). It consisted mainly of galacturonic acid (40.1%, w/w) along with rhamnose, arabinose, galactose, xylose and glucose as minor components, indicating a pectic polysaccharide which was confirmed by FT-IR spectra. The degree of esterification of APS determined by FT-IR method was 68%. The shear-thinning and viscoelastic behaviors of APS have been investigated by steady shear and small amplitude oscillatory experiments, respectively. Steady-shear rheological measurement in a range of shear rate (1–1000 s−1) showed increase in pseudoplasticity (or non-Newtonian shear-thinning flow behavior) with the increase in APS concentration (1.0–10%, w/v). APS itself could not form a gel; however, thermal irreversible gels were obtained in the presence of sucrose at low pH (pH < 3.5). The changes in storage modulus G′ and loss modulus G″ during heating and cooling cycles indicated that G′ was nearly reversible at temperature >30 °C while G″ exhibited thermal hysteresis.