Durcilene A. Silva

Effect of mono and divalent salts on gelation of native, Na and deacetylated Sterculia striata and Sterculia urens polysaccharide gels

The effects of purification and salt addition on gelation of Sterculia striata and Sterculia urens (karaya) gels were investigated. Both gums form thermoreversible and ‘true’ gels. Melting transition temperature (Tm) of Na-polysaccharide sample was lower than that of the native gum. A gel-like mechanical spectrum is observed for two Sterculia gums with G′>G″ and little frequency dependence. The presence of acetyl groups in both gums seems to stabilize the gel. The Tm decreases and the critical gelation concentration increases after deacetylation. The addition of monovalent salts (LiCl, NaCl, KCl) to native S. striata polysaccharide decreases the Tm. However, the gel becomes stronger when monovalent salts are added to Na- and deacetylated gum. The gel strength of Na- S. striata polysaccharide in the presence of divalent salt follows the order: Mg2+>Ca2+>Sr2+>Ba2+, i.e. inversely proportional to the cationic radius. Addition of divalent salts to S. urens polysaccharide promotes a Tm decrease. A model for the gelation was proposed that includes ionic interactions and hydrogen bonding.

Composition and effect of salt on rheological and gelation properties of Enterolobium contortisilliquum gum exudate

The composition, structure and rheological properties and metal ions interaction with Enterolobium contortisilliquum gum were investigated. This gum contains galactose, arabinose, rhamnose and glucuronic acid as main monosaccharide components. 13C nuclear magnetic resonance spectroscopy revealed that the anomeric composition is similar to the Enterolobium cyclocarpum exudate, however no 4-O-methylglucuronic acid was detected for E. contortisilliquum. The rheological experiment resulted in a very similar flow behaviour to that found for E. cyclocarpum gum. The empirical stiffness parameter, B was determined (B=0.090) and suggested that the polysaccharide possesses semi-flexible chains. Cation affinity of the gum aqueous solution was determined by intrinsic viscosity measurements and gelation. The melting temperature of gels pointed to a charge/ionic radius ratio dependence for metal ions. Thermodynamic parameters (enthalpy and entropy) at the melting temperature were also calculated.