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.