Gelation of gellan gum

Abstract

The microbial polysaccharide, gellan gum, was studied in aqueous solution and in the gel state by osmometry, viscometry, light scattering, polarimetry and NMR and by measurements of gel strength and cation-exchange selectivity. In solutions of the tetramethylammonium (TMA) salt of the polymer, increasing concentrations of TMACl induced contraction, ordering, and association of the chains, as revealed by viscometry and changes in optical rotation. Chain ordering occurred at ionic strengths (I) between 0·005 and 0·05 ( m), and light scattering indicated an elongated chain structure in 0·025 m TMACl. Gelation was dependent upon both ionic strength and the identity of the cation. For monovalent cations at I = 0·1 ( m ) , gel strength increased in the order: TMA + < Li + < Na + < K + < Cs + < H + For divalent cations the series was: Mg 2+, Ca 2+, Sr 2+, Ba 2+ < Zn 2+ < Cu 2+ < Pb 2+ The lack of specificity among the alkaline-earth cations distinguished gellan gum markedly from other uronic-acid containing polysaccharides like alginate and pectin. Studies of cation-exchange equilibria likewise showed no selectivity within the alkali-metal or the alkaline-earth metal cations, but for all the divalent ions the affinity series: Mg 2+, Ca 2+, Sr 2+, Ba 2+ < Zn 2+ < Cu 2+ < Pb 2+ was the same as the series for increasing gel strength. Measurements of optical rotation and NMR spectroscopy indicated that chain association and gelation were related phenomena. The results are interpreted as indicating that gelation occurs in two steps, namely, chain ordering and chain association.