Calcium-dependent solubility and rheological properties of some plant polysaccharides

Abstract

Abstract Extraction of pectic material from the leaves of Aloe ferox is largely dependent on the removal of calcium ions. Juice squeezed from leaf, and hot-water extracts of the residual pulp, contain mineral salts and polysaccharides low in uronic acid residues. Treatment with aqueous oxalate, citrate or phosphate permits the extraction from pulp of good yields of polysaccharide that forms viscous solutions and, on restoration of Ca2+ ion, gels. Dialysis of the ammonium salt of the pectic polysaccharide ( 50% -GalEA content) against flowing tap water yielded a gel inside the membrane, the metal ion content of which, in comparison with that in the exterior aqueous medium, showed the relative selectivity coefficients for Ca:Mg:Na:K to be in the approximate ratio 100:20:7:20. Further dialysis against distilled water lowered the percentages of ions present in dried gel to Ca2+ 8.7, Mg2+ 0.14, Na+ 0.32 and K+ 0.23. Steric-exclusion chromatography showed a value of 40 000 for the molecular weight of the soluble pectic salt, raised to 330 000 on addition of Ca2+; this was followed by gel formation. Exudate from Cycad cones, a mannoglucuronoglycan with ramified acidic rhamnoarabinogalactan attached to Man in the core, consists of a weak gel from which a low molecular weight (10 000) fraction is extractable with aqueous ethanol. The residual gel is readily dispersed in aqueous sodium carbonate and then has mol. wt. 180 000, not increased on Ca2+ addition although viscosity lowering does result. The solution properties of the two types of polysaccharide, which resemble those of Khaya grandifoliola gum and gum ghatti respectively, are dependent on the relative dispersions of uronic acid groups in the different molecular structures.