In vitro binding of calcium, iron and zinc by non-starch polysaccharides

The in vitro binding capacity of eight non-starch polysaccharides (agar, κ-carrageenan, gum xanthan, gum arabic, gum karaya, gum tragacanth, pectin and gum guar) was measured by equilibrium dialysis in neutral and acidic (0.1M HCl) solutions in the presence of divalent cations (Ca2+, Zn2+). No significant binding was observed in acidic conditions while, in neutral solutions, the extent of binding was correlated (P<0.1) to the cation-exchange capacity of the polysaccharides. It is apparent that the interactions are essentially electrostatic in nature, due to the presence of ionised carboxyl (uronic/pyruvic acids) and sulphated groups, in polyanionic polysaccharides. By contrast, significant binding occurs with Fe3+ in acidic conditions, presumably due to complexation (chelation). These data provide a clear insight into how non-starch polysaccharides interact with minerals and the potential nutritional consequence in terms of bioavailability.