Mechanism of interactions between calcium and viscous polysaccharide from the seeds of Plantago asiatica L.
ABSTRACT The present study aimed at investigating the mechanism of interactions between calcium and the psyllium polysaccharide. Plantago asiatica L. crude polysaccharide (PLCP) was subjected to ethylenediaminetetraacetic acid (EDTA) to yield calcium-depleted polysaccharide named PLCP-E. There was essentially no difference in the structure between PLCP-E and PLCP. However, PLCP-E exhibited a much lower apparent viscosity compared to that of PLCP. PLCP was treated with sodium hydroxide to deplete ferulic acid. The resultant material was named PLCP-FAS, which also exhibited lower viscosity. Adding Ca(2+) could both increase apparent viscosity of PLCP-E and PLCP-FAS, but only PLCP-E could keep the high viscosity when dialysis was carried out to remove free Ca(2+) in the solution. Thermal analysis showed that the thermal stability of the polysaccharide was reduced after EDTA chelation. Scanning electron microscopy (SEM) analysis showed that PLCP-E was flaky and curly aggregation, while PLCP was mostly filamentous in appearance. The results suggested that there are strong interactions between Ca(2+) and the polysaccharide. The interactions contributed to the high viscosity, weak gelling property, and thermal stability of the polysaccharide.