Material properties of low-methoxyl pectin networks

From a polymeric perspective, pectin is a complex, branched polyelectrolyte where ionic interactions have a relevant importance on material properties. The mechanical properties of films (85% w/w) obtained by means of a casting procedure from low-methoxyl pectin solutions added with increasing amount of Ca++(2.5-100 mM) and pH varying from 3.3 up to 8, were examined. The viscoelastic behaviour of the macromolecular solutions (2% w/w) was studied in order to identify a threshold ion concentration governing the viscosity of the system. A measurement of the binding capacity of pectin for Ca++ was realized by potentiometric analysis and the stability constant of the ligand-ion complex and the number of binding sites were calculated by means of the Scatchard equation. Mechanical properties of films were determined from tensile tests; their isothermal moisture sorption behaviour was evaluated at 25°C and data were fitted by means of the GAB isotherm equation. The pectin films properties showed a strong ion concentration dependence: the stiffness of films increased with the ionic strength of the bathing solution and the extent of film hydration decreased with increasing the counterion concentration.