Rheological, potentiometric and ²³Na NMR studies on pectin-calcium systems

Abstract

Pectins with degree of methylation lower than 50% are able to gel in presence of divalent ions such as calcium. A phase diagram of a sodium pectinate (DM=28%) in wate, pH 7.4, and 20°C was established, evidencing three phases: sol, gel, and syneresis. From rheological experiments, the calcium concentration at the sol-gel transition was determined, [Ca²⁺]_t/[COO⁻] ≈ 0.168, for a polymer concentration of 5.8 g/l and it was shown that this transition was accurately described by the scalar percolation theory. Under the same conditions, potentiometric studies showed that the binding of calcium ions seemed to be independent from physical state (sol or gel). Furthermore, this calcium binding decreased only slightly with the amount of added calcium, whereas the sodium one decreased sharply in the same time. By following the evolution of the ²³Na NMR longitudinal relaxation time, and abrupt increase was found around [Ca²⁺]_t[COO⁻]=0.182, which was close to the results of the rheological study.