Phase diagrams of pectin-calcium systems: Influence of pH, ionic strength, and temperature on the gelation of pectins with different degrees of methylation

Abstract

The effects of ionic strength, pH, and temperature on the gelation of pectin-calcium systems were studied on an apple pectin; the influence of the degree of methylation (dm) was investigated on a family of well-characterised citrus pectins obtained by acid-catalyzed deesterification. The reaction led to a decrease in intrinsic viscosities by hydrolysis of glycosidic bonds and in neutral sugar content by elimination of the neutral fraction. Ion-exchange chromatography indicated that the deesterification did not change the charge distribution along the pectic backbone, which was confirmed by potentiometric and conductimetric studies. These methods showed that the charge distribution was statistical in apple and citrus pectins and that no egg-box can occur. Phase diagrams (sol-gel-syneresis), established as calcium concentration versus reduced polymer concentration, showed that gelation was possible before the overlapping of chains because of the strong interactions of calcium with pectins. An increase in the ionic strength, a neutral pH, and a decrease in the setting temperature and in the dm lower the amount of calcium required to give a gel, which can be explained by changes in the probability of crosslinks. Syneresis appeared at relatively low concentrations of calcium.