Fe2+ adsorption on citrus pectin is influenced by the degree and pattern of methylesterification

Abstract

The present study aimed at gaining insight into the potential of citrus pectin to bind Fe2+, a cation of great importance in several food products. In particular, the role of citrus pectin structural properties, namely the degree of methylesterification (DM) and the absolute degree of blockiness (DBabs – ratio of non-methylesterified GalA units present in blocks to the total amount of GalA units) on the Fe2+ adsorption in aqueous solution was explored using adsorption isotherms. Demethylesterification of high DM citrus pectin enzymatically (using plant pectin methylesterase) or chemically (alkaline demethylesterification using NaOH) generated P- and C-pectins, respectively, characterized by comparable DMs but different distributions of non-methylesterified GalA units (DBabs). Adsorption isotherms of P- and C-pectins in aqueous solutions of various Fe2+ concentrations revealed that both the DM and DBabs influenced the pectin-Fe2+ interactions: the lower DM or higher DBabs, the higher the Fe2+ binding capacity of citrus pectin. The Langmuir adsorption model was used to fit the experimental data for quantification of the maximum adsorption capacity (qGmax) and the pectin-Fe2+ interaction energy (KL) of the P- and C-pectins. It can be concluded that qGmax (mol Fe2+/mol GalA) was mainly determined by the DM and to a lesser extent by the DBabs while the pectin-Fe2+ interaction energy was mainly influenced by the DBabs. As a consequence, pectin modification to obtain targeted DM and DBabs allows optimization of its binding capacity and therefore the associated functional properties.