Fe 2+ 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 Fe 2+ , a cation of great importance in a several food products. In particular the role of citrus pectin structural properties, namely the degree of methylesterification (DM) and the absolute degree of blockiness (DBabsratio of non-methylesterified GalA units present in blocks to the total amount of GalA units) on the Fe 2+ 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 nonmethylesterified GalA units (DBabs). Adsorption isotherms of P-and C-pectins in aqueous solutions of various Fe 2+ concentrations revealed that both the DM and DBabs influenced the pectin-Fe 2+ interactions: the lower DM or higher DBabs, the higher the Fe 2+ binding capacity of citrus pectin. The Langmuir adsorption model was used to fit the experimental data for quantification of the maximum adsorption capacity (q G max) and the pectin-Fe 2+ interaction energy (KL) of the P-and C-pectins. It can be concluded that q G max (mol Fe 2+ /mol GalA) was mainly determined by the DM and to a lesser extent by the DBabs while the pectin-Fe 2+ 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.

Influence of Pectin Structural Properties on Interactions with Divalent Cations and Its Associated Functionalities

“…In addition, Celus et al. (, ) established that for a given DM, the interaction between Zn 2+ , Ca 2+ , or Fe 2+ and enzymatically demethylesterified citrus pectin (higher DB abs ) was stronger than alkaline demethylesterified citrus pectin (lower DB abs ), which was based on the interaction energy estimated by modeling cation binding data (based on the Langmuir adsorption model). In other words, pectins with a comparable DM but higher DB/DB abs exhibit stronger interactions with cations, which can be explained by the formation of dimers between two pectin chains as described for pectin–cation interactions (see Section “Binding Mechanisms Between between Pectin and Divalent Cations”) (Braccini & Pérez, ; Fang et al., ).…”

“…On the one hand, Celus et al. () observed a lower affinity of citrus pectin for Fe 2+ compared to Zn 2+ and Ca 2+ , through an equilibrium adsorption experiment. On the other hand, from the results of a bioaccessibility study by Kyomugasho et al.…”

“…(), and Celus et al. (, ) have determined the maximum Zn 2+ and Ca 2+ ‐binding capacity and associated interaction energy of citrus pectin samples with distinct DMs through equilibrium adsorption experiments and then modeled the results obtained based on the Langmuir adsorption model (Table ). The Langmuir adsorption model defines monolayer binding between a ligand (divalent cations) and a finite number of identical binding sites (GalA units) (Khotimchenko et al., ).…”

“…The influence of pectin DM on its cation binding has mostly been studied through equilibrium adsorption studies in which aqueous solutions of targeted demethylesterified pectin samples are enriched with cations of interest. The number of cations bound to pectin are then estimated from the free cation concentrations, determined either by spectrophotometry (Celus et al, 2017;Khotimchenko, Kolenchenko, & Khotimchenko, 2008), or by potentiometry or conductimetry through establishing the activity coefficients of cations (Dronnet, Renard, Axelos, & Thibault, 1996;Garnier, Axelos, & Thibault, 1994;Ralet et al, 2001;Renard & Jarvis, 1999;Tanhatan-Nasseri, Crépeau, Thibault, & Ralet, 2011;Thibault & Rinaudo, 1985). More recently, isothermal titration calorimetry (ITC) is used to study the thermodynamics of the pectin-cation interactions (Assifaoui et al, 2015;Celus, Lombardo, Kyomugasho, Thielemans, & Hendrickx, 2018b;Fang et al, 2008).…”

“…Despite the limited number of studies described in literature, Ralet et al (2001Ralet et al ( , 2003, Tanhattan-Nasseri et al (2011) andTibault &Rinaudo (1985) reported indeed lower calcium activity coefficients for enzymatically demethylesterified pectins (citrus, sugar beet, or apple pectin) compared to alkaline demethylesterified pectins for a comparable DM, implying a stronger binding of Ca 2+ to enzymatically demethylesterified pectins. In addition, Celus et al (2017Celus et al ( , 2018a established that for a given DM, the interaction between Zn 2+ , Ca 2+ , or Fe 2+ and enzymatically demethylesterified citrus pectin (higher DB abs ) was stronger than alkaline demethylesterified citrus pectin (lower DB abs ), which was based on the interaction energy estimated by modeling cation binding data (based on the Langmuir adsorption model). In other words, pectins with a comparable DM but higher DB/DB abs exhibit stronger interactions with cations, which can be explained by the formation of dimers between two pectin chains as described for pectin-cation interactions (see Section "Binding Mechanisms Between between Pectin and Divalent Cations") (Braccini & Pérez, 2001;Fang et al, 2008).…”