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Modified pectin structure for targeted divalent cation binding and associated functionalities in food model systems

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Category
Ph D Defense
Date
2018-12-10 17:00
Venue
KU Leuven, Aula Jozef Heuts, 00.215 - Kasteelpark Arenberg 20
3001 Leuven, België

Promovendus/a: Miete Celus

Promotor(en): Prof. dr. ir. Marc Hendrickx

Extracted pectin is currently widely used as a food additive in plant-based products, particularly in gelling applications, due to its ability to bind divalent cations, especially Ca2+. This pectin functionality has been reported to be largely influenced by its structural properties, in particular the degree and pattern of methylesterification (DM) and blockiness (DBabs), which are a measure for the percentage and distribution of non-methylesterified galacturonic acid units, respectively. Although the role of these structural properties in Ca2+-binding and gelling has been widely explored, fundamental insights into these interactions are limited. Moreover, scarce information is available on the role of the pectin properties in the interaction with other divalent cations, despite the hypothesized ability of pectin to bind these divalent cations yielding specific functionalities. For instance, given that pectin is a dietary fiber and thus not digestible in the small intestine, its interaction with essential divalent cations, such as Zn2+, could result in reduced mineral bioaccessibilities during digestion, potentially contributing to mineral deficiencies. Additionally, binding of Fe2+ has been considered to confer pectin a lipid antioxidant capacity in emulsion-based products, thereby retarding lipid oxidation. Exploring this pectin functionality could promote its use as a natural antioxidant which is more appealing to consumers compared to synthetic additives, such as ethylenediaminetetraacetic acid (EDTA). Therefore, this doctoral thesis aimed to provide fundamental insights into the effect of pectin DM and DBabs on its interaction with Fe2+, Zn2+, or Ca2+ and associated functionalities, particularly the lipid antioxidant capacity and its role in the Zn2+ in vitro bioaccessibility.

Pectin samples with comparable methylesterification degrees (DM) but different patterns of methylester distribution (DBabs) were generated through enzymatic (using carrot pectin methylesterase) or alkaline (using NaOH) demethylesterification of high methylesterified citrus pectin. First, the interaction of these pectin samples with Fe2+, Zn2+, or Ca2+ was explored through equilibrium adsorption experiments, followed by generation of adsorption isotherms based on the Langmuir adsorption isotherm model to quantify their maximum binding capacities and associated interaction energies. Results of this study showed that decreasing pectin DM or increasing DBabs promoted the Fe2+-, Zn2+-, or Ca2+-binding capacity of pectin, with the maximum binding capacity being mainly determined by the DM and the interaction energy by DBabs. With regard to cation type, the highest maximum binding capacity and interaction energy were exhibited for Zn2+ compared to Ca2+ and Fe2+. Additionally, insights into the thermodynamics of the pectin-cation interaction, particularly pectin-Zn2+ binding, were obtained using isothermal titration calorimetry (ITC). Results obtained complemented those from the equilibrium adsorption experiment. The binding of Zn2+ to pectin was found to be an endothermic interaction, in which a positive entropy change dominated the unfavorable endothermic enthalpy change. Moreover, the pectin-Zn2+ interaction occurred according to a two-step mechanism involving first monocomplexation and the formation of point-like cross-links, followed by dimerization.

The role of pectin DM and DBabs, and associated pectin-Zn2+ interaction, in directing Zn2+ bioaccessibility was subsequently studied through in vitro simulated digestion of Zn2+-enriched pectin solutions. Decreasing DM or increasing DBabs resulted in decreased Zn2+ bioaccessibilities due to higher Zn2+-binding capacities. However, lower amounts of Zn2+ than expected (based on the established maximum Zn2+-binding capacity of pectin) were bioaccessible, suggesting binding of Zn2+ to the bile salts and probably enzymes added during the in vitro simulated digestion. Exploration of the possible competition between Ca2+ and Zn2+ for binding to pectin during digestion revealed that low Ca2+ levels (approximately 0.33 mM) had no clear influence on Zn2+ bioaccessibility.

In view of identifying (potential) natural antioxidants to reduce synthetic ones, the lipid antioxidant capacity of the derived pectins in Fe2+-enriched linseed/sunflower oil-in-water (o/w) emulsions (5% w/v) was explored by determination of the peroxide value as a function of storage time. Low DM pectin and increased pectin concentration promoted the lipid antioxidant capacity, due to an increased Fe2+-binding capacity. However, EDTA exhibited still a higher antioxidant capacity compared to low DM pectin. Moreover, pectin was found to destabilize the o/w emulsions by bridging or depletion flocculation.

This doctoral thesis clearly demonstrated the role of DM and DBabs in directing pectin cation-binding capacity and associated functionalities. The results obtained provide fundamental insights into the pectin structure-function relation which in turn can contribute to optimization of ex situ pectin functionalities in several applications, including product structure build-up while maintaining the nutritional value of the food product. Moreover, these findings form a basis for potential exploitation of pectin as an antioxidant as well as exploring pectin functionalities in situ
 
 

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