Demethylation of a model homogalacturonan with the salt-independent pectin methylesterase from citrus: Part II. Structure–function analysis

Effect of pectin structure on the in vitro bioaccessibility of carotenoids in simulated juice model

The impact of pectin structure on carotenoid bioaccessibility is still uncertain. This study aims to investigate how the different pectic polymers affected the bioaccessibility of carotenoids in a simulated juice model during static in vitro digestion. This study includes homogalacturonan (HG), which is a linear pectic polymer, rhamnogalacturonan-I (RG-I), which is a branched pectic polymer, and rhamnogalacturonan (RG), which is a diverse pectic polymer rich in RG-I, rhamnogalacturonan-II (RG-II), and xylogalacturonan domains. Juice models without pectin had the highest carotenoid bioaccessibility, suggesting pectin has negative effects on carotenoid bioaccessibility. During the intestinal phase, systems with HG showed the highest viscosity, followed by systems with RG and systems with RG-I. Systems with RG-I had lower carotenoid bioaccessibility than systems with HG and RG-II. Both the percentage of RG-I and the average side chain length of RG-I had negative correlations with carotenoid bioaccessibility. RG-I side chains with more arabinose and/or galactose might cause lower carotenoid bioaccessibility in this juice model system. This study offers valuable insights into the relationship between pectin structure and carotenoid bioaccessibility in a simulated juice model, highlighting the importance of considering pectin composition for maximizing carotenoid bioaccessibility and potential health benefits in fruit-based beverages.

Pectin Hydrogels: Gel-Forming Behaviors, Mechanisms, and Food Applications

Pectin hydrogels have garnered significant attention in the food industry due to their remarkable versatility and promising properties. As a naturally occurring polysaccharide, pectin forms three-dimensional (3D) hydrophilic polymer networks, endowing these hydrogels with softness, flexibility, and biocompatibility. Their exceptional attributes surpass those of other biopolymer gels, exhibiting rapid gelation, higher melting points, and efficient carrier capabilities for flavoring and fat barriers. This review provides an overview of the current state of pectin gelling mechanisms and the classification of hydrogels, as well as their crosslinking types, as investigated through diverse research endeavors worldwide. The preparation of pectin hydrogels is categorized into specific gel types, including hydrogels, cryogels, aerogels, xerogels, and oleogels. Each preparation process is thoroughly discussed, shedding light on how it impacts the properties of pectin gels. Furthermore, the review delves into the various crosslinking methods used to form hydrogels, with a focus on physical, chemical, and interpenetrating polymer network (IPN) approaches. Understanding these crosslinking mechanisms is crucial to harnessing the full potential of pectin hydrogels for food-related applications. The review aims to provide valuable insights into the diverse applications of pectin hydrogels in the food industry, motivating further exploration to cater to consumer demands and advance food technology. By exploiting the unique properties of pectin hydrogels, food formulations can be enhanced with encapsulated bioactive substances, improved stability, and controlled release. Additionally, the exploration of different crosslinking methods expands the horizons of potential applications.

A wide diversity exists in pectin structure from thirteen apple cultivars

To emphasize that differences in pectin structure among cultivars play a crucial role in the texture and quality of fruits and vegetables, the sugar content and methyl-esterification of pectin fractions from 13 apple cultivars was studied. Cell wall polysaccharides were isolated as alcohol-insoluble solids (AIS) and subsequently extracted to yield water-soluble solids (WSS) and chelating-soluble solids (ChSS). All fractions contained significant amounts of galacturonic acid, while sugar compositions varied between cultivars. AIS and WSS pectins showed a degree of methyl-esterification (DM) > 50 %, while ChSS pectins had either a medium (~50 %) or low (<30 %) DM. Homogalacturonan as major structure was studied using enzymatic fingerprinting. Methyl-ester distribution of pectin was described by degrees of blockiness and -hydrolysis. Novel descriptive parameters were obtained by measuring the levels of methyl-esterified oligomers released by endo-PG (DB_PGme) and PL (DB_PLme). Pectin fractions differed in relative amounts of non-, moderately-, and highly methyl-esterified segments. WSS pectins were mostly lacking non-esterified GalA sequences, while ChSS pectins had medium DM and many non-methyl-esterified blocks or a low DM with many intermediate methyl-esterified GalA blocks. These findings will be of help to better understand physicochemical properties of apple and its products.

Viscoelastic Analysis of Pectin Hydrogels Regenerated from Citrus Pomelo Waste by Gelling Effects of Calcium Ion Crosslinking at Different pHs

Pectin was extracted from citrus pomelo waste, and the effects of calcium ions (Ca²⁺) on the gelation and hydrogels properties were investigated over a pH range of 3.2-8 by using viscoelastic analysis. The gelatinization of Ca²⁺-pectin was examined at concentrations of 0.9, 1.8, 2.4, and 3.6 M of Ca²⁺ in aqueous pectin solutions of 1%, 2%, 3%, and 4%. The gel transition of Ca²⁺-pectin solution to hydrogels was determined by measuring the storage modulus (G') and loss modulus (G") under mechanical strain from 0.01 to 100%. In a hydrogel of 3% pectin at Ca²⁺ = 2.4 M, as pH increased to 7, the G' at 0.01 strain % was 3 × 10⁴ Pa, and 3 × 10³ Pa at pH 5, indicating that the crosslinking weakened at acidic pH. Due to the crosslinking between the calcium ions and the ionized carboxylic acid groups of pectin, the resulting hydrogel became stiff. When the mechanical strain % was in the range of 0.01-1%, G' was unchanged and G" was an order of magnitude smaller than G', indicating that the mechanical stress was relieved by the gel. In the range of 1-100%, the gel deformation progressed and both the moduli values were dropped. Collapse from the gel state to the solution state occurred at 1-10 strain %, but the softer hydrogels with G' of 10³ Pa had a larger strain % than the stiffer hydrogels with G' of 10⁴ Pa.

Probing the Stoichiometry Dependence of Enzyme-Catalyzed Junction Zone Network Formation in Aiyu Pectin Gel via a Reaction Kinetics Model

We investigate the enzymatic self-catalyzed gelation process in aiyu gel, a natural ion crosslinked polysaccharide gel. The gelation process depends on the concentration ratio (Rmax) of the crosslinking calcium ions and all galacturonic acid binding sites. The physical gel network formation relies on the assembly of calcium-polysaccharide crosslink bonds. The crosslinks are initially transient and through break-up/rebinding gradually re-organizing into long, stable junction zones. Our previous study formulated a reaction kinetics model to describe enzymatic activation, crosslinker binding, and crosslink microstructural reorganization, in order to model the complex growth of elasticity. In this study, we extend the theory for the time-dependent profile of complex moduli and examine the interplay of enzyme conversion, crosslink formation, and crosslink re-organization. The adjusted model captures how the gelation and structural rearrangement characteristic times vary with the polymer and calcium concentrations. Furthermore, we find that calcium ions act as both crosslinkers and dopants in the excess calcium ion scenario and the binding dynamics is determined by Rmax. This study provides perspectives on the dynamic binding behaviors of aiyu pectin gel system and the theoretical approach can be generalized to enzyme-catalyzed ionic gel systems.

Vibrational and electrochemical studies of pectin-a candidate towards environmental friendly lithium-ion battery development

Pectin polymers are considered for lithium-ion battery electrodes. To understand the performance of pectin as an applied buffer layer, the electrical, magnetic, and optical properties of pectin films are investigated. This work describes a methodology for creating pectin films, including both pristine pectin and Fe-doped pectin, which are optically translucent, and explores their potential for lithium-ion battery application. The transmission response is found extended in optimally Fe-doped pectin, and prominent modes for cation bonding are identified. Fe doping enhances the conductivity observed in electrochemical impedance spectroscopy, and from the magnetic response of pectin evidence for Fe³⁺ is identified. The Li-ion half-cell prepared with pectin as binder for anode materials such as graphite shows stable charge capacity over long cycle life, and with slightly higher specific capacity compare with the cell prepared using polyvinylidene fluoride (PVDF) as binder. A novel enhanced charging specific capacity at a high C-rate is observed in cells with pectin binder, suggesting that within a certain rate (∼5 C), pectin has higher capacity at faster charge rates. The pectin system is found as a viable base material for organic-inorganic synthesis studies.

Heat- and shear-reversible networks in food: A review

While nature behaves like an irreversible network with respect to entropy and time, certain systems in nature exist that are, to some extent, reversible. The property of reversibility imparts unique benefits to systems that possess them, making them suitable for designing self-healing, stimuli-responsive, and smart materials that can be used in widely divergent fields. Reversible networks are currently being exploited for applications in tissue engineering, drug delivery, and soft robotics. They are also being utilized as low-calorie fat mimetics with melt-in-your-mouth textures, as well as being explored as potential scaffolds for three-dimensional (3D) printable food, among other applications. This review aims to gather representative examples of heat- and shear-reversible networks in the food science literature from the last 30 or so years, in other words, reversible food gels made either from linear biopolymers or from colloidal, particulate dispersions, including those that have been modified specifically to induce reversibility. An overview of the network mechanisms involved that impart reversibility, including a discussion of the strength and range of forces involved, will be highlighted. A model that explains why certain networks are thermoreversible while others are shear-reversible, and why others are both, will also be proposed. A fundamental understanding of these mechanisms will prove invaluable when designing reversible networks in the future, making possible the precise control of their properties, thus fostering innovative applications within the food industry and beyond.

Synergistic gelling mechanism of RG-I rich citrus pectic polysaccharide at different esterification degree in calcium-induced gelation

RG-I rich pectic polysaccharide is common in fruit and vegetable and possesses health benefits. However, it is removed during commercial pectin production because of poor gelling properties. Synergistic gelation can improve rheological properties of RG-I pectic polysaccharide and expand its application in functional food hydrocolloids. In the study, RG-I rich pectic polysaccharides at different degree of esterification was extracted from citrus membrane by sequential mild acidic (0.4% HCl, 28 °C) and alkaline (0.6% NaOH, 32 °C) treatment. The pectic polysaccharide from acid water (PA) composes of 41% RG-I and 44% HG with DM of 45%, while the pectic polysaccharide from basic water (PB) composed of 63% RG-I and 19% HG with DM of 15%. PA/PB blend gel under CaCO₃-glucono-δ-lactone system showed improved rheological properties compared with pure gels. Ca-bridges connected pectin aggregates and promoted the three-dimensional structure of PA/PB blend gels, while neutral sugar side-chains prompted hydrogen bonds and strengthened gel network.

Impacts of thermal and non-thermal processing on structure and functionality of pectin in fruit- and vegetable- based products: A review

Pectin, a major polysaccharide found in the cell walls of higher plants, plays major roles in determining the physical and nutritional properties of fruit- and vegetable-based products. An in-depth understanding of the effects of processing operations on pectin structure and functionality is critical for designing better products. This review, therefore, focuses on the progress made in understanding the effects of processing on pectin structure, further on pectin functionality, consequently on product properties. The effects of processing on pectin structure are highly dependent on the processing conditions. Targeted control of pectin structure by applying various processing operations could enhance textural, rheological, nutritional properties and cloud stability of products. While it seems that optimizing product quality in terms of physical properties is counteracted by optimizing the nutritional properties. Therefore, understanding plant component biosynthesis mechanisms and processing mechanisms could be a major challenge to balance among the quality indicators of processed products.

Egg-box model-based gelation of alginate and pectin: A review

Alginate and pectin are emblematic natural polyuronates that have been widely used in food, cosmetics and medicine. Ca-dependent gelation is one of their most important functional properties. The gelation mechanisms of alginate and pectin, known as egg-box model, were believed to be basically the same, because their Ca-binding sites show a mirror symmetric conformation. However, studies have found that the formation and the structure of egg-box dimmers between alginate and pectin were different. Very few studies have reviewed those differences. Therefore, this study was proposed to first summarize the intrinsic and extrinsic factors that can influence the gelation of alginate and pectin. The differences in the effect of these factors on the gelation of alginate and pectin were then discussed. Meanwhile, the similarity and difference in their gelation mechanism was also summarized. The knowledge gained in this review would provide useful information for the practical applications of alginate and pectin.

Dissipation and strain-stiffening behavior of pectin-Ca gels under LAOS

Non-linear mechanical responses observed in networks of many biopolymers such as pectin are important for their functioning as biological systems. Additionally, pectins derived from plant sources are also used for several food and biomedical applications. In the present work, the possible contributions of egg-box bundles in the large deformation response of calcium crosslinked gels of low methoxy pectin are explored using large amplitude oscillatory shear (LAOS). The gels exhibit a significant overshoot in the loss modulus (G'') and intra-cycle strain-stiffening, more prominent at greater extents of egg-box bundling. This observation signifies the dissipation characteristics of the egg-box bundles in pectin gels, hitherto not reported. The observed non-linear signatures diminish when the extent of bundling as well as the bundle radius decreases below a critical value. We identify different pectin/Ca concentration regimes based on the semi-flexible/flexible nature of the gel network and the non-linear signatures. Monovalent salt addition prior to crosslinking is shown to modify the extent of bundling, thereby influencing the magnitude of G'' overshoot and strain-stiffening. The intensity of the G'' overshoot and the extent of strain-stiffening are correlated with the radius of the egg-box bundles obtained from small angle neutron scattering (SANS) data. However, analysis using strain-stiffening models indicates the possible contributions from the semi-flexible nature of egg-box bundles and single chains.

Structuring of multiple parallel pectin gel filaments by applied shear

The bundled structure of micron-sized pectin gel filaments was formed by quick shear-induced gelation of the filamentous domains of pectin-polyethylene glycol (PEG) assemblies. Highly concentrated pectin with PEG in a separated pectin-rich phase under aqueous two-phase separation in the pectin/PEG/NaCl system enabled the formation of the pectin-PEG assembly, which was elongated in the flow direction, resulting in the generation of filamentous domains using a microfluidic device. The pectin gel filaments were formed by crosslinking with Ca²⁺ in the presence of shear-responsive PEG assemblies formed in the PEG-rich phase, because the filamentous PEG assemblies prevented fusion of the pectin filaments to form the seamless cylindrical gel. The shear-dependent elongation applied to the pectin-PEG assembly under the aqueous two-phase separation condition enabled the formation of the biomimetic bundled filamentous structure using bio-safe PEG as a sacrificial polymer, without the requirement of a multi-hole nozzle. Potential applications for gel filaments possessing a bundled structure are matrices in the biomedical field, such as a biodegradable scaffold for cell engineering.

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

Pectin is an anionic cell wall polysaccharide which is known to interact with divalent cations via its nonmethylesterified galacturonic acid units. Due to its cation-binding capacity, extracted pectin is frequently used for several purposes, such as a gelling agent in food products or as a biosorbent to remove toxic metals from waste water. Pectin can, however, possess a large variability in molecular structure, which influences its cation-binding capacity. Besides the pectin structure, several extrinsic factors, such as cation type or pH, have been shown to define the cation binding of pectin. This review paper focuses on the research progress in the field of pectin-divalent cation interactions and associated functional properties. In addition, it addresses the main research gaps and challenges in order to clearly understand the influence of pectin structural properties on its divalent cation-binding capacity and associated functionalities. This review reveals that many factors, including pectin molecular structure and extrinsic factors, influence pectin-cation interactions and its associated functionalities, which makes it difficult to predict the pectin-cation-binding capacity. Despite the limited information available, determination of the cation-binding capacity of pectins with distinct structural properties using equilibrium adsorption experiments or isothermal titration calorimetry is a promising tool to gain fundamental insights into pectin-cation interactions. These insights can then be used in targeted pectin structural modification, in order to optimize the cation-binding capacity and to promote pectin-cation interactions, for instance for a structure build-up in food products without compromising the mineral nutrition value.

Structure and Properties of a Non-processive, Salt-requiring, and Acidophilic Pectin Methylesterase from Aspergillus niger Provide Insights into the Key Determinants of Processivity Control

Many pectin methylesterases (PMEs) are expressed in plants to modify plant cell-wall pectins for various physiological roles. These pectins are also attacked by PMEs from phytopathogens and phytophagous insects. The de-methylesterification by PMEs of the O6-methyl ester groups of the homogalacturonan component of pectin, exposing galacturonic acids, can occur processively or non-processively, respectively, describing sequential versus single de-methylesterification events occurring before enzyme-substrate dissociation. The high resolution x-ray structures of a PME from Aspergillus niger in deglycosylated and Asn-linked N-acetylglucosamine-stub forms reveal a 10⅔-turn parallel β-helix (similar to but with less extensive loops than bacterial, plant, and insect PMEs). Capillary electrophoresis shows that this PME is non-processive, halophilic, and acidophilic. Molecular dynamics simulations and electrostatic potential calculations reveal very different behavior and properties compared with processive PMEs. Specifically, uncorrelated rotations are observed about the glycosidic bonds of a partially de-methyl-esterified decasaccharide model substrate, in sharp contrast to the correlated rotations of processive PMEs, and the substrate-binding groove is negatively not positively charged.

Structural behaviour differences in low methoxy pectin solutions in the presence of divalent cations (Ca(2+) and Zn(2+)): a process driven by the binding mechanism of the cation with the galacturonate unit

In this paper, we compare the interactions between low methoxy pectin (LMP) and either Ca(2+) or Zn(2+) in semi-dilute solutions. Intrinsic viscosity and turbidity measurements reveal that pectin-calcium solutions are more viscous, but yet less turbid, than pectin-zinc ones. To get a molecular understanding of the origin of this rather unexpected behavior, we further performed isothermal titration calorimetry, small angle neutron scattering experiments, as well as molecular dynamics simulations. Our results suggest that calcium cations induce the formation of a more homogeneous network of pectin than zinc cations do. The molecular dynamics simulations indicate that this difference could originate from the way the two cations bind to the galacturonate unit (Gal), the main component of LMP: zinc interacts with both carboxylate and hydroxyl groups of Gal, in a similar way to that described in the so-called egg-box model, whereas calcium only interacts with carboxylate groups. This different binding behavior seems to arise from the stronger interaction of water molecules with zinc than with calcium. Accordingly, galacturonate chains are more loosely associated with each other in the presence of Ca(2+) than with Zn(2+). This may improve their ability to form a gel, not only by dimerization, but also by the formation of point-like cross-links. Overall, our results show that zinc binds less easily to pectin than calcium does.

Determination of degree of methylation of food pectins by chromatography

BACKGROUND

In Florida, 90% of citrus is processed into juice, leaving large amounts of peel waste that could be processed into food ingredients such as pectins for other applications. Pectins of low degree of methylation (DM < 50) have important functional properties for use in foods and pharmaceuticals. Thus determination of DM is important, but DM analyses are difficult to perform and assays can have interference due to the presence of salts or neutral sugars.

RESULTS

A chromatography method has been developed for determination of DM of food pectin using alkaline hydrolysis followed by pectin digestion with a commercial pectinase. Analysis was performed by ion exclusion chromatography (IEC) to obtain methanol (MeOH) concentrations and by anion exchange chromatography (AEC) to obtain galacturonic acid (GalA) concentrations. Data were compared with values obtained using other DM analysis procedures. For example, a 55-75 DM pectin assayed as 55.5, 59.6, 60.1 and 57.0 DM by IEC, AEC, colorimetric and conductivity detection methods respectively.

CONCLUSION

The chromatography procedure described in this paper provides an alternative method for determination of DM of food pectins for accurate and efficient analysis when confronted with limited quantities or large numbers of samples.

Utilisation of model pectins reveals the effect of demethylated block size frequency on calcium gel formation

Calcium-mediated gelation of LMP is thought to arise from formation of a dense network of Ca(2+)-cross-linked DMB meeting a required minimum average length along pectin chains. The use of MP containing specific average DMB size (BS) types, in the range of 3-100 and in varying proportion (0-100%), has afforded further insights into the gelling behaviour of pectins with a certain DM in the presence of Ca(2+) ions. It clearly appeared that a required minimum BS and a required minimum average frequency (BSF) of the required minimum BS are conditions that must be satisfied by a pectin for formation of a highly dense Ca(2+)-cross-linked DMB network equaling an elastically stable, strong, and cohesive gel. Furthermore, there is a clear contribution of the pectin branched domains to gelation and formation of a firmer and more cohesive gel. The results suggest that this pectin portion may function, not only as a "maintainer" of the pectin molecular weight to a sufficiently high level which fosters good gelation regarding the gelling rate and the strength and nature of the gel formed, but also as junction-zone-terminating structural elements that limit the appearance of undesirable phenomena, notably turbidity, syneresis, and precipitation.

Hydrolytic and oxidative stability of L-(+)-ascorbic acid supported in pectin films: influence of the macromolecular structure and calcium presence

The hydrolytic and oxidative stability of L-(+)-ascorbic acid (AA) into plasticized pectin films were separately studied in view of preserving vitamin C activity and/or to achieve localized antioxidant activity at pharmaceutical and food interfaces. Films were made with each one of the enzymatically tailored pectins (50%, 70%, and 80% DM; Cameron et al. Carbohydr. Polym.2008, 71, 287-299) or commercial high methoxyl pectin (HMP; 72% DM). Since AA stability was dependent on water availability in the network, pectin nanostructure affected the AA kinetics. Higher AA retention and lower browning rates were achieved in HMP films, and calcium presence in them stabilized AA because of higher water immobilization. Air storage did not change AA decay and browning rates in HMP films, but they significantly increased in Ca-HMP films. It was concluded that the ability of the polymeric network to immobilize water seems to be the main factor to consider in order to succeed in retaining AA into film materials.

Innovative enzymatic approach to resolve homogalacturonans based on their methylesterification pattern

Three series of model homogalacturonans (HGs) covering a large range of degree of methylesterification (DM) were prepared by chemical and/or enzymatic means. Randomly demethylesterified HGs, HGs containing a few long demethylesterified galacturonic acid stretches, and HGs with numerous but short demethylesterified blocks were recovered. The analysis of the degradation products generated by the action of a purified pectin lyase allowed the definition of two new parameters, the degree of blockiness, and the absolute degree of blockiness of the highly methylesterified stretches (DBMe and DB(abs)Me, respectively). By combining this information with that obtained by the more traditional endopolygalacturonase digestion, the total proportion of degradable zones for a given DM was measured and was shown to permit a clear differentiation of the three types of HG series over a large range of DM. This double enzymatic approach will be of interest to discriminate industrial pectin samples exhibiting different functionalities and to evaluate pectin fine structure dynamics in vivo in the plant cell wall, where pectin plays a key mechanical role.

Effect of extraction time and temperature on the characteristics of loosely bound pectins from Japanese plum

The cell wall composition of Japanese plums (Prunus salicina) at six developmental stages was previously evaluated (Ponce et al., J. Agric. Food Chem. 2010, 58, 2562-2570). This fruit is an interesting source of pectins, polysaccharides of valuable functionality for pharmaceutical and food formulations. In the present work it was investigated how the different conditions for the aqueous extraction of pectins from Japanese plums affect the yield as well as their chemical and rheological characteristics. It has been determined that extraction with water at room temperature for periods longer than 2h did not produce additional increment of yield (12%) but decreased the average molecular weights of the extracted pectins. Pectins with a degree of methylation ≈40% with high viscosity in water and with adequate molecular weights (≈72,000) were obtained. Conversely, utilization of boiling water for extraction increased considerably the yields (33-38%) but the extracted pectins showed significant lower viscosity in water in spite of their higher molecular weights. The poorer thickening ability was associated to the lower proportion of arabinose residues present in the hairy regions of the pectin macromolecules extracted by hot water, which led the polymers to interact more transiently in a 2% w/v water solution.