Pectin in Semi-Solid and Fluid Foods: History
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Contributor:
Taíse Toniazzo
,
João Paulo Fabi

Pectin is a versatile polysaccharide produced mainly from natural food sources and agro-industrial wastes, adding value to these by-products. For food applications, it is necessary that pectin first interacts with water for technical purposes. As a food additive, pectin acts as a solution thickener and gelling agent for food formulation, even in concentrations of less than 1 (g/100 mL or g/100 g), and it is sufficient to influence food products’ stability, rheology, texture, and sensory properties.

  • pectin
  • chemical structure
  • chemical properties
  • physical properties
  • semi-solid foods
  • fluid foods

1. Introduction

Food processing is an important scientific area that not only maintains the security of foods through storage time but also improves the sensory and nutritional quality of processed foods [1][2]. In addition, scientific research and the food industry have greatly improved the development of more stable foods, aiming to increase shelf-life by adding food additives [3]. Thus, to currently develop new food additives, their structures, properties, and processes must be considered, as is the case of pectin. It is essential to investigate the molecular structure, chemical and physical properties, functionality, and applications [4][5].
Food can be categorized in different ways, such as solids, gels, homogeneous liquids, suspensions of solids in liquids, and emulsions [6]. Here, the researchers focus on fluid and semi-solid foods, which can exhibit various rheological behavior ranging from Newtonian (water) to non-Newtonian behavior of aqueous dispersion. For example, when packaged, fluid foods retain the shape of the container [6]. Newtonian behavior is likely expected when fluid foods contain dissolved low molecular weight compounds, such as sugars, and a minimum amount of a polymer or insoluble solids. However, adding a small amount of polymer can increase the viscosity and change the flow characteristics from Newtonian to non-Newtonian. Also, many non-Newtonian foods can show viscoelastic behavior, characterized by viscous and elastic properties [6][7].
The food components are part of a complex structure, not simply homogeneously dispersed or in an accessible form [5][8]. Among these components, additives such as polysaccharides (e.g., pectins) can be added to food products during the processes with a technological purpose [8]. The main beneficial effect of pectins as food additives for technical purposes is the ability to interact with water, acting as solution thickeners and gelling agents, foam stabilizers, emulsions, and dispersions [9][10][11][12]. Pectins are added to food formulation in concentrations of less than 1 (g/100 mL or g/100 g) to be sufficient to influence food products’ stability, rheology, texture, and sensory properties. For example, in acidified milk drinks, pectin addition provides a great stabilizing behavior, inhibiting the aggregation of proteins because of the formation of larger electrostatic repulsive and steric repulsive forces [9][10][13][14].
Pectin is commercialized in a dry powder form, and its use as a stabilizer can be applied to food products, such as fruit drinks and fruit and tomato pastes. Also, pectin’s ability to form gels under specific parameters allows its use as a gelling agent in jams, jellies, and marmalades [15]. Pectin—marketed in a powder form—must be completely dissolved in water, and its interaction is essential for food applications [16]. The dissolution of pectin in water can be elucidated as a two-stage phenomenon, as documented in prior studies [16][17], as depicted in Figure 1. Initially, water molecules adhere to the surface and infiltrate the pectin particles, constituting the solvent penetration stage [17]. This process leads to swelling and forming a gel-like layer on the particle surface. Notably, this stage can give rise to the fish-eye effect, a phenomenon characterized by the creation of adhesive and partially undissolved powder aggregates [18][19]. Subsequently, in the second phase, termed chain disentanglement [19], the pectin polymers transit from the core of the powder to the liquid phase, facilitating the overall dissolution process.
Figure 1. Pectin: from powder form to gelation—mechanism of pectin dissolution in water.
Intrinsic and extrinsic factors influence the dissolution process of pectin. Intrinsic factors comprise the number and distribution of hydrophilic groups and neutral sugars. Also, particle properties are part of the intrinsic factors essential for pectin dissolution. The particle properties include size, form, density, porosity, and crystallinity combined with their surface characteristics, such as chemical surface composition, which are mainly soluble component contents in the surface [20][21]. Particularly, the properties of the particles influence the wettability of the powder, which is the time required to complete the wetting and immersion of a powder on a liquid surface [22]. For example, in the first step of pectin dissolution, the formation of lumps can be avoided by modifying the particle properties using the agglomeration process for the pectin, and an instant pectin powder can be obtained with better wettability [23]. The extrinsic factors are mainly temperature and mechanical energy input. The water uptake velocity of powder defines the stirring strength of dry components in water [16], and vigorous stirring is important to prevent any undissolved powder lumps that may form [18]. Regarding temperature, depolymerization of the macromolecules can occur at temperatures above 30 °C with intensive mechanical energy input, influencing the properties of the final food product [16].

2. Chemical Structure of Pectin—A Heterogeneous Polysaccharide

In plants, pectin is naturally located in the cell walls and is restricted to the primary cell wall and middle lamella, remaining almost absent in the secondary cell walls [24][25][26], as depicted in Figure 2. Pectin functionality, combined with cellulose and hemicellulose, provides mechanical strength, contributing to plant growth, morphology, development, and defense [27][28][29]. The primary cell wall composition consists of approximately 35% pectin, 30% cellulose, 25% hemicellulose, and 10% protein, depending on the plant species, ripening stage, and cell differentiation [30]. Note that pectin is the most abundant macromolecule in higher plants, creating valuable opportunities for using pectin to develop new food products. In addition, the circular economy can be increased by stimulating the use of pectin from the by-products of the agricultural, agribusiness, and food industries.
Figure 2. Schematic representation of the localization of pectin in the cell walls.
It is practical to start by defining the chemical structure of pectin, highlighting that it is a group of polysaccharides rich in galacturonic acid (GalA), which is present in two major structural features that form its backbone. There are three polysaccharide segments mainly found in all pectin species: homogalacturonan (HGA), rhamnogalacturonan-I (RG-I), and rhamnogalacturonan-II (RG-II) [26][29][31], as depicted in Figure 3. It is important to understand the changes in the structural characteristics of these three polysaccharide segments for food applications. For instance, HGA is the most abundant and widespread segment of pectin, known as the “smooth” region. The degree and distribution of esterified D-galacturonic acid units in the HGA segment cause relevant modifications in pectin’s gelling and stabilizing properties [32][33]. The degree of methoxylation (DM) of pectin is the ratio of methoxylated D-galacturonic acid units to total D-galacturonic acid units [34][35], and is classified as high methoxylated pectin (HM, DM > 50) and low methoxylated pectin (LM, DM < 50). HM pectin is especially used to prepare jams, jellies, or marmalades due to its ability to form a gel under acidic conditions and high sugar concentrations [34][36]. In contrast, LM pectin can form a gel by interacting with divalent cations, such as Ca2+, following the egg box model. For this reason, LM pectin can produce a gel with less dissolved solids, creating great interest in preparing products with reduced caloric value [34][36][37].
RG-I is a more complex structure than HGA [31], constituting a backbone of repeat units of the disaccharide (1→2)-α-L-rhamnose-(1→4)-α-D-galacturonic acid and neutral sugar side chains attached to the C-4 of rhamnose units [38][39]. Generally, the RG-I structure is not associated with gel formation and is removed by hot acid in commercial pectin production. The low gelling capacity of RG-I is due to the rhamnose inserts on the backbone producing molecular twists, limiting cross-linking. However, studies have reported new perspectives for the branched gelation of RG-I, showing that the side chains of the RG-I region showed strong water binding capacities and stabilized the gel network structures [40]. Liu et al. [41] and Wang et al. [42] showed gel formation stemming from RG-I-rich pectic polysaccharides under divalent ions (Ca2+ and Mg2+), and sucrose can strengthen the gel network [40]. In addition, the RG-I-rich pectic polysaccharide, mainly from fruits and vegetables, is suggested to have potential health benefits, such as modulating the gut microbe and promoting cell adhesion and migration [40][43].
RG-II presents the most complex structure compared to the other segments of pectin [44][45]. Additionally, in smaller amounts, RG-II is present in pectin, and its structure is rather conservative [46], demonstrating its importance in biological functions in plant cell walls [45]. Thus, RG-II is not a common part of commercial pectins for gelling purposes. Commercial pectin is predominantly homogalacturonans and contains low short neutral side chains in RG-I [47]. The impact for the final consumer regarding safety is negligible, and health benefits are related only to the HGA portion, the one presented in high amounts. The chemical structure of the three polysaccharide segments mainly found in pectin is described separately below.
Figure 3. Representation of the three polysaccharide segments found mainly in all pectin species: homogalacturonan (HGA), rhamnogalacturonan-I (RG-I), and rhamnogalacturonan-II (RG-II). Figure 3 was drawn using the symbol nomenclature for glycans guidelines [29][48][49].

3. Pectin and Its Potential Sources

Pectin is usually commercially produced from apple pomace or citrus peel (lemon, lime, grapefruit, orange) and is subsequently obtained from sugar beet pulp as a by-product of sugar production [50][51].
Depending on the application, a suitable pectin with specific characteristics is required. For example, pectin from the apple source is commonly used for fillings in baking and similar products, as this source produces a heavier and more viscous gel. In contrast, pectin from the citrus source results in a lighter color and is more acceptable in confectionery jellies [50]. The sugar beet pectin has poor gelling capacity compared to the apple and citrus sources due to the high content of acetyl groups and neutral sugars and the higher content of proteinaceous materials bonded covalently to the side chains [51][52]. Pectin from sugar beet is commercially used as a food emulsifier, and its capacity to stabilize oil emulsions was verified by Leroux et al. [53]. They found that the molecular weight, protein, and acetyl contents of the sugar beet pectin significantly influenced the emulsifying properties and were able to reduce the interfacial tension between the oil and water phases.
Pectin extraction methods have some variables depending on the technique, such as solid–liquid ratio, acid strength, temperature/power, extraction time, and precipitation method. Pectin yield is affected by these variables but rather affects pectin qualities in terms of the degree of esterification (DE), molecular weight, composition, purity, and color [54]. A study by Kaya et al. [55] demonstrated that pectin extraction methods from citrus peel influence its structure, recovering pectin with different molecular weights and varying RG-I content [54]. Wang et al. [56] studied pectin extracted from apple pomace and citrus peel by subcritical water and found different molecular weight values. The highest molecular weight for citrus peel was ~70 kDa at 120 °C. The highest molecular weight for apple pomace was ~65 kDa obtained at 130 °C.
In addition to the molecular weight of pectin being affected depending on the extraction method, its purity can also be affected. For conventional pectin extraction, high temperatures (80–90 °C), acidic pH (2–3), long time (1–5 h), and a high solid-to-liquid ratio (1:30–1:50) [52] are commonly used. Conventional extraction is initiated using organic acids to break the cell wall fibers (Figure 1), releasing pectin chains. When combined with high temperature, it will accelerate molecular motion, facilitating the dissolution of pectin in an aqueous medium. When working with low methoxylated pectin (LM), citrates, oxalates, and polyphosphates (named chelators) are added to capture Ca2+, allowing the disaggregating of pectin chains [52][53]. Then, the aqueous medium is treated with alcohol to create the pectin precipitate, and filtration is performed to isolate the pectin [52]. Usually, these formed pectin isolates contain a considerable number of contaminants, such as free neutral components, which may include monomeric sugars, oligosaccharides, and high molecular weight polysaccharides [57][58], which can affect the gelling properties of pectin [58]. Studies have reported that the final purity of isolated pectin can be influenced by the purification method used [58], such as ultrafiltration [52], dialysis [58], and metal ion precipitation [58]. For instance, Yapo [58] found that the gel prepared with pectin purified by the metal ion-precipitation procedure formed the gel more rapidly and with much higher strength than non-purified pectin. Muhidinov et al. [59] showed that the di-ultrafiltration (DUF) process was able to separate the pectin oligosaccharides (POS) extracted together with the pectin. Interestingly, oligosaccharides have been classified as a potential prebiotic; for this reason, the authors suggested that the DUF method was preferable to the hydrolysis-extraction method for pectin production. Pectin purification can effectively remove possible contaminants, improving pectin gelling properties [52][59] and creating the possibility of a new ingredient. However, it is essential to highlight that the DUF process can produce pectin at different costs [59].
The pectin from apple, citrus, or beet sources can be considered low-cost and highly available, as it comes from food and agro-industrial waste [60]. According to the Food and Agriculture Organization [61], 17% of global food production is estimated to be wasted, with about 14% of the food produced being lost between harvest and retail. For this reason, the significance of research that explores new alternative sources to produce pectin, mainly from food and agro-industrial wastes, adds value to these by-products. Some relevant factors should be considered to evaluate the feasibility of using new alternative sources for pectin extraction, such as the selection of raw materials and the pectin characteristics. For raw material selection, it is necessary to consider the pectin content, quality, ripeness, availability, seasonality, and logistics (it may be required to dry the raw material to avoid microbial growth and chemical deterioration). Furthermore, it is important to consider pectin's chemical composition, structure, molecular features, and gel-forming capacity [50][60].

4. Pectin as a Thickening and Gelling Agent and Its Application to Semi-Solid and Fluid Foods

The two types of water immobilization by hydrocolloids are thickening and gelling attributes. Pectin has these attributes due to its ability to bind a large amount of water and form a gel. After many years of scientific discussion, a definition was that gel is a system comprising at least two components containing a substantial quantity of a liquid, resulting in soft, solid, or solid-like products [16][62]. Only after using rheological oscillation measurements was it possible to describe the difference among the gels formed in the products [16][63][64].
The thickening behavior of pectin due to the formation of a structure increases the viscosity of products and is the main characteristic of its use as an emulsifying, stabilizing, and bodying agent in foods for determined technological functions [65]. It is worth mentioning that thickening behavior occurs above a certain/critical concentration named overlap concentration (C*) when the product behaves as a non-Newtonian fluid. Otherwise, below this concentration, the product behaves as a Newtonian fluid [10][65].
Regarding the gelling behavior, pectin can form a true gel and, as aforementioned, is applied to foods such as jams, jellies, or marmalades [16]. Pectin forms a three-dimensional network in which a solid matrix involves the liquid phase and immobilizes the liquid within it, forming a rigid structure resistant to flow. A gel is considered a colloidal dispersion; the continuous phase is a solid matrix, and the discontinuous phase is a liquid [66]. In rheology, G′ represents the storage modulus, which characterizes the solid-like elastic attributes of a system, while G″ denotes the loss modulus, quantifying the liquid-like or viscous characteristics of the system [16], and a gel is a viscoelastic system with G′ > G″ [65].

This entry is adapted from the peer-reviewed paper 10.3390/fluids8090243

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