12.1 Introduction

Natural polymers are broadly applied in food, cosmetic, and pharmaceutical systems as edible coating, emulsifying, stabilizing, flavor encapsulating, and viscosifying agents. There is an ever‐growing demand for plant gum exudates due to their structural diversity and metabolic functions. Furthermore, they are non‐poisonous, biocompatible, sustainable, eco‐friendly, and easily accessed [1].

Tragacanth plants belong to the genus Astragalus and are extensively distributed in Southwest Asia (from Greece to Pakistan), especially in the Anatolian region in Turkey and the mountainous regions of Iran [2]. Tragacanth plants have a tap root along with branches characteristic of low bushy perennial shrubs (Figure 12.1). Historically, gum tragacanth (GT) was first introduced by Theophrastus several centuries before Christ. Before the 1970s, Iran exported about 4000 tonnes of GT annually, but in the following years, for some reason, this quantity began decreasing [3]. It has been documented that Astragalus microcephalus is the main source of GT, whereas Astragalus gummifer is the principle gum‐yielding species [4,5]. For well over 2000 years, GT has been applied as thickening and emulsifying agents in cosmetic, food, and pharmaceutical industries [2]. However, due to its high cost and erratic supply, the use of GT decreased from several thousand to 200–300 tonnes per year [5].

From a toxicological point of view, GT is a natural polymer with an established safe history which has been used as a food additive in food products since 1925 (FDA, 1974). On the basis of the FDA code of federal regulations, GT can be used at levels of 0.2% to 1.3% by weight of the product (FDA, 2006). Safety assessments of GT were evaluated in detail by Boyland et al. [6] followed by Green [7], Bachmann et al. [8], Vohra et al. [9], Anderson et al. [10], Eastwood et al. [11], Santos [12], and Hagiwara et al. [13]. It has been demonstrated that GT does not act as a mutagen [7] or tetragen [12]. GT has been introduced as a non‐toxic additive [ 10, 12]. More precisely, toxicity tests reveal that GT (with the maximum advisable dietary levels) has no adverse effect on the ultrastructure of heart and liver tissues of rats. Administration of GT at different dietary levels (1.25% and 5%) to mice indicates that GT is not carcinogenic in mice of either sex [13]. On the basis of the abovementioned studies, GT is neither carcinogenic nor toxic and thus can be used as an additive in food and pharmaceutical industries.

The objective of this chapter is first to review the safety assessments of GT in detail, after which the structure of this biological macromolecule will be characterized. Finally, the rheological behavior and functional properties of various species of this gum under different conditions will be discussed to explore their potential applications as a natural pharmaceutical and food agent.

12.2 Structure

Compositional and structural characterization of hydrocolloids is the first step toward a better understanding of their functional properties. For example, their rheological properties, gelling and emulsifying properties, solubility, and film‐forming properties can be affected by chemical compositions, the sequence of monosaccharides, molecular conformation and position, and the configuration of glycoside linkage [14]. The physicochemical properties of polysaccharide hydrocolloids can be characterized by different techniques (e.g., gel permeation chromatography, gas chromatography, high‐pressure anion exchange chromatography, 1D (¹³C¹H NMR) and 2D NMR (HMBC, HMQC, COSY, and NOESY) spectroscopy, capillary viscometery, and steady/dynamic rheology).

GT is a very complex heterogeneous anionic polysaccharide bonded with a low amount of protein (<4%). It seems that a misinterpretation exists in a significant number of publications [15–18], where reported that GT has two fractions: tragacanthic acid (insoluble fraction) and arabinogalactan (soluble fraction). Furthermore, in some studies, it has been reported that tragacanthic acid and bassorin are the same compounds, or tragacanthin is a trace neutral arabinogalactan. GT consists of two major fractions: bassorin (water‐insoluble fraction) which can swell in water to form a gel, and tragacanthin (water‐insoluble fraction) including tragacanthic acid and a trace amount of a water‐soluble arabinogalactan [19].

The weight‐average molecular weight of GT, determined using Svedberg's method, is 840 kDa [20]. The molecular weight of the soluble fraction of an Iranian species of GT (A. microcephalus), determined by size exclusion chromatography, is more than 3000 kDa [21]. In another study, the molecular weight, intrinsic viscosity, and radius of gyration for tragacanthin were reported to be 502 kDa, 7.2 dl g⁻¹, and 43 nm, respectively. The values of the molar mass per unit contour length, persistence length, ratio R_g/R_h, and stiffness parameter, which are some conformation parameters of tragacanthin, are 1111 nm, 26 nm, 1.87, and 0.013, respectively. A higher value of the stiffness parameter corresponds to a higher level of chain flexibility. A high value of this parameter in tragacanthin reveals the stiff backbone of tragacanthin.

Tragacanthic acid comprises residues of D‐xylose (40%), D‐galacturonic acid (43%), D‐galactose (4%), and L‐fucose (10%). The water‐soluble tragacanthic acid is composed of the residues of linear 1,4‐linked α‐D‐galacturonic acid chains, where the majority of these residues is α‐D‐galacturonic acid carrying xylose‐comprising side chains through C3. The main sugar residues in the outer chains are single β‐D‐xylopyranose, 2‐O‐β‐D‐galactopyranosyl‐D‐xylopyranose, and disaccharide units of 2‐O‐L ‐glucopyranosyl. Arabinogalactan, another water‐soluble fraction, comprises residues of L‐arabinose, D‐galacturonic acid, and D‐galactose with a specific ratio of 75:3:12. Additionally, a trace amount of L‐rhamnose also exists in this fraction [19]. Gas chromatography‐mass spectrometry (GC‐MS), NMR, and ESI‐MS analysis suggested that the arabinogalactan fraction is even more complex than previously reported. The arabinogalactan fraction is mainly composed of a core comprising galactopyranose, arabinopyranose, and some 2‐O‐substituted β‐arabinofuranose units with side‐chain sequences α‐L‐Ara f‐(1 → 2)‐α‐Ara f ‐(1 → 4)‐L‐Arap and α‐L‐Ara f‐(1 → 2)‐α‐Ara f –(1 → 5)‐L‐Araf p.

Proximate analysis for six species of Iranian GT is shown in Table 12.1. It is evident that different varieties of GT possess various chemical compositions. Protein content can be used to differentiate different species from each other [24]. The protein content of different species of GT lies in the range 0.31% to 6.69%, which is comparable to those of other gums like xanthan gum (2.125%) and gum arabic (2.18%) [25]. Moreover, the film‐forming properties as well as the stabilizing and emulsifying capabilities of the gums are mainly dependent on the protein content [ 24,26], and thus it is expected that the abovementioned species have various emulsification and stabilization capabilities, which will be discussed later in this chapter.

As shown in Table 12.2, the moisture content of various species of GT lies in the range 8.79–13.3 g/100 g. This difference has been attributed to different water–polysaccharide interactions [22]. The carbohydrate and ash content of these six species lie in the range 83.81%–86.52% and 1.8%–32%, respectively, which are different from those reported previously for GT from unknown species [28]. Furthermore, Anderson et al. [29] reported that the total ash content and protein content of A. microcephalus were 3.2% and 3.65%, respectively, which are greater than those observed by Balaghiet al. [22]. Various factors such as source, time of exudation, the age of the plant, growing conditions, and contamination of hydrocolloids with other natural compounds affect the compositional properties of gum exudates [30].

Sugar composition is one of the most important factors that have a considerable effect on rheological properties of hydrocolloids. Additionally, characterization of sugar composition may be useful for exploring microstructure–functional relationships. The sugar composition of six species of GT measured by HPAEC‐PAD is presented in Table 12.2. It can be seen that all species of GT are made up of arabinose, xylose, glucose, fucose, galactose, rhamnose, and galacturonic acid; however, various species of GT have a different proportion of each monosaccharide. It has been suggested that arabinose is the major component in the side chains of the 1,4 connected α‐D‐galacturonic acid backbone of GT. Surprisingly, the content of arabinose has an extensive percentage range between 1% and 60% among the six species of GT. The proportions of fucose among these species also are 0% for Astragalus rahensis and 35% for Astragalus compactus. The Spearman correlation indicates that GT with higher galacturonic and fucose content has higher consistency coefficients (as a measure of viscosity). On the other hand, the gums containing a higher amount of arabinose and xylose have lower consistency coefficients (R = −0.943 and p = 0.005) [23]. Astragalus gossypinus contains 37% galacturonic acid, whereas Astragalus rahenisis and Astragalus brachycentrus contain 9% and 2% galacturonic acid, respectively. The uronic acid in a hydrocolloid's composition is a measure of the polyelectrolyte nature and relative proportion of acidic polysaccharides. Uronic acids are a class of sugar acids containing acidic functional groups (carbonyl and carboxylic acid). At pHs above the acid dissociation constant, its acidic functional groups will be largely dissociated, consequently making it negatively charged. The presence of 37% galacturonic acid in the composition of A. gossypinus reveals that this species has a high polyelectrolyte (anionic) nature. Therefore, A. gossypinus can be introduced as an appropriate biopolymer for encapsulation of phytochemicals using the coacervation technique. Coacervation encapsulation can be achieved with interaction between negatively and positively charged biopolymers. For example, at pHs below 6.5, amino groups of chitosan are ionized, thus making it positively charged. Negatively charged macromolecules like different species of GT, particularly A. gossypinus, can interact with positively charged polymers like chitosan through intermolecular electrostatic interaction, generating polyelectrolyte complexation.

Exudate gums contain various neutralized cations and metal ions. These ions influence the viscosifying properties and gelling ability of exudate gums [24]. For example, carboxyl functional groups in the gum structure can act as a binding site for calcium ions, thus improving the rheological and gelling properties of the gums [31]. Furthermore, some ions like iron have a considerable effect on emulsifying properties [32]. The mineral compositions of six species of GT are shown in Table 12.1. The main elements of all the species are calcium and potassium; however, there are some variations in their mineral incorporation. As mentioned before, due to the variation the iron and calcium content in various species, it is supposed that they have different viscous‐enhancing and emulsifying abilities.

Table 12.3 indicates the amino acid profiles of different species of Iranian and Turkish and also five species of Asiatic GT. The proportion of some essential amino acids (threonine, tryptophan, histidine, isoleucine, leucine, lysine, phenylalanine, and valine) is considerably high in the Iranian and Turkish species; cysteine and methionine are the main limiting amino acids in GT's composition. The major amino acids in Iranian and Turkish species are hydroxyproline, proline, serine, and valine. Hydroxyproline is the main amino acid in GT's composition. There are some variations in the amino acid profiles of Iranian and Turkish GT, especially in the content of hydroxyproline.

In another study, Anderson and Grant [27] characterize the amino acid profiles of five species of Asiatic Astragalus, grown in the United States (Astragalus parrowianus, A. echidnae[ormis Sirj.], A. cerasocrenus Bge., and A. microcephalus Willd) (Table 12.3). As observed for the Iranian and Turkish species, the major amino acid present in the composition of these five species of GT is hydroxyproline; its proportion in the A. microcephalus is more than that of other species (623 residues per 1000 amino acid residues). The amino acid profile of A. microcephalus reported by Anderson and Grant [27] differs notably from that cited by Anderson and Bridgeman [29] for a Turkish sample from that species. Similarly, the gums obtained from Prosopis spp. and Acacia senegal (gum arabic) have a very high hydroxyproline content.

The GT powder obtained from the ribbon type is odorless with a color of white to light yellow and has a mucilaginous taste. The color of the flakes varies from yellow to brown to give a cream color. Depending on their application, the particle size and viscosity of both ribbon and flake types are different. A high‐grade commercial GT powder has following specifications (USPC, 2007):

1. ✓ Color: Off‐white to creamy colored fine powder
2. ✓ Viscosity (1%): 800 ± 150 cP
3. ✓ Ash content: ≤3.0%
4. ✓ Moisture content: ≤12%
5. ✓ The value of acid insoluble ash: ≤0.3%
6. ✓ Particle size: 90% through BSS 150 mesh

United States Pharmacopeia USP31 (USPC, 2007) defined minimum safety quality standards for GT to be applied in pharmaceutical and food systems are as follows:

1. ✓ Arsenic content ≤3 ppm
2. ✓ Microbiology: Salmonella/Escherichia coli – absent
3. ✓ Heavy metals content (as Pb) ≤20 ppm

The physicochemical and functional characteristics of GT are discussed next in more detail.

12.3 Thermal Properties

The thermal properties of GT in comparison to different commercial gums (chitosan, gum arabic, and hydroxyethyl cellulose) were investigated by Zohuriaan and Shokrolahi [34]. GT had the highest activation energy of thermal decomposition compared to other industrial gums. For all the tested gums, an endothermic peak was observed in the temperature range 63–134 °C which is related to loss of moisture. No glass transition temperature was detected for GT, which may be due to the interference of its peak with the moisture endothermic peak [34]. Comparatively, based on the magnitude of the integral procedural decomposition temperature (IPDT), the thermal stability of GT is less than that of chitosan, carboxymethylcellulose (CMC), xanthan, and sodium alginate.

From a rheological point of view, with increasing temperature, the energy required for the flowing of molecules, and consequently the viscosity of the GT solution, decreases. Prolonged heating leads to degradation of the gum and decreases its viscosity permanently [35].

12.4 Functional Properties

12.4.1 Rheological Behavior

Viscosity is considered as an indication of the GT quality, and it demonstrates its suspending, stabilizing, or emulsifying ability. The values of viscosity considerably depend on shear rate, concentration, temperature, molecular weight, and presence of sugars and counter‐ions [36]. Furthermore, because the compositional and physicochemical properties of GT exudates are strongly species dependent, understanding of the rheological behavior of GT is necessary for exploring their potential application in pharmaceutical food systems. Therefore, in the following, the rheological properties of GT will be reviewed in both concentrated and gel regions.

12.4.1.1 Steady Shear Rheological Properties

12.4.1.1.1 Effect of Shear Rate

Balaghi et al. [22] investigated the effect of the shear rate (0.0001–900 (1 s⁻¹)) on the apparent viscosity of different species of GT in both the present and absence of NaCl (0.2 M). The results of this study indicate that the values of the apparent viscosity decrease with increasing shear rate, demonstrating the shear‐thinning behavior of GT solutions. This trend has been attributed to the alignment of randomly positioned chains in the direction of flow, which results in a reduction of interaction between adjacent macromolecules chains, producing a solution with less consistency [37]. At low shear rates, the system will be in equilibrium in a fully entangled state, where η = η ₀ . As the shear rate increases and the rate of externally imposed deformation exceeds the rate of new entanglement formation, the onset of shear thinning occurs [38]. However, Balaghi et al. [22] reported that Newtonian plateaus were not detected at low shear rate, and the viscosity increased slightly when the shear rate was decreased. This behavior has been attributed to the fact that GT has two different fractions (bassorin and tragacanthin) which are not chemically bounded and separate easily in aqueous media [22]. These fractions in GT lead to the abovementioned behavior, which is similar to that observed for solutions containing a mixture of polysaccharides [39,40]. The critical shear rate ( ), the shear rate at which the viscosity‐shear rate curve slope starts to change, is different for six species of GT. This difference has been attributed to their different soluble/insoluble fractions, uronic acid content, degree of methoxylation, different polydispersity degrees, and their different configurational and conformational characteristics [40].

12.4.1.1.2 Effect of Temperature

Understanding the influence of temperature on the viscosifying properties of gum solutions is necessary due to the broad range of temperature used in food processing operations [41]. In the Newtonian region of polymer dispersions, an increase in temperature leads to a decrease in viscosity, which can be modeled by an Arrhenius‐type equation [42]:

(12.1)

Here, A is the frequency factor (the viscosity coefficient at the reference temperature) (Pa s), η is the dynamic viscosity (Pa s), T is the absolute temperature, E_a is the activation energy (kJ kgmol⁻¹), and R is the universal gas constant (8.314 kJ (kgmol K)⁻¹).

With increasing temperature from 3 to 45 °C, the flow behavior index and consistency coefficient of different species of GT in the presence and absence of NaCl increased and decreased, respectively, which demonstrates that GT solutions are tending to closer to Newtonian behavior at higher temperatures. The activation energy for the flow process can be used as a useful factor to estimate the chain flexibility of biopolymers. On the basis of Eyring's theory, the activation energy of the flow process is defined as the energy needed for the formation of some extra spaces and consequently moving of the molecules. As the solution's temperature increases, the energy required for the flowing of molecules is provided, and as a result, the activation energy declines. Balaghi et al. [22] reported that an increase in the concentration of GT decreases the activation energy, which is consistent with Eyring's theory; however, the intensity of this reduction is variable depending on the species. Comparatively, the value of the activation energy for A. gossypinus solution (1.5%, in the presence of NaCl) is higher than that for five other species, showing that the temperature sensitivity of this species is more pronounced than that of other species. On the other hand, the activation energy value in A. compactus (1%, in both the absence and presence of NaCl) is lower than that of other studied species. Therefore, A. compactus has a higher stability against temperature than other species. In conclusion, the values of the activation energy in various species of GT are in the range 3.19–22.78 kJ mol⁻¹, which is less than that of most of the hydrocolloids. Accordingly, it can be suggested that GT can be used as a promising ingredient in food formulations that require a high level of temperature stability.

12.4.1.1.3 Effect of Polymer Concentration

The effect of polymer concentration on the steady shear behavior of different species of GT has been studied by Balaghi et al. [22]. When the polymer concentration increases from 0.7% to 1.5%, the apparent viscosity of the different species of GT increases, which is due to the higher solid content at higher concentrations and consequently more intermolecular interaction [43]. The values of the consistency coefficient for various species of GT in the concentration range 0.7%–1.5% were reported to be from 0.19 to 36.60 Pa sⁿ. The flow behavior indices of different species of GT were in the range 0.21–0.80. A broad range of the flow behavior index and consistency coefficient for various species of GT demonstrates that these species have different functional properties, which will be discussed now.

12.4.1.1.4 Effect of Salt

Investigating the influence of salt concentration on the flow behavior of hydrocolloids is important for hydrocolloid function as a polyelectrolyte, as well as for the determination of their rheological and functional characteristics [44,45]. Charged macromolecules have a strong viscosity dependence on ionic strength. As mentioned earlier, GT has a polyelectrolyte nature due to the presence of uronic acid. For example, the consistency coefficient of A. gossypinus in the presence of 0.1 M NaCl (2.89 Pa sⁿ) [46] is considerably more than that obtained in the presence of 0.2 M NaCl (0.7689 Pa sⁿ) [22]. This difference is due to the high uronic acid content of A. gossypinus. A greater ionic strength leads to compactness of biopolymer conformation, and as a result, its viscosity decreases.

Overall, all species of GT mentioned in this chapter exhibit non‐Newtonian behavior under various conditions. The reduction of viscosity in shear‐thinning fluids can be regarded as an advantage during high shear processing operations including pumping. Moreover, as observed above, various species of GT exhibit significantly different rheological behaviors. Thus, it is expected that they can be used in various applications in the food industry.

12.4.1.1.5 Effect of Sugar

The steady shear behavior of GT in the presence of glucose and sucrose at various temperatures was investigated by Silva et al. [47]. They observed that addition of these sugars to GT solution led to improvement in the apparent viscosity. Furthermore, the values of the activation energy at various shear rates demonstrated that GT solutions are mainly temperature dependent at a low shear rate, and this behavior is sharper in the presence of tested sugars.

12.4.1.1.6 Effect of Homogenization

The influence of high‐shear homogenization on the steady shear behavior of three species of GT (A. gossypinus, A. compactus, and A. rahensis) was evaluated by Farzi et al. [48]. All the tested species exhibit shear‐thinning behavior. Before homogenization of the samples, the apparent viscosities of the species were as follows: A. compactus > A. gossypinus > A. rahensis. Surprisingly, the authors observed that the apparent viscosity of the species increased after the homogenization process. This increas was more pronounced for A. gossypinus compared to other species. This effect is in opposition to other polysaccharides such as alginate, xanthan, k‐carrageenan, flaxseed gum, pectin, and methyl cellulose [49–52]. This difference has been attributed to the greater opportunity for GT molecules to contact each other due to the unfolding of their structure and the breaking of the large aggregated particles after homogenization [48]. A. gossypinus with the highest insoluble fraction exhibits the maximum improvement after the homogenization process.

12.4.1.1.7 Effect of Gamma Irradiation

The effect of gamma irradiation on the rheological behavior of two species of GT (Astragalus fluccocus and A. gossypinus) was investigated in a recent study [53]. The authors reported that when the gamma irradiation increased up to 3 kGy, the apparent viscosity of A. gossypinus increased in the range of the tested shear rate. For A. fluccosus, a decrease in the apparent viscosity was observed with increasing irradiation dose from 3 to 10 kGy. The authors observed a rapid decrease in the apparent viscosity of the gum solutions up to a dose of 15 kGy. Teimouri et al. [54] investigated the effect of gamma irradiation on the rheological behavior of Astragalus campactus. From their results, gamma irradiation under different doses increased the consistency coefficient and flow behavior index.

12.4.1.1.8 Synergistic Effect of GT with Other Biopolymers

The steady shear behavior of GT/guar gum mixture as a function of the polymer ratio and the temperature was studied by Silva et al. [55]. They observed that under the tested conditions, guar and GT aqueous systems and their mixture exhibited shear‐thinning behavior, and when the temperature increased, the apparent viscosity of the samples decreased. The rheological properties of the mixed system were mainly dependent on the polymer ratio. The GT/guar gum mixture indicated an interesting synergic influence at the polymer ratio of 1:1.

The strength of polysaccharide/protein interaction has a detrimental effect on the textural and structural characteristics of mixed gels. The effect of three species of GT (A. gossypinus, A. rahensis, and A. fluccosus) as a function of concentration (0.05–0.2% w/w) on a mixed milk protein system during acid gelation was investigated by Nejatian et al. [56]. The results of the rheological study showed that GT addition to milk protein dispersion resulted in a weaker network structure in comparison to the control sample. This weakening impact was eliminated when the A. gossypinus concentration increased up to 0.3%. As reported above, A. gossypinus exhibited higher content of uronic acid and fucose and a greater insoluble/soluble fraction ratio (presence of more hydrophobic groups), which induced a stronger protein–protein interaction which improved the structural strength of the mixed gel.

12.4.1.2 Dynamic Rheological Properties

In a recent study, the dynamic viscoelastic behavior of six species of GT (A. parrowianus, A. fluccosus, A. rahensis, A. gossypinus, A. microcephalus, and A. compactus) in the presence and absence of NaCl were evaluated (1.5% at 25 °C) [23]. The results of this study showed that A. parrowianus solution is at the borderline of liquid and gel‐like states, but the solutions of A. gossypinus, A. rahensis, A. microcephalus, and A. compactus show a weak gel‐like behavior (frequency = 1 Hz) in both the absence and presence of Na⁺ ion. For A. fluccosus, in the presence of NaCl, gum solution exhibits viscous behavior, but with the addition of NaCl, its solution exhibits gel‐like behavior. A. compactus, A. fluccosus, and A. parrowianus have higher values of limiting strain and thus have higher stability against the strain amplitude. As mentioned before, GT consists of two fractions (bassorin and tragacanthin) which differ in various species. On the basis of the Spearman correlation coefficient, the gums with a higher soluble/insoluble ratio have a more limiting strain [23].

According to frequency sweep experiments, the values of tan δ for the gums with a higher bassorin fraction of 50% (A. microcephalus, A. compactus, and A. gossypinus) are less than one, demonstrating the more solid‐like behavior of these species compared to other species. It has been attributed to the greater swelling power and higher Sauter diameter of bassorin compared to tragacanthin, which contributes to the gel‐like character of the gums [23]. Similar values of tan δ for A. microcephalus solution in the presence and absence of NaCl have been reported, showing solid‐like behavior in both conditions. The value of tan δ for A. gossypinus solution in the absence of NaCl is higher than 0.1 and less than one, showing that its solution is not a true gel. Conversely, in the presence of NaCl, A. gossypinus indicates a crossover point at high frequency; this is indicative of a flexible gel. The effect of NaCl addition on the dynamic viscoelastic behavior of A. gossypinus is more pronounced than on A. microcephalus and A. parrowianus, which is due to the higher amount of uronic acid in A. gossypinus composition [23]. On the other hand, tan δ values for A. rahensis dispersion are higher than one, showing a liquid‐like behavior for its dispersion over the frequency range 0.01–100 Hz, and this behavior remains constant at the shortest time scale of the test.

The effect of homogenization on the dynamic viscoelastic properties of three species of GT (A. gossypinus, A. compactus, and A. rahensis) was studied by Farzi et al. [48]. The frequency dependence of G′ and G″ for GT solutions was described. The results showed that homogenization led to the dominance of gel‐like behavior for all tested species. The authors also indicated that the change in the dynamic viscoelastic properties of GT species was highly species dependent.

12.5 Biological Activity

The DPPH method is often used to evaluate the antioxidant potential of plant extracts because it is a reliable, quick, and reproducible assay to assess the in vitro antioxidant activity of plant extracts [72]. The results of antiradical activity are commonly expressed as IC₅₀ values. Lower values of this parameter indicate more antiradical activity.

The magnitude of IC₅₀ for aqueous solutions of two species of GT (A. gossypinus and A. parrowianus) are 0.34 and 1.02 mg ml⁻¹, respectively, demonstrating that aqueous solutions of GT exhibit antiradical activity [73]. There is a significant difference between the antiradical activities of the gum obtained from both species. Comparatively, aqueous solutions of the gum obtained from A. parrowianus exhibit higher antiradical activity. Similarly, the phenol content of A. parrowianus is higher than that of A. gossypinus. According to the literature review, three is a positive relationship between the total phenol content and the antioxidant potential of vegetables and fruits [74–77], which is in agreement with that observed for the antiradical activity of GT.

12.6 Antibacterial Activity

The aqueous solutions of various species of GT (A. gossypinus and A. parrowianus) reveal antimicrobial activities against pathogenic bacteria. Minimum inhibitory concentrations (MICs) of aqueous solutions of GT range from 0.125 to 0.500 mg ml⁻¹ [73]. The minimum bactericidal concentration (MBC) of GT is in the range 0.25 to >0.50 mg ml⁻¹. The effect of aqueous solutions of GT is dependent on the bacterial species. A clear microbial inhibition was observed for the aqueous solution of A. parrowianus against four bacteria, especially Listeria monocytogenes. The antimicrobial activity of GT has been attributed to the presence of phenolic compounds in GT's composition. As stated above, the aqueous solution of A. parrowianus has the highest phenolic compounds (235 mg GAE g⁻¹ gum), produced the highest antibacterial activity.

The antimicrobial activity of the phenolic compounds is associated with the interaction between these compounds and bacterial cell membranes. This interaction changes the cell membrane's permeability, resulting in leakage of intercellular components followed by the death of the cell [78]. Furthermore, these compounds can also penetrate into bacterial cells and coagulate the cell contents [79].

12.7 Effect of Pre‐treatment on GT: Physicochemical Properties

12.8 Food Applications

12.9 Conclusions and Future Trends

The present chapter presented the chemical composition, molecular structure, rheological behavior, functional properties, and potential application of GT in food and pharmaceutical systems. As reviewed above, the physicochemical and rheological characteristics of GT are considerably species dependent, with different species have various functional properties. Furthermore, it was also concluded that any study performed on GT without taking the plant species into consideration will result in misleading results. The future challenge is to elucidate the effect of purification, drying, and further processing conditions on the microstructure, molecular conformation, chemical composition, and functional properties of different species of GT.

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