12 Gum Tragacanth (Astragalus gummifer Labillardiere) – Emerging Natural Hydrocolloids

12
Gum Tragacanth (Astragalus gummifer Labillardiere)

Zahra Emam‐Djomeh*Morteza Fathi and Gholamreza Askari

Transfer Phenomena Laboratory (TPL), Control Release Center, Department of Food Science, Engineering and Technology, College of Agriculture and Natural Resources, University of Tehran, Karaj, Iran

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].

Figure 12.1 (a) Tragacanth plant and (b) gum exuded from tragacanth plant.

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 [1518], 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 Rg/Rh, 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.

Table 12.1 Chemical compositions of six species gum tragacanth.

Source: Data extracted from Balaghi et al. [22] and Balaghi et al. [23].

Composition (%) A. parrowianus A. fluccosus A. rahensis A. gossypinus A. microcephalus A. compactus
Carbohydrate 85.25a) 83.81b) 84.84b) 84.55b) 86.52a) 84.06b)
Protein 3.05 ± 0.01b) 2.59 ± 0.01c) 3.82 ± 0.01a) 0.31 ± 0.01f) 2 ± 0.02d) 1.65 ± 0.01e)
Ash 2.84 ± 0.02b) 3.2 ± 0.04a) 2.55 ± 0.07c) 2.2 ± 0d) 1.8 ± 0.08e) 2.5 ± 0.42c)
Moisture 8.86 ± 0.08c) 10.4 ± 0.03b) 8.79 ± 0.07c) 12.94 ± 0.12a) 9.68 ± 0.06b) 11.79 ± 0.09a)
Soluble/insoluble ratio 1.74b) 3.15a) 1.3c) 0.51f) 1d) 0.84e)
Monosaccharide
Arabinose 39b) 23d) 51a) 1f) 35c) 7e)
Xylose 10e) 24b) 11e) 32a) 19d) 21c)
Glucose 10b) 13a) 1c) 10b) 2c)
Fucose 7d) 8d) 23b) 9c) 35a)
Galactose 8a) 7a) 7a) 1b) 2b) 2b)
Rhamnose 1a) 1a) 1a)
Galacturonic acid 21d) 9e) 9e) 37d) 21d) 30c)
Elements (ppm)
Calcium 8148.54 ± 33.60c) 7450.6 ± 162d) 4842 ± 88e) 9186. ± 276b) 7685 ± 206d) 9758 ± 230a)
Magnesium 32.88 ± 0.70b) 29.83 ± 0.60c) 54.41 ± 0.20a) 34.58 ± 0.80b) 15.03 ± 0.60d) 18.86 ± 0.40d)
Potassium 758.12 ± 23.80e) 2655 ± 25a) 2211 ± 48b) 1394 ± 33c) 889 ± 12d) 780 ± 45e)
Sodium 174 ± 3a 63 ± 2c) 63 ± 2c) 100.58 ± 0.9b) 34.41 ± 2.00d) 103.58 ± 1.00b) 26.58 ± 0.60d)
Zinc 1.35 ± 0.06d) 1.80 ± 0.10d) 1.37 ± 0.40d) 21.91 ± 0.50b) 81.03 ± 0.30a) 14.7 ± 0.2c)
Iron 39.8 ± 0.60b) 24.38 ± 0.11d) 24.13 ± 0.80d) 30.8 ± 0.4c) 75.13 ± 0.20a) 41.5 ± 1.0b)

a–e: Values with different letters in each row are significantly different (p < 0.05).

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].

Table 12.2 Compositional properties of five species of Asiatic gum tragacanth.

Source: Adapted from Anderson and Grant [27] with permission from Elsevier.

A. brachycentrus A. parrowianus A. echidnaeformis A. cerasocrenus A. microcephalus
Loss on drying (%) 10.1 9.7 11.5 10.8 13.3
Total ash (%) 2.3 1.6 3.5 2.9 2.4
Nitrogen (%) 0.52 0.57 0.77 0.19 0.07
Protein (%) 3.31 3.51 4.69 1.26 0.49
Methoxyl (%) 2.1 1.5 4.1 2.9 4.7
Viscosity (1%, cP) 110 55 1325 575 4675
Sugar composition (%) after hydrolysis
Galacturonic acid 2 2 11 6 17
Galactose 16 27 9 14 8
Arabinose 60 50 38 50 16
Xylose 6  5 20 9 22
Fucose 3 5 9 10 18
Rhamnose 13 11 13 11 19

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.

Table 12.3 The amino acid profile of five species of Asiatic Astragalus.

Source: Columns 2–6 from Anderson and Grant [27]. Columns 7 and 8 from Anderson et al. [33].

Amino acid A. brachycentrus A. parrowianus A. echidnaeformis A. cerasocrenus A. microcephalus Average of Iranian samples Average of Turkish samples
Nitrogen (%) 0.52 0.57 0.77 0.19 0.07
Alanine 65 46 29 49 31 35 47
Arginine 22 50 23 32 23 10 12
Aspartic acid 103 87 66 86 49 63 72
Cystine 0 1
Glutamic acid 61 46 29 65 30 33 43
Glycine 74 65 21 72 33 28 36
Histidine 48 44 104 34 17 60 77
Hydroxyproline 124 195 320 252 623 304 224
Isoleucine 43 46 12 33 18 20 27
Leucine 57 46 18 49 25 28 36
Lysine 50 38 47 46 21 45 45
Methionine 6 4 3 8 4 0 3
Phenylalanine 42 40 7 27 11 12 16
Proline 62 47 87 49 21 95 89
Serine 94 93 84 81 39 94 95
Threonine 50 52 34 40 19 41 46
Tyrosine 27 29 44 30 12 45 44
Valine 72 72 72 47 24 85 87

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, Ea 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 [4952]. 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.

12.4.2 Surface Activity

Surface tension shows how strongly the surface molecules in a solution are attracted by the neighbor's molecules [57]. According to previous studies [ 18, 22], GT has an excellent surface activity, and at low concentration (<0.25%) produces a rapid reduction in the surface tension of water. The flake types of GT exhibit more surface activity than the ribbon types. The surface tension values for the ribbon and the flake types (at the same concentration (1%)) are 61.7 dynes cm−1 and 52.5 dynes cm−1, respectively [58]. This difference could be associated with the higher viscosity of the flake types than the ribbon types, which made surface tension determination difficult [59].

The surface activity measurements of six species of GT (A. compactus, A. rahensis, A. parrowianus, A. gossypinus, A. fluccosus, and A. microcephalus) in 0.2 M NaCl solution and deionized water reveal that all the species exhibit surface activity. It has been reported that up to a concentration of 0.4%, A. gossypinus has the least surface activity, but with further increase in gum concentration, its surface activity significantly improves. Among all species of GT, A. parrowianus has the greatest surface activity in NaCl solution, whereas A. rahensis decreases the surface tension more than other species in deionized water. Moreover, the surface activity of A. parrowianus is considerably dependent on NaCl [22].

12.4.3 Solubility

GT swells rapidly as it is wetted in either cold or hot water. On the other hand, GT is insoluble in organic solvents like alcohol; the gum can withstand low amounts of alcohol or glycol. GT solution is relatively stable over an extensive pH range down to extremely acidic media at about pH 2.

12.4.4 Emulsification Ability

GT, as an efficient natural emulsifying agent, is commonly used for acidic oil‐in‐water emulsions. The stabilizing ability of GT is due to the steric repulsion force, and its performance can be controlled by changing the pH [60]. Additionally, Rezvani et al. [61] indicated that the emulsification ability of GT is a result of its residual surface activity and improvement in emulsion viscosity. Another reason for the emulsifying properties of GT is related to its zeta potential (−21 mV). As mentioned before, the presence of a uronic acid which has carbonyl and carboxylic acid imparts a polyelectrolyte nature to GT. The emulsifying ability of GT may arise from negatively charged carboxylic groups of galacturonic acid [62].

The values of the hydrophilic‐lipophilic balance (HLB) in GT are in the range 11–13.9, depending on the grade of the gum. The reason for that has been attributed to the lower interfacial tension between oil and water of the flake type in comparison to the ribbon type [63].

Natural polymers are commonly utilized to formulate emulsions with high stability in the food and pharmaceutical industries. Because of the thickening properties of hydrocolloids and their ability to hold water, these biopolymers are used as a stabilizer. Additionally, some of them like GT eliminate additional surface‐active agent usage to obtain an emulsion [64]. The emulsion activity of GT is species dependent. There is a linear and positive correlation between the emulsion stability of various species of GT and the degree of methoxylation and the amount of galacturonic acid of the gums (A. parrowianus, A. fluccosus, A. rahensis, A. gossypinus, A. microcephalus, and A. compactus). Among different species of GT, A. fluccosus gum shows the best emulsion stability, whereas A. rahensis gum exhibits the lowest emulsion stabilization effect. Also, the emulsion with the A. gossypinus gum has reasonably acceptable stabilization effects [65]. The emulsion stabilization effect of GT is probably due to viscosity improvement of the continuous phase surrounding the oil droplets, which restricts movement of the emulsified oil droplets [65]. Furthermore, this effect may be attributed to adsorption/precipitation of GT at the oil–water interface, leading to a decrease in interfacial tension [66]. On the basis of Gavlighi et al. [65], A. rahensis gum has the minimum viscosity in both solution and emulsion, but A. compactus gum exhibitsthe maximum viscosity, confirming the effect of viscosity increment on the emulsion stabilization for the A. rahensis and the A. compactus gums. Likewise, other studies also reported that GT could be suggested as an excellent candidate for producing high‐stability oil‐in‐water emulsions [67,68]. The authors suggested that the stabilizing effect of GT is because of its ability to improve viscosity as well as to create a network which protects oil droplets from slowed creaming and coalescing.

The emulsification capability of three species of GT (A. gossypinus, A. compactus, and A. rahensis) has been evaluated by Farzi et al. [69]. The results revealed that the three species exhibited various emulsification activities. Comparatively, A. gossypinus had the highest emulsion stability index at 0.5% w/w, so that the value of this parameter was more than 90% after 120 days storage at the ambient temperature. The results of particle size analysis confirmed that the gum is adsorbed at the oil–water interface. Interestingly, the highest consistency coefficients (as a measure of viscosity) were exhibited by A. compactus, but the emulsion with the highest stability was obtained with A. gossypinus. The authors concluded that the emulsification ability of the tested GT species is due to the high insoluble/soluble fractions ratio as well as the large amount of uronic acid, both of which characteristics are reported for A. gossypinus (see the chemical composition section of the study).

Abdolmaleki et al. [70] investigated the influence of salt (NaCl with concentrations of 0.5 and 5.4) and pH (2.5, 4.0, and 5.4) on the physicochemical characteristics and stability of oil‐in‐water emulsions prepared with GT. They reported that the concentration of salt did not have a considerable influence on emulsion stability, but the emulsion stability was strongly dependent on the pH. It was also found that there is no direct correlation between emulsion stability and the droplet size distribution. In pH 2.5 (near to pKa of GT), the prepared emulsions were highly unstable, which has been attributed to the low electrostatic repulsion between the droplets.

Multiple or double emulsions are prepared by emulsifying a single emulsion into another phase. In a recent study, Pulatsü et al. [71] prepared and characterized a water–oil–water double emulsion on the basis of various concentrations of GT and guar gum incorporated in the secondary water phase. Comparatively, the double emulsions containing guar gum had higher viscosity, and their droplet size showed a significant increase at the end of seven days of storage. Since the biopolymers used for the secondary aqueous phase are highly branched and have high molecular weight, it has been suggested that the possible mechanism for emulsion instability is flocculation and creaming [71]. In general, the authors recommended that GT alone or GT/guar gum can be used in double emulsion formulation.

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 [7477], 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

12.7.1 Irradiation

Food irradiation is widely applied as the ultimate processing technology. Irradiation up to 10 kGy has been approved by Food and Agriculture Organization (FAO), World Health Organization (WHO), and International Atomic Energy Agency (IAEA). Additionally, a higher dose of irradiation has also been approved for some products [80]. The effect of gamma irradiation on the chemical structure, particle size characteristics, color attributes, and microstructural properties of two species of GT (A. fluccosus and A. gossypinus) were investigated. In this study, the authors demonstrated that irradiation could not change the chemical structure of GT. On the other hand, the particle size characteristics of both types of GT were affected by the irradiation treatment. The authors also showed that the radiation process increased the blueness and yellowness of GT samples. Scanning electron microscopy (SEM) analysis showed that the irradiated samples had a smoother surface [53].

In a study by Teimouri et al. [54], the influence of different levels of gamma irradiation (0, 4, 8, 16, and 30 kGy) on some physicochemical characteristics of GT (A. campactus) was evaluated. Fourier transform infrared spectrometer (FT‐IR) analysis showed that gamma irradiation did not have a considerable effect on the functional groups of this gum, which was consistent with previous research [53]. On the other hand, this treatment had a significant effect on water absorption capacity, solubility, and color.

In recent research [81], the effect of gamma irradiation on the functional properties of A. gossypinus was evaluated and then used to stabilize an oil‐in‐water emulsion system. The results of their study showed that applying 1.5 kGy gamma irradiation was more effective for obtaining the maximum stability of the emulsion. GT considerably decreased the interfacial tension of the oil‐in‐water system. Therefore, it can be stated that GT has a positive impact on the oil‐in‐water emulsion stability.

Due to the mucoadhesive and gel‐forming ability of polyvinylpyrrolidone (PVP) and polyvinyl alcohol (PVA), the inherent wound healing ability of GT, and PVP, Singh et al. [82] attempted to prepare an antibiotic drug “gentamicin” and analgesic drug “lidocaine” incorporated into TG‐PVA‐PVP hydrogel by the radiation method. Various analyses like 13C NMR, thermogravimetric analysis (TGA), cryo‐SEM, AFM, FTIR, X‐ray diffraction (XRD), differential scanning calorimetry (DSC), and swelling studies were used to characterize these polymers. Network parameters and some properties such as water vapor permeability, microbial penetration, haemolysis, mucoadhesion, oxygen permeability, and antioxidant activities were also evaluated. The authors found that the films of these polymers were permeable to water vapor and O2, blood compatible, and impermeable to the microorganism. Further, it was found that the synergic influences of the antimicrobial, mucoadhesive, and antioxidant nature of hydrogel dressings make them appropriate for wound management.

Mohammadifar and Abdolmaleki [83] used irradiated A. gossypinus at different doses (0, 1.5, 3, and 5 kGy) to stabilize an oil‐in‐water emulsion. Rheological and particle size characteristics were measured to monitor the mechanism of stabilization. According to their results, irradiation had a considerable effect on the emulsion stability, particle size distribution, and rheological behavior of an oil‐in‐water emulsion. The sample irradiated at 1.5 kGy had the most stability.

12.7.2 Heat Treatment

Hatami et al. [84] performed a study on the effects of heat treatment on milk protein–GT mixed gel. Small deformation rheological evaluation demonstrated that the mixed gel network strength increased as follows: heated Na caseinate and heated whey protein with GT alone > heated milk protein dispersion and GT separately > co‐heated GT and whey protein > co‐heated all three biopolymers. Microstructural analysis of the mixed gel showed that the network of heated Na caseinate and heated whey protein with GT alone was much more homogenous, dense, and coarse than when all three biopolymers were co‐heated, while the network of other mixed gels was of intermediate density. The authors suggested that the heat treatment of the biopolymer mixture containing GT is an opportunity to control the rheological and microstructural properties of mixed gels.

12.7.3 High Shear Rate

The light scattering technique is commonly used to observe the reduction in particle size. As mentioned above, GT is composed of bassorin and tragacanthin. The hydrodynamic diameter of bassorin is 200 μm and that of tragacanthin is 0.12 μm [48], indicating GT is a polydisperse system. The effect of high‐shear‐rate homogenization (0–20 min) on the particle size of different species of GT was evaluated by Farzi et al. [48]. Before homogenization of GT solutions, all the solutions showed a wide distribution of various particle size depending on the GT species. The homogenization of GT solutions made the size distribution sharper, and as the time of treatment increased, the particle size shifted to smaller regions. The authors reported that more pronounced changes were observed for A. compactus and A. rahensis than for A. gossypinus after treatment.

12.8 Food Applications

12.8.1 Ice Cream

Kurt et al. [85] investigated the effect of various concentrations (0%–0.5% w/w) of GT on the internal structure and rheological characteristics (dynamic rheology, thixotropy, and creep/recovery behavior) of Salep‐based ice cream. According to this study, incorporation of GT into the ice cream formulation increases both the viscous and elastic behavior of ice cream. Additionally, an increase in GT concentration improves the network resistance against stress. The authors suggested that GT can be introduced as a valuable additive to enhance the structural properties of ice cream.

In another study, the effectiveness and applicability of using guar gum–gum tragacanth (GG–GT) as a stabilizer and polyglycerol poliricinoleate (PGPR)‐lecithin blend as an emulsifier and their effects were evaluated. When GG–GT was used in the ice cream formulation, about 6% improvement in overrun values and 20% increase in overall acceptability and 13% decrease in the melting rate were observed in comparison to the sample prepared with GG. Overall, the authors reported that it is possible to produce an ice cream with 2.8% fat using the double emulsion technique without affecting the quality of the product adversely [86].

12.8.2 Doogh

The major problem during storage of Doogh, a traditional yogurt drink, is phase separation, which occurs because of aggregation of caseins at low pHs. Therefore, several studies focused on the stabilization of this product. In an attempt to promote the stability of non‐fat Doogh, Gorji et al. [87] used GT. On the basis of this study, GT can be used as a suitable additive to enhance the qualitative properties of Doogh. Additionally, GT addition to Doogh leads to prevention of serum separation. The Doogh prepared with GT has a higher complex viscosity, apparent viscosity, and storage modulus, but has a smaller particle size, which has been attributed to the interaction between GT and casein. In another study, Azarikia and Abbasi [62] showed that GT (0.2%) and the soluble tragacanth fraction (0.1%) could prevent phase separation of Doogh. Additionally, they found that with the addition of soluble tragacanth and GT to Doogh, zeta potential values changed from positive to negative. This effect is due to adsorption of soluble tragacanth onto casein and induced stabilization through steric and electrostatic repulsions.

12.8.3 Yogurt

Application of GT (0.25, 0.5, 0.75, and 1 g l−1) as fat replacer has no significant effect on total solids, acidity, total protein, and ash content of non‐fat yogurt. With increasing GT concentration up to 0.5%, the firmness and syneresis of yogurt remain constant, but when the GT concentration increases above 0.5 g l−1, a softer gel is produced, and its degree of syneresis increases. Furthermore, incorporation of GT in yogurt formulations produces a more open and coarser structure than the control sample. Overall, the addition of GT does not enhance the syneresis and textural properties of non‐fat yogurt [88].

12.8.4 Cheese

It has been indicated that with increasing GT concentration in cheese formulations, the hardness of this product decreases, but the level of whiteness increases. However, it has also been reported that interaction of ripening time with gum concentration during 60 weeks of ripening causes deterioration of cheese properties. In conclusion, GT improves the rheological characteristics of cheese, which has been associated with its water‐binding ability [89].

Aminifar et al. [90] investigated the effect of 0.02% GT, milk protein concentrate or sodium caseinate on textural, microstructural, and physicochemical characteristics of Lighvan cheese. The results revealed that incorporation of GT into bovine milk had a considerable effect on the Lighvan cheese properties. The authors proposed that effect of GT on the properties of Lighvan cheese could be related to its heterogeneous, branched, and hydrophilic structure. SEM analysis of GT showed that the addition of cheese samples led to the production of inconsistent black particles containing large white patches, which have been attributed to the water‐binding capacity of this gum. Incorporation of GT into cheese formulation decreased the hardness of the final product, which may be due to a decrease in the moisture content of the samples.

12.8.5 Kashk

The influence of various concentrations of GT (A. gossypinus) on the syneresis, particle size distribution, rheological properties, and turbidity of low‐fat dried yogurt paste (Kashk) were investigated by Shiroodi et al. [91]. The result of their research indicates that an increase in GT concentration improves the shape retention ability and structural strength of Kashk. Furthermore, it was found that the products containing 0.5% GT have the lowest syneresis. The authors suggested that this effect is probably due to the increase in the viscosity of the continuous phase, which leads to trapping of the aggregated casein. On the other hand, the samples containing 0.1% GT have maximum syneresis and polydispersity.

12.8.6 Flavored Milk Drink

Other findings demonstrated that GT has a desirable effect on the physical and rheological properties of flavored milk drinks manufactured with date syrup. According to these results, it can be concluded that the application of an appropriate concentration and type of GT results in an improvement in the mouthfeel and textural properties of milk drink made with date syrup. Due to this desirable attribute of the product, it is suggested that GT can play a major role in altering trends in the consumption of artificially flavored milk beverages [92].

12.8.7 Pasta

In a recent study, Chauhan et al. [93] used whole amaranth flour and various gums such as guar gum, acacia gum, and GT to produce pasta without gluten. Their results indicated that the quality of gluten‐free pasta improved when the abovementioned gums were added at various concentrations. It also was found that the textural properties (gumminess, hardness, and chewiness) and cooking quality of pasta improved when higher levels of the gums were used. Comparatively, the authors reported that guar gum best enhanced the physicochemical properties of gluten‐free pasta.

12.8.8 Ketchup

Nayebzadeh and Mohammadifar [94] investigated the influence of different concentrations of GT, guar gum, and xanthan gum on the sensory and rheological properties as well as the stability of tomato ketchup. When the gum concentration increased, the syneresis values decreased and spreadability, yield stress, and viscosity increased. Surprisingly, ketchup samples containing guar gum or xanthan gum showed thixotropic behavior, but GT created rheopectic behavior. Comparatively, the overall texture acceptability, stability, spreadability, and yield stress of the samples incorporating xanthan were better than those obtained for the samples containing GT or guar gums.

More recently, Komeilyfard et al. [95] studied the influence of Angum gum (AnG) alone and in combination with GT (AnG‐GT) on the rheological, sensory, and textural properties of tomato ketchup. The tomato ketchup formulations were control (without gum), GT (0.5%–1.5%), AnG (0.5%–1.5%), and AnG‐GT (0.5%–1.5%). The result showed that incorporation of the hydrocolloids into tomato ketchup formulations led to a significant decrease in the Bostwick consistency value and serum separation. With hydrocolloid addition, the particle size of the samples significantly increased. Shear‐thinning behavior was observed for all tomato ketchup formulations, and the incorporation of hydrocolloids increased the apparent viscosity. The sensory analysis of the samples indicated that hydrocolloid addition had no significant influence on the color properties (lightness, redness, blueness, hue angle, chroma, and total color differences) of tomato ketchup. The overall acceptability of the samples containing 1.5% AnG, 0.5% TG, and 1% and 1.5% AnG‐GT was considerably higher than that of other samples.

12.8.9 Use of GT as Coating Material

Natural gums are broadly used in food systems; however, the use of these biopolymers as an edible coating to extend the shelf life of fruits and vegetable has been explored recently. To date, the effect of GT as coating materials on the physicochemical properties of some fruits such as banana and mushroom has been investigated. The coated mushrooms have a greater shelf life comparing with uncoated samples. Additionally, GT has a protective influence on the hardness of coated mushrooms. Due to the reduction in the weight loss and preservation of the quality of coated mushroom, the application of GT as a coating agent for maintaining the quality of this fruit during storage is recommended. In another research, the effect of a GT coating on the properties of the banana slice has been evaluated by Farahmandfar et al. [96]. According to this paper, the shrinkage of the banana coated with GT is less than that of the uncoated sample. Additionally, coated banana samples have better textural properties as well as greater weight loss, lightness, and rehydration capacity than uncoated samples. These studies showed that use of GT as a banana coating leads to better preservation of its qualitative properties and saving of energy, expense, and time. Anther finding showed that application of Aloe Vera, GT, and a combination of both as edible coatings can change the physical and textural properties, color attributes, weight loss, and carbohydrate percentage of button mushroom [97]. According to this research, the mushrooms coated with Aloe Vera gel, GT, and a combination of both showed less weight loss, color changes, and softening. The authors suggested that the combination of Aloe Vera and GT is more effective in extending the shelf life of button mushroom. More recently, Nasiri et al. [98] used GT impregnated with Satureja khuzistania essential oil as a coating material to improve the shelf life and postharvest quality of button mushroom. It was found that button mushroom samples had better textural properties and lower microbial count than untreated ones. Furthermore, coating with GT+ S. khuzistania essential oil can reduce decomposition rate of ascorbic acid and phenolic compounds.

In conclusion, GT can be introduced as an appropriate coating material for prolonging the shelf life of fruits and vegetables.

12.8.10 Use of GT as Delivery Carrier

12.8.10.1 Complexation and Coacervation

As mentioned before, TG, a biodegradable anionic polysaccharide with the ability to interact with positively charged polymers, can be used as a wall material in encapsulations of various compounds. Gorji et al. [99] evaluated complexation of sodium caseinate with various species of GT as a function of pH. They indicated that the sodium caseinate/GT complex has the following structural transitions:

  1. pH 7.00 to 5.40: no interaction occurred
  2. pH 5.40 to 4.80: initiation of the complexes formation
  3. pH 4.80 to 4.30: formation of inter‐polymer complexes
  4. pH 4.30 to 4.02: optimum coacervation
  5. pH 4.02 to 2.50: suppression of coacervation

As observed above, the physicochemical characteristics of GT are species dependent, and thus it is expected that the complexation of sodium caseinate and GT is also species dependent. Gorji et al. [99] investigated the effect of different pHs on structural transitions during complexation of sodium caseinate and various species of GT (A. rahensis and A. gossypinus). Their study demonstrated that the system containing different species of GT had various particle sizes and critical pH values, which has been attributed to varying uronic acid content and soluble/insoluble fraction ratios of various species. In another study, the formation of electrostatic complexes between A. gossypinus and sodium caseinate as a function of pH (7.00–2.50), biopolymer concentration, and the ratio of biopolymer mixing was studied [100]. GT addition decreased the pH values of sodium caseinate at which sodium caseinate precipitated, indicating that GT can act as a stabilizing agent for sodium caseinate below the isoelectric pH of casein. The authors also showed that the particle size of GT–sodium caseinate was profoundly affected by the presence of GT, particularly at pH = 4.00. At this pH and in the absence of sodium caseinate, the particle size of GT was large, but in the presence of sodium caseinate, the complexation between these biopolymers led to the formation of nanoparticles which were smaller than the particles of blank GT and sodium caseinate.

Firooz et al. [101] monitored the complexation between GT and β‐lactoglobulin at different pH values. The primary soluble GT–β‐lactoglobulin were formed at pH = 5.3, and the aggregation of the interpolymeric complex was initiated at pH = 4.8. They found that at pH = 4.5, phase separation occurred. The particle size of the assembled structure decreased upon complexation, especially after a pH of 4.5.

12.8.10.2 Encapsulation of Phytochemicals using Coacervation Technique

Astragalus compactus, an Iranian species of TG, in combination with maltodextrin, can be used as an appropriate wall material for encapsulation of 2‐methyl butyl acetate (flavoring agent of strawberry). Addition of A. compactus (0.5% w/w) to maltodextrin solutions (14.55% w/w) can increase the viscosity and glass transition temperature to an optimal level. Additionally, A. compactus can decrease the physical defects of microcapsules and prevent sickness. With the incorporation of this species of GT into maltodextrin solution, as cell material, the release rate of 2‐methyl butyl acetate decreases [102].

TG can form polyelectrolyte complexes and hydrogel, and therefore it can be used as an excipient for protein/peptide delivery. TG particles have a submicron particle size, which contributes to drug delivery. Comparatively, TG has higher mucoadhesion in comparison to chitosan, alginate, and PVP [103]. Nur et al. [103] stated that, on the basis of the particle size, rheological properties, zeta potential value, and loading efficiency of nanoparticles‐based TG, it could be used as a suitable carrier for the encapsulation of insulin, as a loaded peptide. The results of their study demonstrated that both insulin and TG have a gelling point at around pH 4.1. The authors reported that the values of particle size and zeta potential for the insulin–TG complex at pH 3.7 were, on average, 566 nm and −14, respectively.

Hosseini et al. [104] prepared a pH‐responsive nanohydrogel based on TG and various chemical cross‐linkers (glyceroldiglycidylether, 3‐aminopropyltriethoxysilane, glutaraldehyde, and PVA) and then investigated its drug delivery and swelling properties. The authors indicated that in vitro release behavior of indomethacin, as a model drug, can be controlled by pH. With increasing pH, the rate of indomethacin release significantly increased, which has been attributed to THE ionization of hydroxyl functional groups at higher pH values. In another study, a pH‐responsive TG–poly (methyl methacrylate‐co‐maleic anhydride)‐g‐poly (caprolactone) microgel was developed for in vitro release of quercetin. It was found that the developed microgels are dependent on pH, gel content, time of immersion, and temperature. Other findings indicated that TG‐based pH‐responsive microgel protects antibiotics against acidic pH of the stomach and therefore can be utilized as a site‐specific drug delivery to release antibiotic to the colon.

Application of TG, as a wall material for encapsulation of Aloe Vera extract, is a safe and biocompatible technique for the controlled release of this healing compound. A relative inhibition effect has been reported for Aloe Vera extract–GT particles against E. coli, Candida albicans, and Staphylococcus aureus. Additionally, a functional wound healing activity and also a cell viability of around 98% has been observed for this novel prepared wound healing product [105].

In a recent study, TG has been used for the in situ synthesis of ZnO nanoparticles on cotton fabric [106]. According to this paper, ultrasonic irradiation results in easy and clean production of ZnO nanoparticles at low temperature. The cotton fabric produced by TG exhibits a clear inhibition zone against E. coil, S. aureus, and C. albicans. Furthermore, it has a good photocatalytic activity on methylene blue degradation.

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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