8
Qodume Shirazi (Alyssum homolocarpum) Seed Gum

Arash Koocheki1 and Mohammad Ali Hesarinejad2

1 Department of Food Science and Technology, Ferdowsi University of Mashhad (FUM), PO Box 91775‐1163, Mashhad, Iran

2 Department of Food Processing, Research Institute of Food Science and Technology (RIFST), PO Box 91735‐147, Mashhad, Iran

8.1 Introduction

Alyssum, a famous genus of Brassicaceae, the mustard family, is native to the Middle East, especially Iran, Iraq, and Pakistan, and comprises 100–170 related species [1]. Alyssum homolocarpum is well known to Iranian practitioners and folk healers [2]. The plant is traditionally known as Qodume Shirazi or Toodari in Persian, and it is administered for various ailments such as topical inflammation and swellings. It was also reported to be beneficial in respiratory complications, sexual dysfunction, and some neurological disorders [3–5]. As seen in Figure 8.1a, A. homolocarpum comprises annual herbaceous plants clothed with stellate, white hairs, growing to 10–20 cm tall, with oblanceolate, or oblong‐linear, leaves, and white flowers. Each locule of this plant has two broad, round pale pink margined seeds with length 1.5–2.5 mm [1].

Image described by caption.

Figure 8.1 Pictorial view of (a) Alyssum homolocarpum plant, (b) seeds, and (c) the seeds soaked in water.

Alyssum homolocarpum seeds have been used as a traditional medicine for hundreds of years (Figure 8.1b). It has been used to cure a dry cough, whooping cough, asthma, pneumonia, and kidney stones in Iranian traditional medicine [6]. The application of plant extracts for the treatment of several diseases and for minimizing the impact of the chemotherapeutic agent is growing [7]. There is evidence that A. homolocarpum seed extract has antioxidant properties [8]. A. homolocarpum is planted mainly for its mucilage, and the outer layer of seeds absorbs moisture rapidly when immersed in water and produces a viscid, turbid, and insipid liquid (Figure 8.1c). The seeds are known to have plenty of mucilaginous substance [1]. Plant mucilages are applied for thickening, binding, disintegrating, emulsifying, suspending, and stabilizing, and as gelling agents [9]. These properties are relevant to their structural properties and metabolic functions in food, pharmaceutical, and biomedical products [10]. Here, we review these characteristics.

8.2 Gum Extraction Optimization

Response surface methodology (RSM) is applied to optimize the effect of different extraction conditions on the extraction yield and functional properties of A. homolocarpum seed gum (AHSG) [1]. In order to extract mucilage from A. homolocarpum seeds, whole seeds are dispersed in deionized water (water:seed of 30:1) at pH 8. The pH is monitored continuously and adjusted by 0.1 mol l−1 NaOH and HCl, while the temperature of the adjustable water bath is set on 48 ± 1.0 °C. Water is preheated before the seeds are added. The seed–water slurry is mixed with an electric stirring paddle throughout the whole extraction period (1.5 h). Separation of the seed from the liquid is performed using a 27 cm basket centrifuge lined with a 1 mm mesh. The seed slurry is poured into the basket while the centrifuge is running at approximately 1200 rpm. The mucilage is regained from the extract via precipitation in three volumes of 95% ethanol. The precipitates are collected, dispersed in deionized water, and dried overnight in a vacuum oven [1]. Independent extraction variables including temperature, pH, and water:seed ratio have significant effects on the yield, purity, and viscosity of AHSG. Applying the desirability function method, optimum extraction conditions are found to be a temperature of 36.3 °C, pH of 4, and the water:seed ratio of 40:1. At this optimum extraction condition, the consistency coefficient, flow behavior index, extraction yield, and protein content are 8.27 (Pa sn), 0.29, 287.3 (g kg−1), 1.27 (%), respectively. Furthermore, the apparent viscosity of AHSG (3%) is 855.9 (mPa s) at a shear rate of 46.16 s−1 and 25 °C [1].

8.3 Physicochemical Properties

8.3.1 Composition

AHSG has high total carbohydrate content (85.33 ± 0.89, w/w%), which reveals the relatively high purity of the extracted gum [11]. The gum consists of a small amount of uronic acids (5.63 ± 0.18, w/w%), illustrating its weak polyelectrolyte nature [11]. This polysaccharide is mainly composed of galactose (82.97 ± 0.64), glucose (5.70 ± 0.06), rhamnose (5.04 ± 0.29), mannose (3.04 ± 0.37), xylose (2.72 ± 0.07), and arabinose (0.53 ± 0.01), which is different from most other gums and is probably a galactan‐type polysaccharide and not a galactomannan or glucomannan type [11]. The zeta potential (ξ) of AHSG solution (0.1% w/w) is −25.81 ± 0.04 mV at neutral pH, meaning that AHSG has a negative charge and so is an anionic hydrocolloid. The polysaccharide backbone composes chiefly of 1,2‐rhamnose, 1,3‐ and 1,3,6‐ galactose glycosidic linkages [12]. Measuring the molecular weight (Mw) at room temperature (25 °C) indicates that AHSG has a small molecular weight (3.66 × 105 Da) compared to other gums [11]. AHSG (0.1%) has an average particle size of 225.36 ± 31.06 (nm). A smaller particle size can lead to a more stable suspension, showing that AHSG solutions might be more stable than those of other hydrocolloids [11,13].

8.3.2 Fourier Transform Infrared Spectroscopy (FTIR)

The main absorption peaks of AHSG and their tentative assignments are summarized in Table 8.1 [11]. The Fourier transform infrared spectroscopy (FTIR) spectrum of AHSG is dominated by a broad band at about 895 cm−1, resulting from the presence of β‐D‐mannopyranose units. The absorptions at the wavenumber range of 950–1150 cm−1 are ascribed to vibrations of CO, COC glycosidic, and COH bonds. The peak at 1621 cm−1 is because of the asymmetrical COO stretching vibration, while the band at 1426 cm−1 is caused by the symmetrical COO stretching vibration [14,15]. The 3000–2800 cm−1 wavenumber range is related to the stretching modes of the CH bonds of ethyl groups (CH3). The bands around 2900–2950 cm−1 refer to CH absorption including CH, CH2, and CH3 stretching and bending vibrations, symmetric, asymmetric, and occasionally double overlapping with OH [16].

Table 8.1 The main absorption peaks and their tentative assignments of FTIR spectrum of purified Alyssum homolocarpum seed gum.

Source: Adapted from Hesarinejad et al. [11] with permission from Elsevier.

Wavenumber (cm−1) Fragment
3460.27 Hydroxyl (OH) stretching
2902.71 CH stretching of CH2 and CH3 groups
2369.74 Alkynes CC Stretch or C≡C terminal alkynes
1621.34 COO asymmetrical stretching of the hydrogen‐bonded carboxylic groups
1426.57 CH deformations/COO symmetrical stretching of carboxylic groups
1066.03 CO, COC glycosidic and COH bonds
895.10 CH out of plane
777.62 CH out of plane

8.3.3 Chain Flexibility

The value of the Huggins constant (KH) shows the polymer–polymer interactions in a dilute regime and is related to the molecular architecture and the extent of polymer coil expansion. The KH value of AHSG in deionized water at 25 °C is 0.301, which approximated its value in good solvents (0.3). In addition, when the temperature is increased from 25 to 65 °C, the KH value decreases, indicating weaker intermolecular polymer–polymer interactions or deterioration of the solvent quality at high temperature [11,17]. The chain flexibility parameter (Ea/R) and activation energy (Ea) for AHSG are 618.54 and 0.51 × 107 J (kg mol)−1, respectively. These results indicate that the flexibility of the AHSG chain is relatively low compared to some stiff chain hydrocolloids like chitosan, xanthan, and sage seed gum [11,18].

8.3.4 Shape, Swollen Volume, and Hydration Parameters

Some molecular parameters of AHSG are summarized in Table 8.2. The shape function (ν) is used with an anhydrous macromolecule which essentially expands when suspended or dissolved in solution because of its associations with the solvent. The swollen specific volume (νs) is a measure of a (aqueous) solvent associated with the macromolecule and is defined as the volume of the macromolecule in solution per unit anhydrous mass of macromolecules. The hydration parameter (δ) is considered as the level to which an aqueous solvent can be added to a dry macromolecule beyond which there is no change in a macromolecular property other than dilution of the sample. The shape of the AHSG macromolecules is spherical at temperatures between 25 and 65 °C, and it has a universal shape function. It is further observed that the hydration value of AHSG is temperature dependent and decreases when the temperature is increased up to 55 °C. Results relating to the AHSG hydration parameter suggest the plausible reduction in the associated solvent through hydrogen bonds and/or physical entrainment, leading to an enhancement in the intermolecular interactions (i.e., aggregation) between unsolved chains [11].

Table 8.2 Some molecular parameters of Alyssum homolocarpum seed gum.

Source: Adapted from Hesarinejad et al. [11] with permission from Elsevier.

Temperature (°C) νs (dl g−1) ν (−) δ (−) Rcoil (nm) Vcoil (nm3)
25 7.21 2.55 6.55 11.10 5732.50
35 6.98 2.53 6.32 10.77 5231.69
45 6.80 2.30 6.14 10.56 4935.10
55 6.89 2.48 6.20 10.44 4777.08
65 6.96 2.61 6.26 10.57 4956.98

νs: swollen specific volume; ν: shape function; δ: hydration parameters; Rcoil: coil radius; Vcoil: coil volume.

8.3.5 Coil Radius and Volume

The hydrodynamic coil radius is based on the Einstein viscosity relation, and the coil volume depends on the assumption that the shape of the AHSG coil is sphere‐like. The alterations in the hydrodynamic coil radius (Rcoil) and thereby in the corresponding volume (Vcoil) of AHSG are caused by the temperature rise (Table 8.2). The temperature could depress the coil dimensions; however, a small increase is observed by a subsequent increase in temperature to 65 °C. The decrease in the AHSG coil dimensions by heating could be attributed to the reduced stability of the hydrogen bonds between AHSG and solvent molecules and a small increase in the stability of the intramolecular interactions between the polymer segments of AHSG. The values obtained for the AHSG coil radius and volume at 25 °C are 11.10 nm and 5732.50 nm3, respectively [11].

8.3.6 Partial Specific Volume

A particle's buoyancy in food systems is an influential factor that affects the sedimentation phenomenon. As the partial specific volume (images ) increases, the buoyancy of a specific particle is increased, and therefore the greater the partial specific volume of a polymer, the less the sedimentation [19]. The partial specific volume of AHSG is 0.44 ml g−1, which is smaller than that of most gums [11].

8.4 Rheological Properties

8.4.1 Intrinsic Viscosity

The intrinsic viscosity [η] is calculated by measurement of the solution viscosity at very low concentrations. The result of the AHSG intrinsic viscosity [η], calculated using five models (Huggins, Kraemer, Tanglertpaibul and Rao, Higiro 1, and Higiro 2) at different temperatures (25–65 °C), shows that the Tanglertpaibul and Rao model gives the highest determination coefficient. It is also observed that on the basis of this model, the intrinsic viscosity diminishes from 23.11 to 19.65 dl g−1 when the temperature is increased from 25 to 55 °C. Increasing the temperature may result in decreasing hydrogen‐bonded hydration water of glucose, dextran, and so on. The decrease in hydrogen‐bonded hydration water may decrease the specific volume, and therefore the intrinsic viscosity decreases. The intrinsic viscosity of AHSG at a temperature of 65 °C increases to 20.39 dl g−1, which could be attributed to the increased chain dimensions of AHSG [11].

The solutions of sucrose and lactose are poor solvents for AHSG as indicated by a decrease in the intrinsic viscosity. As the sucrose and lactose concentrations increase, the coil radius of AHSG decreases. The reduction in the shape and swollen volume parameters in the presence of sucrose and lactose, as compared to the sugar‐free solution, indicates the negative effect of the chosen sugars on the molecular volume of AHSG. Evaluations of the dilute solution properties of this gum in sucrose and lactose solutions reveal the existence of a conformation which tends to an ellipsoidal shape and the probability of a random coil conformation with no molecular entanglements in AHSG solutions [20].

8.4.2 Steady Shear Rheological Properties

AHSG exhibits a non‐Newtonian shear‐thinning behavior. The rheological behavior of AHSG solutions is well described by the power‐law model with a high coefficient of determination. Increasing the gum concentration decreases the flow behavior index values while increasing the consistency coefficient. The values of the consistency coefficient (k) vary between 1.48 and 29.80 Pa sn for upward and between 1.47 and 31.07 Pa sn for downward curves as the gum concentration increases from 1.5% to 4% [21]. The increase in k values is probably related to the increase in the water binding capacity [22]. The apparent viscosity of AHSG decreases with increasing shear rate, and it has a direct dependency on the gum concentration too [21]. The flow behavior of AHSG shows that after a sharp reduction, the viscosity change is smoothened at high shear rates. Shear thinning is the result of an orientation effect. As the shear rate is increased, the long chain of polymer molecules and randomly positioned chains become increasingly aligned in the direction of flow, resulting in less interaction between adjacent polymer chains [21]. The concentration of AHSG in solution is known to affect the apparent viscosity and the degree of pseudoplasticity [23]. Increasing the AHSG concentration increases its apparent viscosity. This is due to the presence of higher solids content, which generally cause an increase in the viscosity owing to mainly molecular movements and interfacial film formation [21].

8.4.2.1 Effect of Temperature

The examination of flow properties as a function of the shear rate designates the non‐Newtonian behaviors of AHSG at different temperatures. The values of the flow behavior index are less than 1, indicating the pseudoplastic (shear‐thinning) nature of the gum at different temperatures. By increasing the temperature, the flow behavior index increases, and the consistency coefficient decreases. Furthermore, no differences are found between the flow behaviors indices of the upward and downward curves. The effect of temperature on the flow behavior index (n) is negligible until 45 °C at gum concentrations of 1.5% and 2%. The concentrated solution of AHSG (4%) at 5 °C is the most pseudoplastic among the other concentrations of AHSG (1.5%–4%). A decrease in the consistency coefficient is observed with increasing temperature, which indicates a decrease in the apparent viscosity at higher temperatures [24]. AHSG has lower k values compare to carrageenan and xanthan at similar concentrations and higher values compare to that of pectin, starch, and the rounded‐tuber salep [25,26].

The apparent viscosity of AHSG decreases with increasing temperature [24]. This effect is reversible and is due to the interactions of the molecules in solution which become weaker at a higher temperature [27]. Viscosity is a function of the intermolecular forces and water solute interactions that restrict the molecular motion. Therefore, as temperature increases, the thermal energy of the molecules increases, and the intermolecular distances increase as a result of thermal expansion [24,28]. The temperature dependence of the viscosity is assessed by applying the Arrhenius‐type model. The activation energy for AHSG decreases when the gum's concentration increases from 1.5% to 4% (from 8229.81 J mol−1 to 4520.57 J mol−1), indicating that the solution with lower concentration has the greater viscosity sensitivity to temperature. A higher activation energy value signifies a more rapid change in viscosity. Therefore, temperature control is more critical when 1.5% AHSG is used [24].

8.4.2.2 Effect of pH

AHSG is sensitive to pH. Increasing the pH of the AHSG solution from three to seven augments the pseudoplasticity and the consistency coefficient. These effects have been explained by the induction of electrostatic repulsion by functional groups, which tend to keep the molecules in an extended form, thus producing a highly viscous solution, and therefore the consistency coefficient values increase [29,30]. In the vicinity of pH 7, at the point where the carboxyl groups are ionized to a degree that the consistency index reaches a maximum, its molecular chains in the solution are in rod conformation [21]. The apparent viscosity of AHSG also increases when the pH is increased to 7. For pHs above 7, the viscosity values do not change. The pH has relatively little effect on the apparent viscosity over the range 7–9 [21]. This could be explained by the ionization of the mucilage carboxyl groups for pHs above 7.

8.4.2.3 Effect of Salt

Since the gum extracted from AHSG behaves as a polyelectrolyte, the solution viscosity is affected by the addition of salt. If no intermolecular interaction occurs, the viscosity of a dilute gum solution decreases due to the screening of charge and contraction of the macromolecule in presence of the counterions. In a more concentrated solution, the presence of multivalent ions may promote interactions between chains, resulting in an increase in the viscosity [21]. The values of the flow behavior index increase progressively when the salt concentration is increased from 0.035 to 0.172 M for NaCl and KCl. Addition of CaCl2 at 0.01 M and MgCl2 at 0.039 M augments the flow behavior index, but decreases afterward and remains constant at higher salt concentrations. The consistency coefficient of AHSG decreases with the addition of NaCl and KCl. Also, increases in CaCl2 concentration from 0 to 0.01 M and in MgCl2 concentrations from 0 to 0.039 M decrease the consistency coefficient value. The results of surveying the effects of some edible salts on the flow behavior of AHSG confirm that KCl can decrease the apparent viscosity of AHSG more than other salts [21].

8.4.2.4 Effect of Sucrose

The values of the consistency coefficient and flow behavior index of AHSG are influenced by sucrose concentrations. The consistency coefficient value of AHSG increases in the presence of sucrose, while the flow behavior index value decreases. The lower flow behavior index indicates that at higher concentration of sucrose, the solutions are less pseudoplastic [21].

8.4.3 Time Dependency (Thixotropy)

The thixotropy and rheopexy of a polymer solution are interpreted as the continuous breakdown or rearrangement of network links formed by the associations between polymer chains during shearing, respectively [24]. The hysteresis loop between the upward and downward curves indicates time dependency. The extent of thixotropy increases with increasing AHSG concentration and decreases with increasing temperature and shear rate [24]. Three models are generally used to predict time‐dependent rheological behavior, namely, the first‐order shear stress decay, second‐order structural kinetic, and Weltman models. The AHSG concentration has a considerable effect on the degree of the thixotropic behavior. On the other hand, the solution becomes somewhat less thixotropic at high temperatures. The rupture rate of AHSG association increases at high shear rates, temperatures, and concentrations. Among the three models used, the thixotropic behavior of AHSG is well described by the first‐order shear stress decay [24].

8.4.4 Dynamic Rheological Properties

AHSG dispersions exhibit viscoelastic properties at any given temperature (5°–85°). The storage modulus (G′) is always higher than the loss modulus (G″) at all concentrations and temperatures. The mechanical spectra of AHSG are classified as weak gels on the basis of the frequency sweep, complex viscosity, and loss tangent results. The frequency sweep test reveals that viscoelastic moduli have a very low‐frequency dependency, indicating that AHSG is a cross‐linked network. At higher concentrations (3%) and a heating‐cooling rate of 1 °C min−1, AHSG forms a gel during cooling. At high gum concentrations (2.5%–3%), as the temperature increases from 50 to 85 °C, the storage modulus starts to increase, whereas for low AHSG concentrations, an increase in temperature has no significant effect on the storage modulus [31,32].

8.5 Biological Activity

Antioxidants are vital substances which possess the ability to protect the body from the damage caused by free‐radical‐induced oxidative stress [33]. A variety of free‐radical‐scavenging antioxidants are found in dietary sources like fruits, vegetables, and tea. The characteristics of A. homolocarpum and the inhibitory effects of its methanolic extracts on linoleic acid peroxidation are expressed as IC50. Considering the IC50 values (94.25 µg ml−1), the antioxidant activity of A. homolocarpum is low (IC50 > 75 µg ml−1). The phenolic content of A. homolocarpum, calculated as the gallic acid equivalent, is 165.68 mg/100 g of the dry weight, which is moderate (100–300 mg) according to Souri et al. [33]. The cytotoxic activity of ethanolic extracts of A. homolocarpum is studied against three different cancer cell lines: colon carcinoma (HT‐29), colorectal adenocarcinoma (Caco‐2), and breast ductal carcinoma (T47D). In addition, Swiss mouse embryo fibroblasts (NIH 3T3) are used as normal nonmalignant cells. The MTT assay (3‐(4, 5‐dimethylthiazol‐2‐yl)‐2, 5‐diphenyltetrazolium bromide) is utilized for calculating the cytotoxicity of extracts on cell lines [34]. The anti‐proliferative effects of the ethanolic extract of A. homolocarpum are found to have no cytotoxic activity on colon, colorectal, and breast cancer cell lines [8].

8.6 Applications

Gum extracted from AHSG can be used as thickening, fat replacing, and stabilizing agents [1,21,35]. In the following sections, the applications of AHSG in the formulation of different food products are reviewed.

8.6.1 Emulsions

AHSG could be used as a potential stabilizing agent for food emulsions, although it does not show any noticeable emulsifying property or surface activity [35]. On the other hand, AHSG is a poor emulsifier owing to its high hydrophilic nature, low molecular flexibility, and consequently low surface activity. However, it may contribute to the stability of O/W emulsion by increasing the medium viscosity, forming a network in which oil droplets are entrapped and thus reducing the mobility of droplets and delaying their coalescence [35–38].

Increasing the gum concentration enhances the stabilizing capacity and improves the heat stability of ultrasonically prepared corn oil‐in‐water emulsions. A low degree of droplet polydispersity is observed when higher concentrations of AHSG are used. The negligible surface activity observed for this gum can be attributed to the proteins co‐extracted from the seed that are not completely removed during the gum purification process [35]. The number of large droplets present in these emulsions increases during storage. These alterations are more pronounced in the emulsion containing 0.25% AHSG over the first 3 weeks, and thereafter no significant changes occur. In general, emulsions made with higher concentrations of AHSG (>0.5%) are stable during the storage period of 4 weeks, and the size distribution of droplets does not show any remarkable variation [35]. The droplet diameter of AHSG stabilized emulsions varies over a wide range of sizes from 0.39 to 6.04 µm for the fresh emulsion and 0.39–15.81 µm for the sample stored for 4 weeks. During storage, the median diameter increases from 2.14 to 2.48 µm within the first 2 weeks and remains unchanged afterward [35]. For the freshly prepared emulsion, the Sauter diameter (D32) decreases gradually from about 1.92 to 1.59 µm when the concentration of gum increases from 0.25% to 0.75% [35].

The stability of emulsions against creaming and phase separation can be enhanced by increasing the AHSG concentration. At lower concentrations of gum (0.25% and 0.5%), distinctive creaming is observed within 7 days. On the other hand, at higher concentrations (0.75% and 1%), no visual changes are observed, and the emulsions retain their initial integrity during storage. This is due to the increase in the viscosity of the aqueous phase as well as the small size of droplets at higher proportions of the gum that drastically reduce the mobility of the oil droplets and hence their upward movement. The cream layer in the emulsion with 0.25% gum is evidently more compact, indicating free mobility of droplets and a higher degree of their interactions with each other due to weak forces in the aqueous phase. The emulsions stabilized with 0.75%–1% AHSG; on the other hand, they show no distinguishable serum separation during 28 days of storage [35].

Briefly, the incorporation of AHSG in an O/W emulsion greatly enhances the stability against flocculation, coalescence, and gravitational phase separation. In addition to its beneficial effect on the emulsification stage, it also reduces the droplet–droplet interactions during the storage of emulsion and thus delays the occurrence of destabilizing phenomena. These effects are found to be all dependent on the concentration of gum. AHSG with 0.75% concentration is more effective for producing small droplets during ultrasonic homogenization. However, energy input considerations should be taken into account when higher levels of the gum are to be used. Considering the excellent stability of emulsions against phase separation as well as their improved texture and consistency, AHSG could be accounted as a potential stabilizer for O/W emulsions [35]. Figure 8.2 illustrates changes in the overall appearance of emulsion samples after 4 weeks [35].

Image described by caption.

Figure 8.2 Overall appearance of emulsion samples containing different levels of Alyssum homolocarpum seed gum after storage for four weeks. From right to left: 0.25%, 0.5%, 0.75%, and 1% AHSG, respectively.

Source: Adapted from Koocheki et al. [35] with permission from Elsevier.

AHSG and emulsifier (Tween 80) and NaCl greatly affect the quality attributes of an O/W emulsion and its overall stability during storage. The contribution of AHSG, however, seems to be more prominent than that of the emulsifier once the droplets form. At higher AHSG content, the addition of salt up to 1% has no effect on the physical properties of the emulsion, except for the viscosity, which is slightly influenced. Taking into account the nature of the surfactant and the behavior of the gum, the changes observed in the presence of salt are postulated to be mainly due to the competition between gum and NaCl for water. The optimum emulsion formulation for the maximum specific surface area and viscosity as well as the minimum D32, span, and creaming index are achieved by setting the AHSG concentration, Tween 80 content, and NaCl content at 1%, 0.96%, and 0%, respectively. This emulsion is greatly stable against flocculation and gravitational phase separation. It shows pseudoplastic behavior (n < 1) with no significant changes in the flow behavior index and consistency coefficient during storage [39].

For emulsions stabilized with an AHSG–whey protein concentrate mixture, increasing the gum content increases particle size, the negative charge on the droplet surface, the consistency coefficient, yield stress, and hysteresis between the forward and the backward flow diagrams. The particle size distribution curve is monomodal, and emulsions stabilized with this mixture show non‐Newtonian shear‐thinning behavior. No creaming phenomenon is observed in these emulsions [40].

8.6.2 Encapsulation

Studies on the nano‐encapsulation of bioactive compounds are taken into consideration because of the unique properties of materials at the nanometer length scale. Nanostructures exhibit a greater surface area to volume ratio, which improves encapsulation efficiency, solubility, bioavailability, and controlled release of the encapsulated compounds [41]. AHSG could also be used for fabrication of nanocapsules containing D‐limonene through the electrospraying process. However, the morphology of these nanostructures is mainly affected by the physical properties of AHSG solution, particularly its rheological behavior, surface tension, and electrical conductivity. AHSG solution (0.5% w/w) produces aggregated structures with a wide size range owing to its high surface tension and high electrical conductivity. Addition of 10% and 20% D‐limonene and 0.1% Tween 20 increases the solution apparent viscosity, while it decreases the surface tension and conductivity of the solution, allowing the formation of round and smooth capsules. The size of these particles, ranging from 35 to 90 nm, increases with increasing D‐limonene content. The transition from particles to fibers occurs for 30% D‐limonene emulsion, due to its low storage modulus (G′), high loss modulus (G″), and very low surface tension. High encapsulation efficiency (around 87%–93%) and the low surface of D‐limonene reveal the stability of 10% and 20% D‐limonene emulsions, which do not separate during the electrospraying process. D‐limonene‐loaded nanocapsules have a fully amorphous structure. The increased thermal stability of D‐limonene after encapsulation holds promise for the protection of sensitive bioactive compounds in thermally processed food [41].

8.6.3 Edible Film

AHSG has great potential to be used as a new source of biodegradable film due to its proper thickening/gelling function [42]. AHSG can produce films with excellent appearance and can easily be cast with satisfactory mechanical properties. The films formulated without plasticizer or with plasticizer content lower than 25% (w/w) are brittle and difficult to handle and remove from the casting plate. An increase in plasticizer concentration increases the transparency, flexibility, and homogeneity of the films, as well as producing smooth surfaces without pores or cracks. An over 50% (w/w) increase in glycerol concentration makes AHSG films very soft and sticky [43]. The density of AHSG films decreases upon adding the plasticizer, and a slight variation is evident. In this condition, the moisture content of the films increases, and this rise is likely caused by the water‐holding capacity of glycerol [44]. The increase in moisture content is consistent with the increase in film thickness. Increasing the moisture and glycerol contents decreases the contact angle and density. Given the hydrophilic nature of glycerol, reducing the hydrophobicity would consequently decrease the contact angle of AHSG films [43,45]. Increasing the glycerol content from 25% to 45% augments the film solubility. These are directly related to the hydrophilic nature of glycerol in the AHSG films. If the packaged film would be consumed simultaneously with food, then a high degree of solubility would be acceptable [43].

A significant increase in both water vapor permeability (WVP) and oxygen permeability (OP) is observed after adding glycerol. The predominantly hydrophilic behavior of biopolymers, such as polysaccharides, results in poor water barrier characteristics. The WVP of films is a characteristic that depends on the diffusion rate and solubility of water in the film. Thus, the WVP of plasticized AHSG film would increase by adding a plasticizer to the film matrix. Increasing the glycerol concentration decreases the intermolecular forces among the biopolymer chains, and increases the segmental motions and free volume, allowing water molecules to diffuse more easily and providing a higher WVP [43,45,46].

Given the significance of oxygen in the oxidation of food, the OP of food packaging materials is very important in food preservation. The oxygen and aroma permeability of polysaccharide‐based films are very low because of large amounts of hydrogen bonds. The biopolymer chain structure and plasticizer distribution within the biopolymer matrix are important in the permeability of films. Increasing the glycerol levels from 25% to 45% augments the OP values of AHSG film drastically. This finding could be attributed to an increase in chain mobility of the AHSG and the creation of void spaces in the film matrix [45]. The OP of AHSG films at 50% RH is 67.36–102.27 (cm3 µm m−2 d atm) [43]. Incorporating glycerol into the AHSG films mostly increases the lightness index (L*) [43].

One of the most important parameters required for food packaging is the mechanical stability of films during shipping, handling, and storage [47]. Thus, these properties are critical for the evaluation of the industrial biodegradable film. Tensile strength (TS), Young's modulus (YM), and elongation at break (EB) are the most common parameters used to evaluate the mechanical properties of films [48,49]. The TS and YM of the plasticized AHSG films decrease when the plasticizer content increases from 25% (19.39 MPa and 0.438 GPa) to 45% (11.87 MPa, 0.279 GPa), respectively. In addition, by adding glycerol, the EB of such film increases from 25% to 43%. Therefore, the increase in glycerol concentration in AHSG films weakens the films but enhances their flexibility [43]. The glass transition temperature (Tg) of the AHSG film is around 43–58 °C. The addition of glycerol increases the moisture content of films and then decreases the Tg of films from 58 to 43 °C. The AHSG films surface without plasticizer appears smooth, but that of the plasticized AHSG films exhibits certain differences in terms of the surface microstructure. This is caused by the high moisture content absorbed by glycerol, which is hydrophilic in nature. Increasing the concentration of glycerol from 25% to 45% results in a rougher film surface compared with that of control films. The pores on the film surface could be the binding site for water during moisture uptake. This qualitative result may explain the increase in gas permeability and moisture content of AHSG films containing plasticizers [43].

To improve the physical, mechanical, barrier, thermal and optical properties of AHSG film, Marvdashti et al. [42] blended AHSG with polyvinyl alcohol (PVA). They investigated the interactions among two polymers and their effect on the properties of blend films. Films made from AHSG have poor mechanical and barrier (to oxygen) properties. The addition of PVA to this film significantly increases the moisture content, solubility, EB, and transparency, while it decreases the density, OP, Chroma, water contact angle, and Young modulus. The films with higher AHSG to PVA ratios have a lower WVP. The light barrier measurements present low values of transparency at 600 nm for PVA/AHSG films, indicating that films are very transparent while they have excellent barrier properties against UV light. The moisture content of these blend films is affected by the PVA to AHSG ratios. Since PVA is a polar polymer containing many hydroxyl groups, it can impart higher hydrophilicity to the film. Therefore, as the PVA to gum ratio increases in PVA/AHSG films, the number of possible hydrogen bonds that interact with water molecules increases. The moisture content of the film negatively correlates with their density. The moisture content of these films affects their permeability to water and gas; therefore, a film with high moisture content cannot act as a good barrier against water vapor and gases. The moisture sorption of pure AHSG films at 0.9 a w is 2.7 times lower than that for pure PVA films. As the AHSG ratio increases in PVA/AHSG blend films, their water sorption ability decreases. Due to the higher hydrophilic property of PVA, any increase in the AHSG content enhances the water resistance of the blended films. The hydroxyl groups of PVA can form hydrogen bonds with hydroxyl and/or carboxyl groups of AHSG, which might decrease the free hydroxyl groups and reduce the number of hydroxyl groups available for binding with water molecules in the blend films. AHSG film has the lowest solubility; therefore, with the addition of PVA to AHSG film, the water solubility increases. This might be due to the distribution of hydroxyl groups and their higher density in AHSG film. Pure AHSG film has a smooth surface and reveals a homogeneous structure, where polysaccharide chains aggregate to form a continuous and dense network. AHSG film shows a surface with no obvious phase separation, crack, or pores. Incorporation of PVA into AHSG film forms a homogeneous structure, due to the high compatibility of both polymers [42].

AHSG film has a rough cross section due to its brittle structure, which fractures when liquid nitrogen is used for the preparation of film for the scanning electron microscopy (SEM) test. SEM analysis shows that at higher PVA to AHSG ratios, complex networks with high homogeneity are formed. The color parameters of films, including lightness (L*), redness or greenness (a*) and yellowness or blueness (b*), the total color difference (E), Chroma (C), yellowness index (YI), and whiteness index (WI), significantly change when the AHSG/PVA ratio increases. Chroma values decrease with the addition of PVA to AHSG film, probably due to the reduction of film compactness. The E and YI of AHSG/PVA blend films decrease with increasing PVA content, while their WI increases. These trends might be due to the variation of L* and b* with the addition of PVA. The lowest OP is observed for PVA film, while AHSG film has the highest OP. The oxygen permeability of the blended films decreases with increasing PVA to AHSG ratios. The oxygen permeability value of these films is below 10 cm3 µm−2 d−1 kPa−1. The WVP of AHSG film can be changed by the integrity of the film, the hydrophilic‐hydrophobic ratio, and the water vapor diffusivity rate [50]. Pure AHSG film has the lowest WVP. On the other hand, AHSG film resists water molecule transfer through its matrix; therefore, AHSG could reduce the WVP of PVA‐AHSG films. The water contact angles (WCAs) of AHSG/PVA blend films lie between 30° and 90°. The WCA of AHSG film is 74.6°. Addition of PVA to AHSG film decreases the WCA of the blend films. A conceivable reason for this phenomenon might be the difference in the nature and chemical structure of film‐forming materials which leads to diverse WCA films [42].

The glass transition temperatures of PVA/AHSG blend films are in the range −78.52 to −67.6 °C. Tg increases when the PVA to AHSG ratios decrease. This increase indicates that the mobility of PVA film decreases with the addition of AHSG. This trend could be due to the higher molecular weight of AHSG in the blend matrix, which increases the interactions among polymer chains and hence Tg [51]. The thermal stability verifies due to the difference in interactions between AHSG and PVA polymers. This result indicates that more intermolecular interactions through hydrogen bonding between PVA and AHSG improve the thermal stability of the final films. AHSG film has a significantly higher YM (3526.5 MPa) and TS (37.23 MPa), but a lower EB (1.4%) compared with PVA film. The higher elongation indicates that the film is more flexible when subjected to tension and mechanical stresses [52]. The high TS and low EB of AHSG film are probably due to the presence of higher intermolecular interactions along the polysaccharide chains (SEM), solubility, density, and moisture content. Intermolecular interactions and hydrogen bonds between polymer chains make a rigid and brittle matrix with low elongation. An increase in the number of intermolecular crosslinks and decrease in the intermolecular distance in AHSG film increase its TS but decrease the EB. The results of FTIR show a clear interaction between PVA and AHSG, forming a new material. These results indicate that PVA/AHSG blend films have good compatibility [53].

Film transparency provides information about its ability to protect the packaged material from the disadvantages of light, especially UV radiation. Ultraviolet light is a powerful oxidative agent, so light transmission resistance of films can determine their application for different conditions. The light barrier properties of films are controlled by the amount of AHSG in the final blend. Transparency values tend to decrease in films with higher PVA to AHSG ratios compared to AHSG film. Transparency and opacity are inversely related, so high transparency leads to low opacity [47]. High AHSG content in films produces high opacity values. The transparency of PVA/AHSG blend films is high, so these blend films are clear enough to be selected and used as a see‐through packaging material [42].

8.6.4 Application in Dairy Products

AHSG has been considered as a substitute for fat in ultrafiltrated Iranian white cheese (UF cheese). By decreasing the fat content, the moisture content increases, and protein plays a more effective role in cheese texture. These changes affect the sensory, functional, microbial, and chemical properties of cheese. AHSG can technically be a good replacer for fat in Iranian white cheese. Increasing the AHSG concentration enhances the moisture content of cheese, and thus the texture softness increases. The most acceptable concentration of AHSG is found to be 0.3%, while the texture of samples containing 0.4% AHSG is too soft and have an undesirable mouthfeel [54].

AHSG can be also used as a natural stabilizer in yogurt drink. Addition of AHSG to yogurt drink changes its flow behavior from a Newtonian fluid to a shear‐thinning one, and the apparent viscosity increases from 4.5 to 8.8 cP. Adding AHSG causes stabilization in yogurt drink. The highest stability is observed for the sample containing 0.3% AHSG, while sensory evaluations show that 0.2% AHSG is the most desirable concentration [55].

8.6.5 Application in Bakery Products

Sponge cake is a bakery product with an approximate shelf life of four week. Retarding the staling rate of baked goods is one of the biggest issues and is of nutritional and economic importance. The application of AHSG in bakery products increases the shelf life and postpones its staling. The addition of AHSG up to 0.75% improves the characteristics of the sponge cake in terms of sensory, shelf life, cake batter specific gravity, volume, and apparent density. During the storage time, the highest moisture content and the lowest firmness are observed in sponge cake containing 0.75% AHSG [56].

The addition of AHSG can improve the rheological properties of wheat flour and increase the quality of bread. The gelatinization temperature of flour containing this gum is reduced. AHSG ameliorates the properties of bread and its softness and therefore can be added as a natural supplement to wheat flour to improve its quality [4].

8.7 Conclusion and Future Trends

The physicochemical properties of A. homolocarpum seed gum have been reviewed in this chapter. The benefits of AHSG solutions depend on their extraction methods (temperature, pH, and seed to water ratio) and the measurement conditions (concentration, temperature, shear, pH, and the attendance of additives such as salts and sugars). The biological activity, physicochemical, functional, and rheological attributes of A. homolocarpum seed gum demonstrate a potential for their usage as a novel food hydrocolloid and herbal medicine sources.

The cost of commercial gums is steadily increasing due to drought and unexpected increase in demand. Therefore, their usage by food manufacturers will be very limited. In this situation, if a new replacement is found, the new gum will be welcomed. Also, the food industry will always substitute new texturizing gums to the old ones. AHSG can be applied because of its unique properties in the food industry, such as dairy, bakery, and other food processing industries. Middle East countries can help to export AHSG in the future to support the productivity in the food industry. Subsequently, AHSG can affect the global economy gradually. It can be one of the importing commodities in the world market. Future and options for AHSG application can be high in the coming years.

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