11 Persian Gum (Amygdalus scoparia Spach) – Emerging Natural Hydrocolloids

11
Persian Gum (Amygdalus scoparia Spach)

Soleiman Abbasi

Food Colloids and Rheology Laboratory, Department of Food Science and Technology, Faculty of Agriculture, Tarbiat Modares University, 1411713116, Tehran, Iran

11.1 Botanical Aspects and Importance

Almond botanically belongs to the Rosaceae family and Prunus genus. Prunus encompasses a large group of deciduous and evergreen trees and shrubs, including Amygdalus, Prunophora, and Cerasus [1]. The Amygdalus L. itself consists of 26 well‐defined species and a long list of additional binomials (including 20 different botanical names). The principal diagnostic trait of Amygdalus is that it flowers before the appearance of leaves and bears fruit in which the pericarp (hull) dries out at maturity. In addition, Amygdalus L. was originally found in the hot and dry climate of the Middle East, in all probability the Iran plateau (Figure 11.1a), and subsequently spread over the other regions (Figure 11.1b), namely, Southeast Asia, Southeast Europe, the central Mediterranean basin, south Mongolia, and Western China [2,3].

Figure 11.1 Illustration of (a) different ecological zones of Iran and (b) natural distribution area of Amygdalus L., excluding cultivation.

Source: Adapted from Browicz and Zohary [2] with permission from Springer.

Amygdalus scoparia Spach (Synonym: Prunus scoparia Spach after the French Botanist Edouard Spach, 1801–1879), is one of the wild (mountain) almond species of Amygdalus L. which can be found along with other wild species in the Iran‐o‐Touranian (Figure 11.1a) zones [2]. From the phenotypic point of view, it is an upright broom‐like shrub (3–4 m tall) with non‐angled branches (Figure 11.2). The diameter of a well‐developed trunk is around 10 cm, and this plant can grow on loose conglomerates, limestone cliffs, loose volcanic rocks, crevices in rock slopes, and in clay and sandy soils. It most commonly can be found at high altitudes (1200 up to 2700 m). This plant is appreciated mostly due to its economic importance (the usage of its products and by‐products in the food, pharmaceutical, and chemical industries), and its use as a natural resource as well as in controlling soil erosion control. It can generally tolerate dryness (drought), salinity, low soil fertility, wind, and high and low temperatures. For these reasons, the germination, germplasm, and genetic characteristics of A. scoparia Spach are nowadays being widely studied for application as rootstocks in breeding programs [410] to generate domestic species that could adapt to arid and semi‐arid conditions and climate changes as well as for planting in mountainous areas and deserts to control the desertification process (Figure 11.2a), which is rapidly under way in Iran [ 2, 3].

Figure 11.2 Illustration of (a) Amygdalus scoparia Spach shrub, (b) unripened fruit with pericarp plus leaves, and (c) bitter almond kernels plus hard shells (hull).

Source: Reproduced from [4] with permission.

As mentioned earlier, it is primarily planted for desertification control; however, it is capable of producing various products and by‐products which are of commercial interest too. For instance, its fruits (Figure 11.2b) are traditionally debittered, as it contains amygdalin, a bitter compound, and roasted by indigenous people for consumption as nut. In line with this application, the effect of microwave roasting, a new and rapid technique, on its physicochemical properties had been recently addressed [11]. Apart from that aspect, the bitter almond kernel retains some 35%–50% (w/w) high‐quality oil (65% PUFA, 25% PUFA, 12% SFA, polyphenols, antioxidants, and many other bioactives) which had been already extracted and characterized by various methods [1218]. This oil can be potentially used as an edible oil, in formulations of cosmetics (creams and lotions) and nutraceuticals as well as a herbal remedy. The kernel also contains almost 25%–40% w/w of high‐quality protein, which makes it a good candidate for supplementation and nutraceutical applications [1921]. In addition, the hard shell of the fruit (Figure 11.2c) is traditionally used by indigenous people as a fuel for heating and cooking, but quite recently some researchers have examined its capability as a natural activated carbon which can be used as an adsorbent to remove lead from aqueous solutions and decontaminate water by removing heavy metals [22,23]. It needs to be noted that the aerial parts of A. scoparia Spach is also of great interest, from pharmacologic and pharmacognosic point of views, as the leaves (Figure 11.2b) contain invaluable biocompounds, antioxidants, and acetylcholinesterase inhibitory activities, which can potentially be used against diabetes and common cold, and prevent β‐amyloid‐induced cytotoxicity, which is the path to treating Alzheimer's disease [2427]. Last but not least, the small trees and shrubs of wild or mountain almond also secret a natural exudate gum, Persian gum (PG), which is the main subject of this chapter.

The abovementioned features make it very clear that A. scoparia Spach can be considered a reliable and sustainable source for providing PG alongside the other products (bitter almond, kernel oil, kernel protein and protein fractions, pharmaceuticals, timber, and ornamentals). Iran, owing to its geographical and environmental situation, has traditionally been the main and probably the only source for PG, and at present, Iran annually exports over 400 000 kg of PG, and it is predicted that this number will tremendously increase in a matter of a few years or so once an artificial population of seedlings is established [ 1, 3,28].

In recent years, the growing demand in the food and pharmaceutical industries for safe, low‐price, easily accessible and sustainable, biodegradable, biocompatible, ecologically friendly, and renewable natural sources for stabilizers and emulsifiers has attracted scientists to investigate the potential capabilities of existing natural gums and mucilages as alternatives to the current ones, which are becoming more and more expensive (e.g., gum tragacanth), unsustainable (e.g., gum arabic) due to the periodical crisis in the producing countries (e.g., Senegal, Sudan, and Somalia), or with health concerns (e.g., bio‐based or biotechnologically produced ones). To this end, PG is a relatively unknown natural hydrocolloid but with abundant production and countrywide distribution, so that it has been recently considered as a potential alternative, and its characteristics and applicability have recently been extensively studied [3]. Hence, this chapter summarizes the existing information with regard to its production, chemical structure, rheological, thermal, surface properties, and potential applications.

11.2 General Specifications

As indicated above, it is clear that there are quite a number of Prunus species, including almond (Prunus dulcis), apricot (Prunus armeniaca), blackthorn (Prunus spinosa), cherry (various Prunus species), cherry laurel (Prunus caroliniana and Prunus laurocerasus), chokecherry (Prunus virginiana), plum (various Prunus species), nectarine and peach (Prunus persica) and their species with exudates that can potentially be used for several purposes, some locally and others at industrial levels. However, the focus of this chapter is on A. scoparia Spach exudate, namely, PG, due to its abundant production in comparison to the others.

PG is the natural exudate of mountain or wild almond (A. scoparia Spach) shrubs (Figure 11.3a) or small trees (Figure 11.3b), which naturally grow in Irano‐Touranian and Zagros regions (Figure 11.1a). The gum, with a semi‐transparent appearance and sweet smell, usually exudes from the trunks or branches (Figure 11.3c) as a result of mechanical injuries, insects attack, microbial diseases, as well as a climatic adaptation (in drought conditions) called physiological gummosis. PG is also known as gum Zed, Zedu, Zedo, Ozdu, Farsi, Shirazi, Angum or Angom, Arzhan, Arjhan, gomme notras, and gum gharacia [ 3, 28].

Figure 11.3 Illustration of (a) Amygdalus scoparia Spach shrub, (b) small tree, (c) PG tears on well‐developed trunk, (d) natural size, color, and shape of PG.

Source: Reproduced from [3] with permission.

PG is commercially available in different colors (white, light yellow, dark yellow, light brown, dark brown, and amber or red), shapes, and sizes (Figure 11.3d). In terms of color indices, the higher the visual redness, the higher a* (redness), b* (yellowness), and C* (chroma) and the lower H* (hue angle) and L* (lightness) values [28]. It also disperses rather easily in warm water, the color of the dispersion strongly depending on the color of the original PG granules. PG is classified as a partially soluble gum since it can be easily separated, particularly at low concentrations, by gravitational acceleration (up to 5% w/w) or by centrifugation, into two distinct phases that contain soluble (25%–30% w/w) and insoluble (70%–75% w/w) fractions [ 2840].

Nevertheless, it has been recently reported that the soluble fraction itself consists of 35 and 65% w/w alcohol‐soluble and alcohol‐insoluble fractions, respectively [32]. The former fraction possibly contains different mono‐, di‐, and oligosaccharides along with small molecules or low‐molecular‐weight polysaccharides, while the latter includes various high‐molecular‐weight polysaccharides; the higher the degree of polymerization (DP) or molecular weight, the lower the solubility in alcohol. In contrast, it has been claimed that only 7 [40] or 8.4% w/w [41] of PG can be considered soluble. However, these numbers are debatable as Raoufi and co‐workers (2017) [40] employed NaCl solution (0.2 M) as the eluent to dissolve PG. In this regard, it has been already confirmed [33] that in the presence of NaCl (particularly over 0.02 M), PG is not dispersible anymore and easily precipitates. This is likely why certain findings [40] on fractionation and molecular weight might be divisive. On the basis of this evidence, one may speculate that the existence of different structures (oligosaccharides and short‐chain polysaccharides) and their possible interactions under various conditions (pH, ionic strength, and temperature) should be carefully considered prior to any functional characterization [ 3, 28,34]. With regard to a recent report [41], it needs to be emphasized that the authors did not consider the fact that alcoholic precipitation of the water‐soluble fraction caused loss of part of truly water‐soluble low‐molecular‐weight fractions which were soluble in ethanol. For this reason, the reported number for the water‐soluble fraction is incorrect. The author of this chapter believes there are possibly even more distinct fractions which can be fractionated if various alcohols, acids, bases, and salts at different ratios and concentrations are used.

11.3 Production, Collection, and Processing

The high‐quality gum is usually collected on the trunks of small trees as striated nodules or tears (Figure 11.3c,d) through deliberate incisions made by indigenous people or farmers – the thinner the incision, the more superior the quality – rather than by natural exudations (i.e., insects attack, microbial or fungal diseases, adaptation to harsh climate, and soil conditions). Collection of soft and adhesive lumps is manual, and they are usually dried under direct sunlight during the harvest season (from June to September). Each square meter of the wild almond tree's canopy can produce some 20–50 g of dried PG [39]. The collection and drying processes are followed by some operations where the gums classified into different colors and physical characteristics (large granules, sugar crystals, and powder like) groups (Figure 11.3d). Similar to most other exudate gums, raw PG is also pre‐cleaned to remove barks, sands, and fines in order to improve its quality. It is either sold in its natural state without any separation, purification, or pulverizing or after some physical treatments such as grading, grinding, sieving, dissolving, decantation of soluble and insoluble fractions, and drying by conventional, spray drying, or freeze drying as powder. As described earlier, technological treatments such as sieving, decantation, fractionation, centrifugation, concentration, thermal processing, and drying can also yield a product with no insoluble matter that hydrates much better than in its natural state. However, the thermal process may affect the color indices and transparency of PG solutions [ 28,30, 32, 39].

11.4 Physicochemical Properties

Similar to the most of the other exudate gums, the natural pH of aqueous dispersions of PG is acidic (4.30–4.90), and the growing region has no significant effect on this property. However, a significant difference between the natural pHs of various color groups is perceived: the lighter the color, the higher the pH, and this behavior can possibly be related to the presence of pigments or other metabolites, particularly tannins or tannic acid at various concentrations [ 28, 39]. Moreover, PG, especially its soluble fraction, is an acidic and anionic polysaccharide which contains a high number of negative‐charge‐bearing groups so that its functional and rheological properties are strongly pH dependent [ 3,29,42].

With regard to the moisture content, PG contains around 5%–13% w/w water, and the effects of color and growing region are significantly more likely due to the different climates in which they were dried and collected. It is noteworthy that the moisture content and a w have profound effects on the shelf life, physicochemical characteristics, and chemical (such as non‐enzymatic browning), microbial, and enzymatic deterioration [33]. However, generally speaking, the total microbiological count of PG does not exceed ∼1000 CFU/g, and no pathogen has yet been reported, likely due to its low moisture content and consequently low a w as well as direct sun drying [ 28, 39].

The reported total ash content for PG varies widely, ranging over 1.20%–3.60% w/w, probably due to the differences of the cultivation climates, especially the soil, harvesting season, as well as the occurrence of gusts of wind during the secretion period as the fine aerosols and dusts can be easily adsorbed on fresh exudates; for that reason, part of the measured ash content was acid‐insoluble [ 28,37, 39]. However, the ash content of different color groups of PG was almost similar, which implies the importance of the aforementioned reasons rather than color [28]. The major monovalents (Na and K) and divalents (Zn, Fe, Mg, and Ca) also widely vary in different reports.

The average nitrogen (N) content of PG and its soluble and insoluble fractions is also quite low and negligible (0.01% w/w); however based on some reports, it could even contain up to ∼0.2% on the dry basis [ 37,38]. Therefore, the protein content of PG could vary over the rage 0.06%–1.10% w/w; however, in one report, no significant difference is seen in regard to protein content, growing region, and color. The reported lipid content is even lower than protein, and the maximum value is less than 0.20% w/w [ 3, 28].

Tannins are astringent or bitter polyphenolic biomolecules with brown color that easily bind to proteins, amino acids, and other organic compounds and precipitate. They can be accumulated in leaves, fruits, stems, barks, and hulls, so that plant‐based foods and ingredients such as gums may retain some tannins. Therefore, its presence and content are important from the health, technological, and functional points of view. Despite its importance, its quantification is neglected in the majority of existing reports – except one [ 3, 28, 39], which has shown a direct and inverse correlation between its content (0.50%–1.50% w/w) and PG color and its dispersion's pH, respectively; the higher the tannin content, the darker the color (tending to more brownish or reddish) and the lower the pH. The tannins could enter the exudates due to its long‐term contact with the bark, the tough exterior covering of the trunk or branches, and can be considered as an indication of age and purity rather than of origin: the higher the tannins, the older the gum, and the more reddish the color [43]. In contrast, some researchers disagree with any correlation between tannin content and color [44]. Considering these, the tannin content is potentially a restricting parameter for its application in food and pharmaceuticals. Therefore, in case of high tannin content (if >0.6% w/w), the tannins either need to be removed (by chemical, enzymatic, or filtration methods), or PG should be utilized in non‐food industries, for example, textiles, papers, chemical, leather, and ink [ 3, 28, 34, 39].

Considering the negligible content of lipids and proteins and taking into account the ash and tannin contents, it can be obviously concluded that the major fraction of the dry matter of PG should be carbohydrates, particularly polysaccharides. On a wet basis, PG contains 82%–90% w/w polysaccharides. In addition, 98% of polysaccharides are composed of sugars (arabinose, galactose, mannose, xylose, and rhamnose), of which 2%–10% w/w as uronic acids (e.g., galacturonic) on the basis of the existing reports [ 3, 28,35, 37, 39, 41].

The minimum gelation concentration (MGC) and water absorption capacity (WAC) are around 11% w/w with diverse correlation with color: the darker the gum, the lower the MGC and the higher the WAC. It should be noted that PG is not capable of converting to a true gel, as takes place, for instance, in pectin or tragacanth. As with most other hydrocolloids, the dispersibility and solubility of PG are strongly temperature dependent, as upon heating, its solubility could significantly improve, from 45% in cold water (30 °C) to almost 75% in hot water (90 °C), with a profound effect on the insoluble fraction. Furthermore, around 94% of PG precipitates in alcohol, which implies that only one third of the soluble fraction (SFPG) is soluble in alcohol comprising mono‐, di‐, or oligosaccharides [ 28, 32, 37, 39].

11.5 Structural Characteristics

Generally speaking, there is a direct relation between the functional properties of polysaccharides (gelling, stabilizing, texturizing, emulsifying, and binding) and their chemical structures and molecular weights. Therefore, the structural elucidation is a very crucial step to understand their behaviors under different circumstances. There are several techniques to qualify and quantify the constituent monosaccharides (chromatographic methods, e.g., high performance liquid chromatography, HPLC, or gas chromatography, GC), the chemical groups (Fourier transform infrared spectroscopy, FTIR), bondings (nuclear magnetic resonance, NMR), molecular weight (e.g., sedimentation, gel permeation chromatography, gel electrophoresis, and dynamic light scattering) as well as other properties [ 4346], and ideally a combination of these techniques needs to be used in the case of any unknown hydrocolloid. It needs to be emphasized that due to the complexity and inclusion of various oligo‐ and polysaccharides of different molecular weight and structure, any unknown hydrocolloid (e.g., PG) should be fractionated prior to any further characterization via these techniques [ 43, 44]. Otherwise, the collected data could be too confusing and misleading. Considering these criteria and for the sake of the readers, the available data related to characterization of the structural features of PG and its fractions will be discussed in the following subsections. Most of these data could be also found in the previous reports and reviews written by the author [ 3, 28, 34].

11.5.1 Monosaccharide Composition

Using qualitative HPLC measurements, the presence of galactose, arabinose, and rhamnose as the major monosaccharides of whole PG has been confirmed [ 3, 28, 39]. Its water (WE) and alkaline (0.1, 0.5, and 1 M NaOH) extracts (AE) and final residues (RES) as well as their Smith degradation products have also been analyzed [ 28, 39]. The uronic acid content is around 10% w/w, distributed in 2, 2, 1, 1, and 4% w/w in the WE, AE0.1, AE0.5, AE1, and RES fractions, respectively. No uronic acid was detected in their Smith degradation products except for RES (2% w/w). This implies that the majority of uronic acid was not located in the backbone, since they were removed by chemical degradation. The total sugar content of the alkaline extracts was 10%–15% w/w, higher than that of their Smith degradation products, owing to the removal of the low‐molecular‐weight components and oligomeric species by dialysis during Smith degradation [ 3, 28, 39].

In another study [35], GC/MS was employed to investigate the monosaccharide's composition. It is not clear from the report whether they examined the soluble fraction or the whole gum, but it is likely that they analyzed the soluble fraction. Apart from this ambiguity, they also detected arabinose (Ara) and galactose (Gal) with an Ara:Gal ratio of 2:1 along with some other monosaccharides, namely, xylose, rhamnose, and mannose (6.8, 1.1, 0.3 mol%). The sum of galacturonic acid and 4‐ortho‐methyl‐galacturonic acid was 2.3 mol%, which is much less than what was reported earlier [ 28, 39]. On the basis of this evidence, it was concluded that PG can be considered to be an arabinogalactan, like most of the exudate gums from Prunus genus as well as gum arabic. However, the adverse ratio of Ara:Gal, the low protein content, and the existence of xylose and mannose were highlighted as a distinguishing feature compared to gum arabic. It is noteworthy that owing to the high price of gum arabic and the competitive lower price of PG, there are serious concerns regarding the adulteration of gum arabic with PG. On this basis, it is suggested that these differences could likely employed as potential clues to verify any adulterations or contaminations; however, this is not confirmed yet [35].

In a very recent report [41], the soluble fraction of PG (SFPG) was first extracted by hot water. Then the alcohol‐precipitated fraction (alcohol‐insoluble fraction of SFPG) was purified with DEAE‐cellulose A52 and Sephacryl S‐400 HR columns. The monosaccharide composition was analyzed by GC‐MS, and the chromatograms indicated that the alcohol‐insoluble fraction of SFPG was an arabinogalactan containing arabinose, galactose, xylose, and rhamnose with a relative molar ratio of 20.0:17.9:5.2:1.1. The uronic acid content of the fraction was about 6%.

Considering the differences in preparation and fractionation methods and the analysis techniques utilized in the abovementioned reports, it can be concluded that arabinose and galactose are the major constituent monosaccharides in the chemical structure of PG, and its soluble (SFPG) and insoluble (IFPG) fractions as well as the alcohol‐insoluble fraction of SFPG; therefore, they can be classified as an arabinogalactan. However, a distinct and comprehensive compositional analysis on various fractions (water, alcohol, acid‐ and alkaline‐soluble and insoluble fractions) is necessary to obtain a clear picture of their structures.

11.5.2 Chemical Structure

The 1H–NMR spectra of the aforementioned fractions (WE, AE0.1, AE0.5, AE1, and RES) showed high numbers of β–anomeric protons (δ 4–5 ppm) and a fine chemical shift signal over the range 1.0–1.1 ppm attributable to the methyl group protons of rhamnose. The existence of a large number of β‐anomeric carbons (δ 100–105 ppm) in the 13C–NMR spectra also confirmed the dominance of β links among the building blocks of PG. It was also shown that rhamnose is most likely located in the backbone, since its corresponding signal was detected (signal at 16.5–17.5 ppm) in the 13C–NMR spectra of all fractions, including Smith degradation products. A comparison of the chemical shifts of 13C–NMR spectra with other polysaccharides suggested the existence of β‐D–Galp‐(1 → 3) and (1 → 6), α‐L–Araf‐(1 → 3). It was concluded that the backbone of PG is most likely constructed from (1 → 3) linked β‐D‐Galp and rhamnose residues, whereas the branches are composed of (1 → 6) linked β‐D‐Galp, (1 → 3) linked α‐L‐Araf, and terminal α‐L‐Araf. On the basis of these findings, a tentative chemical structure is proposed as depicted in Figure 11.4a [39].

Figure 11.4 Representation of chemical structure of the soluble fraction of Persian gum (SFPG).

Source: (a) Proposed by Rahimi [47] and (b) Molaei and Jahanbin [41]; adapted with permission.

The deuterium‐exchanged 1H NMR and 1H‐1H COSY spectra of SFPG also showed the anomeric region of t‐α‐Araf and variously linked α ‐Araf residues, t‐ α ‐Galp, t‐β‐GlcA, and 4‐OMe‐β‐GlcA resonances. The anomeric signal of α ‐L‐Rha residue was not distinguished due to signal overlap, but the chemical shift of methyl proton of Rha and its correlation with H5 of Rha were easily identified. The authors concluded that Ara and Gal accounted 54–62 and 27–31 mol% of SFPG, respectively. Chemical shifts of t‐β‐GlcA and 4‐O‐Me‐β‐GlcA residues were tentatively assigned from the NMR spectra [35].

On the basis of another NMR study [41], the alcohol‐insoluble fraction of SFPG (only one third of SFPG is soluble in alcohol, and two third is considered insoluble in alcohol; see Ref [32]) is classified as an acidic branched polysaccharide with a backbone consisting of →3,6)‐β‐d‐Galp‐(1→, →3)‐β‐d‐Galp‐(1→ and →3)‐α‐l‐Araf‐(1 → residues with side chains attached to the O‐3 and O‐6 positions of 1,3,6‐linked β‐d‐Galp. The side chains consist of β‐d‐Xylp‐(1 → 3)‐α‐l‐Araf‐(1 → 3)‐α‐l‐Araf‐(1→), α‐l‐Rhap‐(1 → 6)‐β‐d‐Galp‐(1→), and β‐d‐GlcAp‐(1 → 6)‐β‐d‐Galp‐(1→). This report also proposed a different chemical structure (Figure 11.4b). It seems that the researcher did not consider the partial solubility of SFPG in alcohol; consequently, the fraction precipitated by alcohol has been reported as the soluble fraction, which is misleading [41].

From a review of these findings, it is obvious that there is no good agreement between these reports, and extensive investigations need to be conducted by an expert group of researchers in a well‐equipped laboratory on various fractions of PG with special attention to details. Otherwise, these findings cannot be considered as a solid platform for describing its functions under various conditions.

11.5.3 Functional Chemical Groups

In the field of carbohydrate analysis, FTIR spectra are usually used to determine their major organic functional groups. The full spectra of PG and its soluble (SFPG) and insoluble (IFPG) fractions have been already considered by researchers [ 3, 28, 39, 41 4749]. Despite some differences, five distinct peaks could be highlighted in all the examined samples regardless of their differences (i.e., source, solubility, insolubility, measuring device, and resolution levels). For instance, the spectral bands falling between 1427 and 1448 cm−1 are attributed to the CH3 bonding vibration or to the presence of COOH as the carboxylic group of uronic acids. Peaks at 1614–1633 cm−1 were also attributed to carboxyl groups, anhydride components produced via carboxyl groups, or the presence of bound water. Moreover, peaks over the wave numbers of 2920–2935 cm−1 were indicated as clues to the symmetric and asymmetric stretching of CH bonds, overlapping of the double bonds with OH, as well as the existence of asymmetric CH2 functional groups. The two other peaks (2140–2146 and 3424–3441 cm−1) were not clearly described. In a very recent report [41] on the characterization of the alcohol‐insoluble fraction of SFPG, apart from peaks at 3433, 2925, and 1637, other peaks were detected at 1736, 1379, 1144, 1080, and 1033 cm−1, and some others at lower wave numbers. Therefore, it seems the FTIR spectra of PG and its fractions were not very well described by the authors for further elucidation of their structural features; however, this technique has demonstrated its effectiveness for monitoring any structural or chemical changes or interactions which happen due to chemical or enzymatic treatment [ 47,48,50].

In terms of gum modification, in an interesting study based on response surface methodology (RSM), using acryl amide, the solubility of the insoluble fraction (IFPG) was improved for utilization in broader applications. A significant increase in the FTIR spectra intensity at 1100–1300 cm−1 and the presence of a peak at 1575 cm−1 were attributed to the formation of etheric (C–O–C) bonds as well as to the stretching and bending vibrations of CO and NH bonds in the esterified structure (IFPG–O–CH2–CH2–CONH2), respectively. The latter peak did not exist in the original IFPG. Using the FTIR spectra alongside the molecular weight and intrinsic viscosity measurements, the occurrence of esterification was approved [48].

In order to improve the emulsifying capability of PG, the influence of octenyl succinic anhydride (OSA), gum concentration, pH, temperature, and reaction time on the esterification of PG, SFPG, and IFPG was investigated. On the basis of the FTIR spectra, an absorption peak emerged at 1726 cm−1, indicating ester carbonyl bond formation in OSA‐PG and OSA‐SFPG but not in IFPG, due to its insolubility. The absorption peak at 1569 cm−1 was also attributed to the asymmetric stretching vibration of carboxylate ions (RCOO–) as indicative of the esterification reaction [47].

FTIR spectra have been also employed to investigate the incidence of the Maillard reaction or covalent linkage in wet‐heated complexes of β‐lactoglobulin and PG conjugates [50]. The peaks at 3311 and 2923 cm−1 were attributed to the hydroxyl stretching vibration and CH stretching and bending vibrations in PG, respectively. In addition, the normal peaks of amide II and III were shifted to higher wavelengths upon wet heating without any considerable change in the amide I peak. In addition, the fall in spectra between 3600 and 3200 cm−1 corresponded to the hydroxyl stretching vibration, and the one at 1031−1 indicated a hydroxyl‐bending vibration in the wet‐heated complex. These FTIR spectra, as evidence of β‐lactoglobulin–PG conjugation, were supported by SDS‐PAGE chromatograms.

11.5.4 Molecular Weight

The molecular weight (M w ) of a definite polysaccharide is another parameter which could affect its interactions with neighboring, like or unlike, macromolecules or ions, and hence the rheological properties, functionalities, and applications. Therefore, over the past few years, the M w and dispersity index of PG and its fractions have been investigated. For instance, it was attempted [35] to separate SFPG of various color groups (white W, yellow Y, and red R) by centrifugation followed by freeze drying. The freeze‐dried powders were then dissolved in 0.1 M NaCl and filtrated (1 µm filter), and the permeate was analyzed with a GPC‐MALLS system. An inverse correlation was found between the color of PG powder and the M w : the darker the color, the smaller the M w . It was concluded that the white PG consisted of high M w and homogenous molecules, while yellow and red ones were composed of very heterogeneous polysaccharides with lower M w . The average M w of the soluble fraction of the white PG was estimated as 4.74 × 106 Da. However, in a recent report from the same researchers on similar samples [51], the molecular weights were quite different (3.14, 2.72, and 1.66 × 106 Da for white, yellow, and red color groups, respectively).

In another study [40], a similar procedure with minor modifications was employed for molecular weight investigation. The PG was initially dissolved in 0.2 M NaCl, centrifuged, and the supernatant was filtrated (0.22 µm filter). It was noticed that the dry matter content of the SFPG filtrate was only 7% w/w, indicating that PG is mainly composed of large water‐insoluble particles (93% w/w). The weight‐average molecular mass M w (2.82 × 106–5.11 × 106 Da), number‐average molecular mass M n (0.43 × 106–2.01 × 106 Da), and dispersity index M w /M n (2.54–6.64) of the SFPG filtrate were relatively high.

Using a capillary viscometer, the intrinsic viscosity of various fractions (WE, AE0.1, AE0.5, AE1) of PG at distinct concentrations (0.2 g dL−1) was also determined [39]. The approximate intrinsic viscosities for these fractions were 7.08, 9.22, 11.33, and 13.33 dL g−1, respectively. It was concluded that these viscosities are directly correlated with solubility: the smaller the intrinsic viscosity, the higher the solubility. Similar measurements [48] were conducted on various concentrations (0.18, 0.018, 0.0018, and 0.00018 g dL−1) of the ordinary SFPG and the SFPG, which were separated from chemically modified (esterified by acryl amide) IFPG. Their molecular weights were also examined using static light scattering. The recorded intrinsic viscosities were 8.76 and 4.41 dL g−1, and molecular weights were 134 and 42.4 kDa, respectively. These differences were in very good agreement with their rheological properties, and it was pointed out that IFPG is probably depolymerized by acryl amide esterification, as a result of which the solubility increased. A molecular weight of 99 kDa for PG, which increased to 333 kDa upon irradiation at 4 kGy, was also recently reported [52]. The reported intrinsic viscosity (7.14 dL g−1) for SFPG [51] is in good agreement with what were reported earlier: 7.08 and 8.76 dL g−1, respectively [ 39, 48].

Comparing the reported molecular weights [ 35, 40, 41, 51] with other gums reveals that the molecular weight of SFPG is at least 30‐fold higher than those reported, for instance, for P. persica (1.3644 × 105 Da) [53] with similar botanical origin (see Section 11.1). Moreover, the reported M w of gum arabic (1.56 × 106 Da) [ 35, 51] is also five times larger than what was already reported (3.2828 × 105 Da) [54]. It should be noted that in these studies [ 35, 40, 51], NaCl (0.1 and 0.2 M) was utilized (for preparation of solutions and as an eluent for GPC) without considering the possible detrimental interaction of PG and SFPG with monovalent ions and their precipitations. It had been already confirmed [33] that PG may precipitate in the presence of NaCl, and its concentration in these studies was very high. Therefore, the molecular weights determined may be more associated with the non‐indigenous (i.e., aggregated) SFPG than with the native SFPG. On the other hand, the reported numbers [ 48, 52] are several fold smaller but in good agreement with the aforementioned hydrocolloid. Despite these efforts, there is still ambiguity and uncertainty regarding the molecular weight of PG and its fractions, which therefore needs to be extensively clarified in the future.

11.6 Rheological Properties

Polysaccharides, due to their high molecular weights, chain entanglement, and polymer–solvent interactions, are usually used to control the rheology (flow, deformation, and texture) of solutions, dispersions, and emulsions [ 43 46, 55,56]. Thus, understanding the rheological characteristics of PG dispersions and their soluble fraction (SFPG solutions) is of great importance and needs to be extensively investigated. Due to the complexity of the existing reports, for the sake of the readers, in the following paragraphs, first the effects of concentration (PG or SFPG), temperature, pH, and ionic strength (type, concentration, and valence of salts) on the steady and dynamic shear rheological properties will be discussed. Later, the influences of irradiation or chemical modifications on the rheological properties will be described. It needs to be emphasized that the interaction of PG and SFPG with proteins and other hydrocolloids as well as their applications in emulsions and their subsequent effects on rheological characteristics is another interesting subject that has drawn the attention of researchers. However, they will be discussed in the following subsection.

In regard to the influence of gum concentration (up to 5% w/w) on the apparent viscosity of PG dispersion, a direct but nonlinear correlation is reported [ 33, 37, 51]. From the rheological point of view, these results cannot be fully relied on as two thirds (∼70% w/w) of PG is insoluble, which potentially could cause serious problems during measurement. Considering this, there are other studies which have reported on the effect of SFPG concentration (up to 3% w/w) on apparent viscosity, in which they have also shown direct relationship: the higher the SFPG content, the higher the viscosity [ 33, 39, 42, 47, 48]. It is obvious that the apparent viscosity of SFPG solution, at any distinct concentration, was almost half that of PG dispersion. It is worth emphasizing that the viscosity of PG was also reasonably smaller than that of gum tragacanth, xanthan, and karaya, but more than gum arabic and ghatti at comparable (e.g., 1% w/w) concentrations.

In terms of temperature (up to 90 °C), similar to many hydrocolloids, it has shown an adverse effect on the apparent viscosity of PG dispersions as well as of SFPG solutions over a wide range of concentrations [ 33, 37, 42, 51]. It is noteworthy that the temperature dependency of viscosity is reversible, and upon cooling, the viscosity regains its initial value. Moreover, a strange behavior has been reported [33] at 60 and 90 °C, where the viscosity of PG dispersions increased but their color diminished (from brownish to colorless). In regard to SFPG, at 60 °C, its viscosity increased, but at 90 °C it was reduced. These observations have not been confirmed by later reports.

The natural pHs of PG and SFPG are reported as acidic (∼4.60), and they are classified as anionic polysaccharides [29]. In this regard, researchers have also investigated their stability under various pHs (highly acidic, neutral, and basic) as well as their influences on the apparent viscosity. In spite of its natural pH, the highest viscosity of PG dispersions and SFPG solutions occurred in an almost neutral range (∼7.0), but decreasing (down to 2.0) or increasing (up to 12.0) the pH diminished the viscosity [ 33, 37, 42, 47, 48].

Owing to their acidic and anionic nature, PG dispersions and, more importantly, SFPG solutions should have been very sensitive to ionic strength. From this point of view, the effects of monovalent (KCl and NaCl), divalent (MgCl2, CaCl2, and FeCl2) and trivalent (FeCl3 and AlCl3) salts over a wide range of concentrations (up to 500 mM) have been evaluated [ 33, 37, 42]. It can be generally concluded that the valence, type, and concentration of salts strongly affected the apparent viscosity and flow behavior of PG and SFPG: the higher the valence and concentration of the ions, the lower the apparent viscosity. However, Khalesi et al. [37] noticed an increase in viscosity (above 100 mM) in the presence of CaCl2. Nevertheless, over certain concentrations (>100 mM), Dabestani [33] also observed precipitation and gritty (sandy) texture, particularly in IFPG, during storage (after few days). Therefore, it seems that the PG dispersions and SFPG solutions cannot withstand high ionic strengths and precipitate.

The rheological behavior of PG dispersion and SFPG solution is non‐Newtonian, shear thinning, thixotropic, and the best‐fitting model to describe their time‐independent behavior is the power law [ 29 31 33, 35, 39, 42, 47, 48, 52, 57]. However, under certain circumstances, some other mathematical models have shown better fitting [ 32, 35, 47, 48]. For instance, the effect of pH (2–7) on the flow behavior of SFPG (0.6% w/w) revealed some clear differences. Over the mild pH range (4–6), the rheology followed the power law and Herschel–Bulkley models, but at lower pH (2–3) the rheology followed the Bingham model. It was argued that this was due to the protonation of carboxyl groups at low pH; the electrostatic repulsion decreased, the flexibility of the SFPG backbone increased, and as a consequence, the apparent viscosity reduced too [32]. Anyhow, Hosseini et al. [42] have extensively discussed how the concentration, temperature, pH, and salts could affect the power‐law parameters. They have also noticed thixotropic behavior as a result of a hysteresis loop between the upward and downward curves of the shear stress versus the shear rate.

Dabestani [33] also examined whether sonication could improve the solubility and consequently the rheology of PG dispersion. It was found that high‐power sonication profoundly increased the solubilization and eased the phase separation, though the viscosity decreased. However, the influence of the duration of sonication was much more pronounced than the intensity.

The effect of irradiation doses (up to 30 kGy) on the apparent viscosity of PG dispersion (0.5% w/w) has been investigated [52]. On the basis of this report, by irradiation at 4 kGy, the apparent viscosity and consistency coefficient increased (33.5 mPa s and 0.7 mPa sn), but when treated at higher doses (up to 30 kGy) it significantly diminished. These findings revealed the detrimental effects of irradiation on the structure and the rheological properties by altering the shear‐thinning behavior of the un‐irradiated samples to Newtonian behavior in the case of irradiated (irradiated at 30 kGy) ones. It was argued that irradiation (4–8 kGy) likely disrupted the side branches, cross‐links, and somehow hydrogen bonds which contribute to the apparent and intrinsic viscosities.

Chemical modification and esterification are other processes that showed significant effects on the rheology of PG, SFPG, and IFPG. As an example, the flow behaviors of PG, OSA‐PG, SFPG, and OSA‐SFPG dispersions (1% w/w) were shear thinning. Nevertheless, the apparent viscosity of PG was lower than that of OSA‐PG, in contrast to the higher apparent viscosity of SFPG compared to its esterified counterpart (OSA‐SFPG). The partial solubilization of IFPG, depolymerization of large molecules, and their consequent esterification with OSA were highlighted as the main reasons for these rheological differences [47]. Similar to the ordinary SFPG, the SFPG which was separated from the chemically modified IFPG (esterified by acryl amide) showed shear‐thinning flow behavior at 1.8% w/w [48]. However, the apparent viscosity of the latter was reasonably lower than the former one, probably due to partial depolymerization under alkali pH.

The majority of the discussed rheological data were achieved on the basis of steady shear measurements. Nonetheless, there is another approach to investigate the viscoelastic and structural properties, which is called the oscillatory (dynamic) method. Using the latter method [35], PG dispersion showed a predominantly viscous behavior (i.e., the loss modulus was larger than the storage modulus); however, it showed elastic behavior at moderate time scales as its complex viscosity was high and loss tangent was small. The frequency sweep curves were also very similar to those of dilute solutions of unlinked polymers. In addition, the differences in cross‐over frequencies were attributed to their M w : the higher the M w , the smaller the cross‐over frequency.

11.7 Interaction with Other Macromolecules

From the practical point of view, the compatibility and interactivity of PG and SFPG with other macromolecules (proteins and polysaccharides) is very critical and needs to be clarified. Hence, the following subsections will discuss their interactions with these two major macromolecules.

11.7.1 Polysaccharides

The application of PG:maltodextrin (5:35–1:39, 40% w/w) for emulsification of D‐limonene has shown that they are most likely compatible, particularly at high ratios of PG, since the resulting emulsions were completely stabilized [58,59].

The compatibility of SFPG with the soluble fraction of gum tragacanth (SFGT) at various ratios (20:80, 50:50, 80:20), concentrations (0.5, 0.75, 1% w/w), pH (3, 5, 7), and salts (NaCl, CaCl2, AlCl3) at different concentrations (0.002–0.10 M) has also been extensively studied [33]. It was shown that these two polysaccharides were very compatible regardless of their mixing ratios, concentrations, and pH. Moreover, due to the significantly higher viscosity of SFGT, compared to SFPG at the same concentration, it was noticed that by mixing a small portion of SFGT with SFPG, the viscosity of the mixture can be greatly increased, which could be very beneficial for industrial applications, bearing in mind the large differences in their prices (GT is currently 50–100 times expensive than PG). The effects of salts, pH, and heating were generally similar to those of SFPG and SFGT individually. Surprisingly, in the presence of AlCl3 (1–5 mM), the mixtures gelled. The higher the SFGT ratio, the stronger the gel, according to oscillatory measurements. It is noteworthy that for the first time the gelling effect of AlCl3 (1–5 mM) on GT, SFGT, and IFGT were noticed; nevertheless, GT and SFGT gels were absolutely reversible, but IFGT gel was fragile. This could be very interesting for industrial applications.

11.7.2 Proteins

Generally speaking, proteins and polysaccharides usually play important roles (as emulsifiers, foaming agents, carriers, stabilizers, thickeners, and gelling agents) in food formulations. However, their functionalities strongly depend on the quality of the solution (concentration, pH, ionic strength, and temperature), their intrinsic properties (molar mass, molecular structure, polydispersity, and charge density), and their interactions with solvents and co‐solutes (e.g., sugars) via intra‐ and inter‐molecular interactions. As a result, under certain conditions (typically, near the isoelectric point, at high ionic strength, or at high temperature), functionality can be totally or partially lost, particularly for proteins. To overcome such issues and in order to produce novel emulsifiers, the covalent and electrostatic complexation of different proteins with various polysaccharides had been already addressed [56 6062]. In line with this idea, the interaction of PG and SFPG with four different proteins, namely, gelatin, whey protein isolate (WPI), casein (CN), and β‐lactoglobulin (β‐L) has also been studied and will be reviewed in the following subsections.

11.7.2.1 Gelatin

A study on the compatibility of PG and its fractions with gelatin, from the standpoint of partial replacement in the formulation of jellies and gummy candies, has shown that despite the gelation capability of IFPG, PG and SFPG cannot gel in the presence of even high sugar content (35% w/w). Nonetheless, they showed compatibility with gelatin at various mixing ratios (10:90–90:10). However, only at an IFGT:gelatin ratio of 40:60 (10% w/w) were the mechanical properties similar to those prepared with gelatin alone (10% w/w) [30].

11.7.2.2 Whey Protein Isolate

In a recent report [32], it was attempted to find appropriate ratios of WPI:SFPG and WPI:SFGT to avoid precipitation over a wide range of pH (2–7). According to the optical density, zeta potential, and rheological measurements, the desired ratios were 0.4:2 and 0.4:1 (%w/w), and the responsible mechanisms for the stability were electrostatic and steric repulsion as well as high bulk viscosity. Despite the anionic nature of the polysaccharides, SFGT was more efficient than SFPG, probably due to the higher steric repulsion caused by overlapped side chains as well as inter‐ and intramolecular interactions of the SFGT chains. Finally, it was pointed out that these complexes can be used as surface‐active agents in the formulation of emulsions.

The interaction of WPI (up to 8% w/w) with SFPG (up to 1% w/w) at neutral pH (7) has also been studied [36]. On the basis of methylene blue spectrophotometry, surface tension measurement, and zeta potentiometry, the occurrence of weak electrostatic interactions, even when both biopolymers were net negatively charged, was confirmed. The WPI:SFPG ratio showed its importance, and the most significant effect was observed at 1:1 ratio. In the next report [38], by examining the effects of various pH (3–7) and mixing ratios (1:3, 1:1, 3:1, 6:1, and 9:1% w/w WPI/PG), it was revealed that pHc and pHφ1 moved toward higher pH values when the WPI:PG ratio was increased. In addition, solubility was increased at pHc > pH > pHφ1 for all mixtures, and lowering the pH toward pHφ1 decreased the size of complexes, while any further decrease led to larger particles. Heating also had no significant effect on pHc and pHφ1 at neutral pH, although the turbidity of heated mixtures was higher than that of unheated ones due to the formation of larger aggregates. It was concluded that these findings could be potentially useful for designing novel microstructure for special functionalities in food systems.

The effects of the mixing ratio (0.1:1, 0.2:1, 0.5:1, 1:1, 2:1, 5:1, 10:1, total biopolymer concentration 0.5% w/w) and pH (2–7) on the complexation of WPI:PG was also investigated [40]. Upon decreasing the pH and increasing the mixing ratio, the net neutrality shifted to the higher pH values, and neutralization occurred at pHopt (the formation of neutralized complexes). The maximum precipitation yield occurred at pH 3.4 for the 1:1 ratio. However, again this work was conducted on PG that had not been fractionated.

11.7.2.3 Casein

The phase behavior of sodium caseinate (3%–8.5% w/w) with SFPG (1%–3.5% w/w) at neutral pH and in the presence of NaCl (0.05 M) was investigated [63]. The plotted phase diagram clearly demonstrated the prevailing effect of SFPG on the incompatibility of biopolymers as the mixture of the two at different locations retained various microstructures. The flow behavior of mixtures was strongly dependent on the composition of the equilibrium phases and the corresponding microstructure of the system. The composition of equilibrium phases also showed a minor contribution of sodium caseinate to the thermodynamic incompatibility.

The interaction of SFPG and SFGT with sodium caseinate (Na‐CN) over a wide range of pHs (2–7) in order to provide soluble complexes was investigated [32]. According to the optical density, zeta potential, and rheological measurements, the desired ratio for both was 0.4:0.6 (% w/w). It was argued that at these concentrations, the anionic polysaccharide content was not only adequate to neutralize the positive charge of protein at acidic pH, but also was sufficient to hinder the approach of protein particles via electrostatic repulsion, due to the prevalence of negative charges in the system.

The efficiency and mechanism of PG, SFPG, IFPG, and SFPG:SFGT mixtures on the stabilization of milk–orange juice mixture (pH = 4.2) were also studied [29]. On the basis of visual observations, the zeta potential, and rheological measurements, the mixtures were effectively stabilized by PG, SFPG, and SFPG:SFGT (80:20) at 2.20, 1.00, and 0.37% w/w, respectively. Therefore, it was concluded that SFPG, as an anionic‐adsorbing hydrocolloid, adsorbed onto the caseins at the interface, giving rise to enhanced electrostatic and steric repulsion. It is interesting to note that the ratio of SFPG:casein in this study (1:1.5) was similar to that mentioned in a recent report [32]. SFPG and SFGT also proved to be highly compatible adsorbing hydrocolloids, where the presence of the latter improved the effectiveness of the former one. Similar results on the stabilization of milk–sour cherry juice mixture have been reported [57], while in another study [52], it was noticed that, due to some structural changes by irradiation (at 4 kGy), the PG requirement for stabilization of milk–sour cherry juice mixture decreased (from 2.2 down to 1.5% w/w).

11.7.2.4 β‐Lactoglobulin

The influence of the PG: β‐L ratio (1:2, 1:1, 2:1, at 0.15% w/w) and pH (2–7) on the formation of electrostatic complexes was also investigated [49]. According to the optical density analysis, particle size measurement, and microscopic observations, seven pH‐delimited zones were detected, and the maximum solubility was observed at pHc > pH > pHϕ1, though this pH was dependent on the biopolymer mixing ratios. pHc, pHϕ1, and pHopt denoted the formation of soluble complexes, interpolymeric complexes, and neutralized complexes, respectively. The smallest particle (d = 27 µm) was recorded for a 1:2 ratio of PG: β‐L at pHopt. The emulsifying properties of the complexes (0.75% w/w) with soybean oil (20% w/w) showed that only the emulsions made with soluble complexes (pH after mixing > pH > pHc and pHc > pH > pH ϕ1) in all ratios were stable (no changes in droplet size over 48 h). It should be noted that PG containing 70% w/w IFPG was used, and for this reason, it was impossible to obtain fully soluble complexes. This is why droplet size distributions were bimodal in some reports [ 58, 59].

In other studies [ 50,64], β‐L–PG conjugation at various ratios (1:2, 1:1, 2:1) was studied using the wet Maillard reaction method as a function of time (1–14 days). The covalent linkage of β‐L–PG was confirmed by SDS‐PAGE. Moreover, the emulsifying activity of β‐L:PG (1:1) conjugates as a function of Maillard reaction time showed no significant effect on the emulsion activity index; however, the stability index significantly increased. The emulsification performance of all conjugates (1:1, 1:2, and 2:1) was also much better than that of PG and gum arabic alone at the same emulsifier/oil ratio (1.5% w/w total biopolymer/40% v/v oil). However, the usage of PG without fractionation could be problematic, as already argued in some existing studies [ 49, 58, 59].

In an extensive study [65], the effects of pH, β‐L:SFPG ratios (8:1–1:4), total biopolymer concentration (0.1%–0.6% w/w), salts (NaCl and CaCl2), ionic strength (0–100 mM), and temperature (25, 40, and 55 °C) on the electrostatic interaction of β‐L:SFPG were elucidated. Similar to the previous reports [ 32, 36], soluble complexes were formed above the isoelectric point (pI) of β‐L; however, pHϕ1 and pHopt were significantly affected by the mixing ratios and concentration. They shifted toward lower pH values as a result of decreasing the mixing ratio (at a constant concentration) or concentration (at a constant mixing ratio), though pHc (formation of soluble complexes) remained constant. The inhibitory effect of CaCl2 was much more pronounced than that of NaCl, and the higher the ionic strength, the lower the interaction. The roles of electrostatic interaction, hydrogen bonding, and hydrophobic interactions in the complexation process were investigated using isothermal titration calorimetric (ITC) analysis.

11.8 Surface Activity and Emulsifying Properties

Surface activity, or the ability to reduce the interfacial tension, is a special characteristic which can be seen usually in proteins and surfactants but not in polysaccharides. However, it is reported that polysaccharides may also show surface activity if they contain either hydrophilic auxiliary (e.g., methyl and acetyl) groups or are linked to the glycolipids and/or proteinaceous fractions, as in gum arabic [55, 59,66]. In this regard, it has been shown [ 28, 39] that PG can diminish the surface tension of distilled water (from 72.5 to 63 mN m−1) at low concentrations (0.1% w/w). At higher concentrations (0.9% w/w), the surface tension can drop even lower (56 mN m−1). Referring to PG's chemical composition, one could speculate that its protein and lipid content (∼0.20% w/w each) is almost negligible, which therefore does not explain its surface activity. Fadavi et al. [51] also examined the effect of various concentrations (up to 3% w/w) and color groups (white, yellow, and red) of PG dispersions and observed similar trends; however the behavior of red color was irregular. There is also one report [67] on foaming properties (capacity and stability), which showed that even at 3% w/w, it was unable to create foam, and at high concentrations (>4% w/w), the foam immediately collapsed.

The emulsifying property of hydrocolloids is an interesting topic which has been examined by many researchers. With regard to PG, these studies can be classified into two distinct groups: those on PG and those on SFPG. With reference to the latter one (SFPG = 0.5% w/w), it was shown [39] that it is capable of emulsifying up to 5% w/w oil (average droplet size 1.13 µm), although within a day during storage (4, 25, 50 °C), they easily phase separated. When the SFPG concentration increased (2% w/w) but the oil content decreased (1% w/w), the resulting emulsions were quite stable (at 4, 25, and 50 °C) too. The optimum emulsification capacity was 2:1 (the SFPG:oil ratio), when emulsions were stable against thermal processing at 80 °C for 30 min (average droplet size 0.80 µm). However, apart from the possible contribution of the inherent protein and lipids, the emulsifying activity and capability were mostly attributed to the viscosity increase and the possibility of weak surface activity due to methyl and acetyl groups on the sugar residues. The emulsification activity, capacity, and heat stability of SFPG was significantly better than those of gum arabic.

Regarding the emulsifying property of SFPG, it is also noticed [48] that the emulsification properties of SFPG which was separated from chemically modified IFPG (esterified by acryl amide) were not as good as those of ordinary SFPG, since its emulsions were partly phase separated in a matter of a few hours. The lower emulsifying activity of SFPG was attributed to its lower molecular weight and viscosity as well as to possible structural alterations.

In another study, the effect of SFPG (0.25%–1%) and oil (5%–20%), pH (3.5–8), and storage time (up to 20 days) on the stability, rheological properties, particle size distribution, as well as microstructure was investigated [68]. Accordingly, the emulsions, particularly at higher SFPG concentrations (0.75 and 1% w/w), were stabilized via the bulk viscosity increase. In addition, in mildly alkaline conditions (pH = 8), the stability increased (20% w/w oil, 1% w/w SFPG, was fully stable after heating at 80 °C for 30 min), whereas at pH 3.5, phase separation occurred. The droplet size distributions were unimodal, and average droplet sizes were <2 µm. Moreover, the significant effect of SFPG (0.25% w/w) on the stability, particle size, and rheological properties of orange peel essential oil nanoemulsions was reported [69].

Some researchers [ 37, 67] also attempted to evaluate the emulsifying capacity and emulsion stability of PG (up to 2% w/w) using 7% w/w orange peel essential oil and 20% w/w sunflower oil, respectively. They concluded that the emulsifying properties are almost comparable with gum arabic and sweet almond gum, respectively. In contrast, it is reported [58] that PG, even at 5% w/w, was unable to emulsify D‐limonene (5 and 10% w/w) but in combination with maltodextrin (35% w/w) thorough stability was achieved. Interestingly, the size distributions were clearly bimodal with large droplets, probably due to the presence of the insoluble fraction (IFPG). Thus, it is recommended that PG should be fractionated, and only the soluble fraction (SFPG) should be utilized for emulsification applications.

11.9 Thermal Characteristics

The thermal properties of PG have also attracted scientific interest from the standpoint of finding out how its physicochemical characteristics could change during heating and the stability of PG to thermal decomposition by observation of the rate of weight loss using differential scanning colorimetry (DSC) and thermogravimetric analysis (TGA), respectively. On the basis of DSC analyses (−50 to 150 °C) over a wide range of concentrations (up to 8%), PG was classified as an endothermic (peaks at 95–120 °C) compound, demonstrating a stable and ordered organization. Moreover, due to the lack of thermal peaks between 20 to 90 °C, it was concluded that PG cannot create any true gel [37]. On the other hand, the TGA analyses (30–600 °C) revealed two distinct regions (40–140 °C and 250–300 °C) of weight loss which were attributed to dehydration and thermal decomposition, respectively [35]. It can be concluded that PG, and probably its fractions, could withstand conventional thermal processing in food industry, and decomposition would not occur.

11.10 Potential Applications

For centuries, PG has been traditionally used as herbal medicine (e.g., bandage for swollen joints, an anti‐parasite, teeth pain healer, appetizer, anti‐cough, hair conditioner, and skin glazer) in Iran and some other countries. However, over the past decades, mostly owing to its comparably low cost and natural origin, Iranian scientists have shown great interest in elucidating its potential uses in commercial food or pharmaceutical products too. For instance, its applicability in dairy, as prebiotic or edible coating, emulsions, sauces and dressings, snacks and drinks, and bakery products are some of the fields that have been already documented.

In the field of dairy products, stabilization of milk–orange juice [29] and milk–sour cherry juice mixtures [57] has been recently investigated. PG and SFPG can be utilized for stabilization of acidic‐milk‐based drinks where stability to thermal treatment (pasteurization) is a challenge. Addition of PG has also shown no significant effect on the viability of probiotics in yoghurt [70]. PG, as a fat replacer in low‐fat Iranian white cheese, effectively increased the moisture‐to‐protein (M:P) ratio, leading to a significant reduction in the hardness and textural parameters. It also promoted yield and proteolysis rate [71].

The viability of Lactobacillus acidophilus La5 in tomato juice in the presence of PG and SFPG as prebiotic agents was also assessed during fermentation, storage, and exposure to simulated gastric juice. The findings confirmed their prebiotic properties; however, SFPG was superior, and no unpleasant characteristics were observed on the basis of sensory analysis [72]. PG and its combination with gum arabic and skimmed milk powder at various ratios and concentrations have also been examined for the survival of microorganisms (Lactobacillus plantarum, Escherichia coli, Xanthomonas axonopodis, and Saccharomyces cerevisiae) during freeze drying, and the best protective effects occurred with the former two microorganisms during 14 days of storage [73]. The antimicrobial and antioxidant (cinnamon and thyme essential oils) effects of adhesive coatings prepared by SFPG on the refrigerated shelf life of rainbow trout fillets effectively reduced total viable, psychrotrophic and lactic acid bacteria counts. It was concluded that SFPG was a novel edible coating agent for introduction of essential oils [74]. The effect of PG, as a coating material, on the oil content and quality parameters of shrimp after deep‐fat frying showed its potential for the production of low‐fat shrimps [75].

SFPG also has shown a reasonable capability to extend the stability of orange peel essential oil nanoemulsions against coalescence [69]. A combination of PG (5% w/w) and maltodextrin (35% w/w) has been utilized to stabilize D‐limonene emulsions for a month or so [ 58, 59]. The emulsifying and stabilizing abilities of PG (2.5 and 5% w/w) on the production of crocin (biocompound of saffron) double emulsion, its stability, rheological changes, and release behavior have been studied [76].

PG and SFPG have also demonstrated reasonable film‐forming ability in the presence of glycerol, though SFPG was superior to PG in terms of qualitative parameters [ 39,77]. The efficacy of SFPG as an edible coating to improve postharvest quality of “Valencia” orange showed that SFPG, despite its glossy appearance, was sticky, and visible cracks occurred during storage [78].

Behbahani and Abbasi [79] also examined the stabilizing effects of PG and SFPG in London rocket seed drink (a refreshing Iranian or Persian drink). It has been reported [30] that gelatin can also be partially replaced by PG and IFPG in the formulation of jelly or gummy candies.

Golkar et al. [80] also tested the capability of PG in the production of reduced fat mayonnaise sauce; however its quality parameters were inferior to those of commercial mayonnaise sauce. PG has been used individually and in combination with GT as a stabilizer in tomato ketchup [81] as well.

In bakery products, PG at various concentrations showed different effects on the rheological characteristics of the dough, dough baking properties, staling, as well as sensory attributes [82].

There are also some other reports addressing the potential application of PG and its fractions in milk and dark chocolate, and some other areas, but these have not yet been documented (personal communications).

11.11 Concluding Remarks

PG, as a natural polysaccharide, is widely being produced in Iran. It is a competitively low‐cost emerging hydrocolloid which can potentially be used as an alternative to the existing biotechnological or other natural but likely expensive plant‐based hydrocolloids. Presently, its chemical structure and physicochemical and functional properties have been somehow characterized, but there are ambiguities which require an expert team of researchers to consider all aspects to characterize its true structure. In addition, its compatibility with other macromolecules and potential applicability in many food formulations have been verified. However, its partial solubility (70% w/w insoluble) is a major challenge, especially when it is needed to be utilized in liquid or semi‐liquid formulations (e.g., foods, pharmaceuticals, and cosmetics) or as an emulsifier for which fractionation (separation of the soluble and insoluble parts) and solubility are necessary. Therefore, further investigation on how to solubilize its insoluble fraction by various methods (chemical, enzymatic, and physical, e.g., sonication) is necessary, which could expand its functionality. Furthermore, the preservative function of special fractions of PG, as a natural hydrocolloid, also needs to be investigated. At last but not least, in order to be used in food products, its toxicological aspects and approval by the regulatory organizations need special attention too, where, on the basis of existing knowledge, the tannin content could be a challenge.

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