Physicochemical, rheological and structural characterisation of Bene gum exudates from Pistacia eurycarpa Yalt

Abstract Bene gum was investigated for its physicochemical and structural properties in comparison to gum arabic and gum tragacanth. Elemental analysis of the Bene gum sample revealed sodium and potassium content to be 57.21 and 0.95 ppm, respectively. According to the rheological properties, all gum solutions exhibited non-Newtonian, shear-thinning in the concentration of 1% (w/w). Both storage modulus G′ and loss modulus G″ of all gums demonstrated elastic properties. The GC–MS analysis of dichloromethane extract confirmed the presence of eight constituents and α-pinene is the predominant constituent. The NMR spectra of the gum sample showed resemblance in individual sugar ingredients and the specific patterns of gum were observed. FTIR spectra of the studied gums displayed the presence of the same functional groups in the three gums. XRD studies revealed the amorphous nature of Bene gum. DSC and TGA thermograms were characteristic for each gum. Major thermal transitions were determined. Graphical abstract


Introduction
Nowadays, there is a special interest in the unique opportunity of using the natural gums which exude from trees and shrubs due to their non-toxic characteristics, easy availability, and their low price. Natural gums are hydrocolloids polysaccharides that are used in foods, industry, cosmetics and biomedical. Wild pistachio (Pistacia) belongs to the Anacardiaceae family and mainly grows in West Asia and the Mediterranean region. Three Pistacia species originally grown in Iran are P. vera Linnaeus, P. khinjuk Stocks, P. eurycarpa Yalt. and P. atlantica Desf. The height of P. eurycarpa trees is up to 60 ft. It can generally tolerate drought and alkaline soil circumstances, winds, and desert heat (Delfanian et al. 2018). One of the most beneficial and economic exudates in western parts of Iran is the oleoresin of P. eurycarpa Yalt. (Synonym: P. atlantica subsp. kurdica) which is locally named 'Bene'. Bene gum is usually collected in the clay container that is placed under the wounds made on the trees' trunk and branches in June and July for a month or more. It should be noted that environmental items such as temperature, atmospheric oxygen and sunlight influence the exuded oleoresin during the harvest time. Furthermore, Bene gum includes volatile oil and resins. The significant components of its oil are sabinene, limonene and pinenes. Its exudate gums are usually used to make chewing gum (Mohammadi et al. 2019). The conventional uses of this gum are appreciated due to their positive roles of the plant and gum in gastrointestinal diseases and different digestive problems like gastritis, diarrhoea, intestinal upsets and peptic ulcers. Bene has acceptable effects as an anti-Helicobacter pylorus, also the anti-inflammatory potential on colitis, and as an antibacterial agent against S. aureus (Taleghani et al. 2021). Although most research on P. eurycarpa has been devoted to antioxidant activities and gum biological, some limited publications have been reported about the structural properties of the exudate gums from this species. Over the past years, the new knowledge about the chemical properties of polysaccharide gums makes a good alternative for the preparation of a perfect biopolymer for the commercial and industrial aims of gums. To our knowledge, there is no rigorous study available on exudate from Bene trees, and the chemical characterisation of this gum has not been investigated so far. Therefore, the major aim of the present study is the physicochemical characterisation of this gum using several physicochemical methods. Furthermore, in order to have a better understanding of the functional properties of Bene gum, we compared differences in its physicochemical properties with gum arabic and gum tragacanth. Hence, the current study can be used as a database in the food and pharmaceutical industries. Table S1 shows the chemical composition of the Bene samples, gum arabic and gum tragacanth. All the samples had different values for Na and K. Potassium and sodium levels of the Bene gum were lower than the ones found in gum arabic (Yebeyen et al. 2009) and gum tragacanth exuded by Astragalus parrowianus (Balaghi et al. 2010). Figure S1 displays the measured viscosity for three gum solution samples. There is a decline in apparent viscosity with the shear rate rises, which shows that all solutions have shear-thinning characteristics. The viscosity of gum tragacanth ( Figure S1c) indicates a different flow behaviour from other gums. Gum tragacanth sample illustrates the maximum apparent viscosity followed by gum arabic ( Figure S1b), while Bene Gum ( Figure S1a) demonstrates the lowest viscosity between all samples. The shearthinning property is because of the alteration in the molecular network triggered by the alignment of polysaccharide chains in the line of shear and the interruption of molecular entanglements under shear stress forces. The various shear properties are due to molecular weight and the capability of the combination of water molecular and also there is the impact of intermolecular electrical field force in the gum tragacanth (Miao et al. 2018). Therefore, this gum displayed a different flow curve. Figure  S2 illustrates when the frequency was increased to the specific points both G 0 and G 00 moduli increased, which might be because of growing the velocity of the solid particles at the larger frequencies. It is also displayed that G 0 was slightly higher than G 00 over the whole experimental frequency range but, they reached each other at higher frequencies. So, a solid-like treatment was detected at the low frequency (G 00 < G 0 ). There were some crossover points for all samples especially in the high range of frequency, also at a speedy motion, their behaviour was more inflexible and stricter. Frequency sweep design of the polymer solutions associated with several factors such as molecular weight, the concentration of the polymer, quality of solvent, dispersity, structural properties. It appears that all the gums had a frequency sweep pattern like unlinked polymers, which usually happened in dilute solutions. If the concentration of unlinked polymers solution is in height enough, the polymer chains start to create entanglements. These muddles seem more mechanical interactions than chemical or physical-chemical bonds. Therefore, macromolecules can transfer gradually, even under weak shear, and slide along each other, revealing whole or partial disentanglement (Razi et al. 2020).

GC-MS analysis
As shown in Table S2, about 8 constituents were identified from the sample, representing 99.98% of the total essential oil, a-pinene (41.40%) was the predominant component. The most abundant chemical structure within components was monoterpene hydrocarbons (41.4%) and phenylpropanoid (20.97%), followed by hydrocarbons (14.97%) and diterpene alcohol (13.12%). The exist of high amounts of a-pinene which is recognised to be active against Helicobacter pylori, may clarify some of the old-fashioned therapeutic uses of this species in the treatment of peptic ulcers, and to strengthen teeth gum. Comparison with previous reports on the most closely related species, P. terebinthusm P. khinjuk and P. palaestina, shows a-pinene as the major component of this plant. For the other compounds, 4-ethyl-2-methoxyphenol is an extremely weak basic compound and it has a smoky odour, while hexadecanoic acid is a fatty acid known to have potential antibacterial and antifungal activity. It is interesting to say that non-volatile phenylpropanoid-derived compounds are in charge of the pungent taste. Extract derived from Bene includes trans-6-shogaol as the main compounds which are responsible for the spicy taste of extract. In addition, it is of interest to note that the phytol was recognised in the dichloromethane extract of Bene as a major compound which is diterpene alcohol present in the essential oils of various aromatic plants. A lot of biological and therapeutic properties have been reported for phytol, such as anticonvulsant, anti-inflammatory, antispasmodic activities and antibacterial (Saikia et al. 2010). Figures S3 and S4 show 1 H and 13 C NMR spectra of Bene gum. Figure S3 depicts crowded signals in the 1 H NMR spectrum among 3 and 6 ppm that is characteristic of polysaccharides and reveals the existence of sugar residues, for this gum an upfield peak at around 1.11 ppm assigned to the methyl group of rhamnose sugar which on high resolution illustrates triplet of triplets. The chemical shift at 2.14 ppm in Bene gum spectra shows the existence of an acetyl group (COCH 3 ). The signal that arises at 3.58 ppm was caused by the existence of -O-CH 3 . In comparison, some researchers reported that in the NMR spectrum of gum arabic the peaks observed at 3.3-3.8 ppm resulted from the presence of -O-CH 3 . From 13 C spectra of Bene gum ( Figure S4), the peak at 20.38 ppm, is linked to the carbon of methyl group of rhamnose, and this shows that this gum contains deoxygenated sugars. While the signals because of nonanomeric carbons C 2 -C 5 appear between 60.37 and 84.35 ppm, signals from anomeric carbons of Bene gum appear in the 103.01-109.61 ppm (Cui 2005). Figure S5 shows the comparison of FT-IR absorption spectra of Bene gum, gum arabic and gum tragacanth, respectively. The broadband at 3429 cm À1 , 3488 cm À1 , 3437 cm À1 corresponds to the O-H stretching band of the hydroxyl group. The band in the region of 3400-3500 cm À1 for the N-H band has been overlapped by the O-H band. The comparison of band intensities in the spectra shows increases and broadens for gum arabic and gum tragacanth compared to Bene gum. These increases in intensity recommend the hydrogen bonding becomes stronger. N-H stretching band can be attributed to galactopyranose and glucopyranose rings. The peak at 2941 cm À1 , 2891 cm À1 , 2930 cm À1 is the characteristic of the C-H stretching band which is showed the presence of carbohydrates (Daoub et al. 2018). A weak peak was observed at 2145 cm À1 (gum arabic), 2155 cm À1 (gum tragacanth) was related to the characteristic band of C ¼ C stretch, whereas 2480 cm À1 (gum tragacanth) and 2360 cm À1 (Bene gum) were assigned to C-H stretch of the -CH 2 . The FTIR spectrum shows a strong peak at 1701 cm À1 ,1459 cm À1 (Bene gum) and 1751 cm À1 , 1445 cm À1 (gum tragacanth) and 1430 cm À1 (gum arabic) which is corresponded to C ¼ O stretching of the COOH group also the band at 1628 cm À1 (gum tragacanth) and 1613 cm À1 (gum arabic) which is attributed to C ¼ O stretching of -COO group may be because of the presence of carboxylic acid or carboxylate anion form d-galacturonic acid. The band at 1378 cm À1 (Bene gum) and 1375 cm À1 (gum tragacanth) is attributed to the alkane CH 3 bend. The band between 1253 cm À1 and 1022 cm À1 is due to the C-O-C stretching and between 1022 cm À1 and 586 cm À1 represent the C-H band from polysaccharides (Zohuriaan and Shokrolahi 2004).

XRD analysis
The investigation of the crystalline structure of gums is carried out by XRD analysis. The XRD patterns ( Figure S6) indicate a sharp diffraction peak centered at 2h % 15.1508 with the intensity of 97.63, which reveals the amorphous nature of Bene gum. Also, researchers found the same amorphous nature of gum arabic with the maximum intensity of 2h ¼ 19.895 (Azzaoui et al. 2017). Hence, the structure of Bene gum is more similar to gum arabic.

DSC analysis
The DSC thermogram ( Figure S7) representatively shows a comparison of the DSC thermogram for Bene gum, gum arabic and gum tragacanth, respectively. There is not glass transition temperature (Tg) in these thermograms. Gum arabic showed an exothermic event at around 18.7 C probably because of crystallisation. The endothermic peak at the temperature range of 57-68 C is due to the removal of the bounded water in these gums because of the hydrophilic nature of functional groups. The wide exothermic peak at around 220-278 C is related to the decomposition of the gums. H 2 O, CO 2 , CH 4 are formed due to depolymerisation, dehydration, and pyrolitic decomposition in these high-temperature levels. Nevertheless, different structures and functional groups will cause a difference in route structures or consequential fragments.
For example, polysaccharides like these tree gums have consisted of carboxylic acid or carboxylate functional groups. Hence, thermal changes of the carboxylate groups and the evolution of CO 2 from the related carbohydrate backbone can be a likely mechanism for the thermal evolutions (Zohuriaan and Shokrolahi 2004). Figure S8 illustrates TG/DTA curves for Bene gum, gum arabic and gum tragacanth. All samples have a similar double steps trend: desorption of moisture and major decomposition stage. Regarding Bene gum, the first event occurs from 42 C to 150 C with 27% of mass loss which is due to water loss. This stage arises in the other two gums with 7% of mass loss. The second stage happens at above 200 C with 97% mass loss is attributed to the polysaccharide decomposition process. It seems that because the polysaccharide decomposition occurs at 284 C, these gums have thermal stability (Daoub et al. 2018).

Experimental
This part is described in the Supplementary Material.

Conclusion
Comparison analysis of Bene gum with gum tragacanth and gum arabic was performed in order to characterisation its nature and exploitation of this novel exudate as an additive in food industries. The outcomes of flame photometer showed that the maximum concentration Na, while Li was not determinable (N.D.) in all samples. All mixture solutions had non-Newtonian shear-thinning properties and the frequency sweep tests were similar to those of branched polymers. The GC-MS spectra of dichloromethane extracts showed a high amount of monoterpene hydrocarbons. The NMR results led to the identification of main functional groups for the first time. FTIR spectra presented the characteristic bonding between the functional groups of polysaccharides and XRD analyses reiterated the amorphous structure of Bene gum. DSC thermograms showed decomposition of amorphous nature during heating of the gums. It seems that the thermal behaviour of Bene gum is more similar to gum tragacanth. The TG curves depicted similar thermal stability for them. This biopolymer may be a good initial point fascinating not only for pharmaceutical manufactures but also for food and other industries.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
The author(s) reported there is no funding associated with the work featured in this article.