Investigation and identification of pectin substances of wild fruits of sea buckthorn (Hippophae rhamnoides l.)

Abstract

Introduction

Plant materials are valuable natural raw materials used in everyday life such as food products, food additives, aromatic components, pharmaceuticals, etc. A large and important task is the study of local wild-growing plant resources with the aim of their rational use and involvement in the production of organic food ingredients in the creation of functional food products [1]. Among the variety of wild plants growing on the territory of Azerbaijan, the sea buckthorn (Hippophae rhamnoides L.) stands out which gives a consistently high yield of fruits (the annual potential of harvesting fruits exceeds 3,000 tons). This is not only one of the most beautiful components of the republic's landscapes which has an extensive growing area, but also a valuable food, vitamin and medicinal plant, extracts and other food products having high antioxidant, antibacterial, anticarcinogenic and anti-radiation activity [2]. Therefore, the creation of integrated technology for processing the fruits of wild-growing sea buckthorn in order to obtain functional food ingredients of high technological and biological value is an urgent direction. The fruits of wild sea buckthorn are a unique raw material in terms of the quantitative and qualitative content of biologically active substances and their effect on the physiological functions of the human body [3]. In recent years, the growing interest in food products enriched with biologically active components ensure the normal functioning of the human body which increase its resistance to viral diseases, stress, negative environmental influences and prolong life has been noted [4]. Therefore, it is urgent to create new technologies for the production of functional food ingredients of natural origin with a complex of physiological and technological functions. Among the many studied the natural chemical components of plant raw materials, plant polysaccharides, in particular, pectin substances, are of the greatest interest [5]. Pectin substances (pectins) include protopectin, pectin polysaccharides and related arabinans, galactans and arabinogalactans, which generally have a complex chemical structure and composition. Pectin substances are a multifaceted family of complex plant polysaccharides, which form a functionally important part of the primary cell walls, together with cellulose and hemicellulose, provide plant cell strength, plant resistance to drought and low temperatures. They also provide water-salt metabolism, and characterized by high gel-forming ability and play an important role in human nutrition as components of "dietary fiber" [6]. In addition, pectins have a wide range of physiological activity, including immunomodulatory and gastroprotective effects and they are an integral part of human food at all stages of its evolutionary development, which led to the almost ideal adaptation of the human body to them [7].
In wild forms of sea buckthorn, depending on the climatic conditions of growth, there is a significant difference in the chemical composition of the fruits, including the content of pectin substances [2], therefore, to scientifically substantiate the integrated technology of processing sea buckthorn fruits, it is necessary to carry out systemic studies on the structure and properties of pectin substances.
The purpose of this work is to study and identify pectin substances isolated from the secondary waste of processing the fruits of wild sea buckthorn.

Objects and research methods

The object of the research is the fresh pomace of sea buckthorn fruits, which are formed after the extraction of juice and seeds of the fruit. Previously, we studied the full chemical composition of sea buckthorn fruits collected in the period September-November 2019 in the Babek administrative region of Azerbaijan, including the carbohydrate profile of the fruits [8]. The composition contained 10.32% (on absolutely dry weight) of pectin substances in the form of a water-soluble form (hydropectin) and a protopectin fraction. These data indicate that the secondary waste processing sea buckthorn fruits can be used for industrial processing in order to obtain pure pectin and pectin products.
Elemental microanalysis was used to identify the isolated pectin substances, IR spectra on an IR-Fourier spectrometer (Impact 410 "Nicolet") in the range of wave numbers 400-4000 cm-1 in KBr pellets with a spectral resolution of 2 cm-1, NMR spectra 1H and 13C on a Bruker Avantes instrument. Optical rotation was determined on a Perkin-Elmer 141 instrument in water at a temperature of 20 °C. The sorption capacity of pectin substances in relation to lead ions were determined by complexometric titration (back titration method) [9]. The results of experimental studies were evaluated using Microsoft Office Excel 2013 and Statistica 8.0 for Windows software packages.

Results and discussion

Modern trends in the development of pectin substances technology provide not only economic aspects, but also environmental ones, associated both with the reduction of the emission of harmful chemicals into the environment and with the creation of favourable working conditions. The intensification of individual resource-determining stages is another important aspect of the technological improvement of the processes for obtaining pectin substances. The hydroacoustic processing of pectin-containing raw materials in the rotary-cavitation extractors method is one of the effective technological solutions [10]. Cavitation treatment of an aqueous extractant changes its physicochemical properties, increases the pH of water, contributing to its activation, as a result of such treatment, water temporarily becomes an active solvent with acidic properties without introducing chemical reagents [5]. In an extractor of a rotary-cavitation type, under optimal conditions, the processes of grinding pectin-containing raw materials (the area of ​​the solid phase increases by 60-75 times), hydrolysis of the protopectin (water-insoluble) fraction of pectin substances and the actual extraction (diffusion) of pectin substances into the aqueous phase occur simultaneously. Another important technological method allowing to preserve the nativeness of pectin biomolecules is the use of membrane processes for their purification from ballast substances and concentration since these processes take place at ambient temperature and without phase transitions [11].
To obtain pectin substances, freshly obtained pomace of wild-growing sea buckthorn fruits were used. They were crushed to a size of 2-3 mm to carry out the extraction processes under equal conditions for the entire plant mass, and the extraction process itself was carried out with the following parameters: hydromodule 5: 1 - (deionized water: pomace); cavitation index 0.6; temperature -65 оС, process time -20 min. The entire technological process was carried out in accordance with the established standard procedure for using the cavitation-membrane technology for extracting pectin substances [7]. At the output, a milky-white-beige achromatic powder was obtained without absorption in the range 400-700 nm with parameters L = 90-92, a = (-3.7) - (-1), b = (+2) - (+ 15) colorless both in dissolved form and in gels and emulsions.
Microanalysis of pectin substances isolated from the pomace of wild sea buckthorn showed the following chemical parameters: C-28%, H-42%, O-24%, which corresponds to the gross formula C14H21O12, i.e. it is really pure pectin [12].
The obtained pectin from the wild-growing sea buckthorn was tested for its physicochemical characteristics, according to [13], and the data obtained are shown in Table 1.

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Table 1. Physical and chemical characteristics of sea buckthorn pectin.

According to the degree of esterification, the obtained pectin belongs to the group of low esterified pectins, and according to the rather high content of free carboxyl groups (21.80%) it should have a high complexing ability. This is confirmed by the results obtained in the course of research: the complexing ability of sea buckthorn pectin in relation to Pb2 + ions was 284.5 mg Pb/g, which can serve as a basis for creating functional products on its basis with a high antidote potential in relation to heavy metals and radionuclides [fourteen].
To identify the obtained pectin substances, FTIR spectra and 1H and 13C NMR spectra were recorded. The results are shown in Figure 1-2.

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Figure 1. IR spectra of sea buckthorn pectin samples.

From the detailed examination of the IR spectra, it can be concluded that pectin contains a large amount of galacturonic acid (intense absorption bands in the region of 1010-1150 cm-1). The band in the region of 1374 cm-1 is due to bending vibrations of the C-H groups of the pyranose ring, and in the region of 1610-1740 cm-1 absorption bands are observed, indicating the presence of free carboxyl groups. The available vibration bands of CH3 groups indicate the partial esterification of carboxyls. The assignment of bands in the experimental IR absorption spectra of sea buckthorn pectin is presented in Table 2 based on a comparison of absorption bands with data from the NIST spectral database (ASTM).

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Table 2. Assignment of bands in IR absorption spectra.

In figure 2 the information obtained during the NMR study of the sea buckthorn pectin sample confirms the high purity of the obtained pectin. Possible assumptions about the structural features of pectin obtained by mathematical processing of NMR spectroscopy data are shown in figures 2-3.

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Figure 2. The NMR spectra of sea buckthorn pectin.

In the proton spectrum of the sample in the high-field region (δ = 0.97-1.47; δ = 1.47-1.73 ppm), the manifestation of signals from CH groups located in position 4 is observed. Protons of the carbon atom of position 1 of the pyranose ring resonate at 3.94-3.38 ppm .d., and chemical shifts 3.48-3.59 ppm. refer to the carbon protons at positions 2 and 3 of the galactopyranosyluronic fragment. The methoxyl and carboxyl groups are characterized by the manifestation of signals at 3.67 and 3.91 ppm, respectively. Analysis of the 13C NMR spectrum showed the presence of carboxyl (δ = 103.03 -103.64 ppm), methoxyl (δ = 57.40 ppm) and methine groups in the structure. Carbon atoms in position 1 of the pyranose fragment give a signal in the high-field region at 63.78, 72.50, and 77.65 ppm. From figures 2 and 3 carbon atoms, the manifestation is characteristic at 77.33, 81.01 and 76.92 ppm. The carbon of position 4, which participates in the connection of pyranose fragments by the oxygen bridge, resonates at 44.05, 40.44 ppm.

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Figure 3. The mathematical processing and interpretation.

We can say that the doubling of the signal group is clearly manifested in the NMR spectra, which speaks in favour of the predominance of two different regions of the polymer pectin molecule (Figure 4), the structure of the oligosaccharide fractions, based on the data obtained, is uniquely α -1,4-D-glucans.
The abundance of signals from the carbonyl group indicates a very complex and heterogeneous structure of pectin, which has a fairly branched structure. The presence of a large number of signals in the region of acetal carbon also indicates the presence of a significant amount of neutral sugars mainly rhamnose and galactose.

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Figure 4. Prospective structure of sea buckthorn pectin.

Based on the obtained and published data [15, 16], it can be concluded that pectin from the fruits of wild sea buckthorn is low esterified and is a mixture of linear and highly branched polymers, mainly high molecular weight, α-D-galacturonan and other polysaccharides, the macromolecules of which include residues of galacturonic acid and neutral sugars.

Conclusion

Under optimal technological conditions, samples of pectin substances were obtained from the pomace of the wild-growing sea buckthorn fruits of Azerbaijan. Their physicochemical characteristics was studied, indicating that it was a low-esterified pectin. A high complexing ability of the obtained pectin in relation to lead ions has been established, which allows it to be used as an active ingredient in products for therapeutic and prophylactic purposes. Identification of the isolated pectin substances was carried out using physicochemical methods - elementary analysis, IR and NMR spectroscopy.

References

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