Influences of extraction methods on physicochemical characteristics and activities of Astragalus cicer L. polysaccharides
Graphical abstract
Introduction
In recent decades, polysaccharides have attracted attention from researchers due to their biological antioxidation, immunological regulation, antitumor and anti-inflammatory activities [[1], [2], [3]]. Extraction techniques exert significant influences on the extraction yields, physicochemical properties and bioactivities of polysaccharides. Currently, several extraction techniques are used to extract polysaccharides. When considering the extraction cost, operation convenience, environmental factors and extraction efficiency, each method has both benefits and drawbacks. Hot water extraction (HW) is an easy-to-operate and environmentally friendly extraction technology. However, HW generally has the disadvantages of long operation times, high extraction temperatures and low extraction yields. Ultrasonic-assisted extraction (UA) can enhance the extraction yield of polysaccharides through the generation of cavitation effects [4]. The release of polysaccharides from plant cells can be promoted by enzyme-assisted extraction (EA) due to the degradation of cell walls by enzymes such as cellulase, papain and pectinase [5]. Enzyme-ultrasonic-assisted extraction (EUA) has the benefits of both UA and EA. Microwave-assisted extraction (MA) is based on the direct application of electromagnetic radiation, which has the ability to absorb electromagnetic energy and transform it into heat, to a material. Compared with traditional methods, MA has many advantages because it consumes less time and solvent, provides a better extraction rate, and ultimately produces a higher substance yield. However, it requires higher equipment cost and operator skill [6]. Moreover, different extraction techniques can lead to differences in the biological activities of polysaccharides.
Astragalus L., the largest genus in the family of Leguminosae, is widely distributed throughout temperate regions in the world. Several species of Astragalus L. are considered to have potential anhidrotic, diuresis, antidote and tonic effects [7]. Certain species of Astragalus L. are also used for the treatment of nephritides, mellitus, leucocythemia and metrocarcinoma. The active ingredients of Astragalus L. include flavones, saponins, alkaloids, polysaccharides, sterols and phenols. The extraction efficiency of polysaccharides from Astragalus cicer L. (ACPs) has great effects on the utilization of this plant for food and pharmaceutical purposes.
Therefore, the aim of this study was to evaluate the effects of different methods (HW, UA, EA and EUA) on the extraction yields, physicochemical characteristics and antioxidant activities of ACPs. The physicochemical properties of ACPs were measured based on a series of indicators, including the chemical composition, solubility and Fourier transform infrared spectroscopy (FT-IR) results. The antioxidant activities of ACPs were determined based on an analysis of the free radical scavenging activities and ferric reducing power of these molecules. It is of interest to prepare ACPs with better antioxidant activities and analyze and characterize their physicochemical characteristics to provide a theoretical basis for future use of these ACPs in related fields.
Section snippets
Materials
The aerial parts of Astragalus cicer L. at anthesis were harvested at Jilin Agricultural University (Changchun, China) and dried at 50 °C. Subsequently, the dried plant materials were ground into powder (1 mm) and prepared for ACPs extraction. The chemicals cellulase, papain, pectinase, monosaccharide standards, 2,2-azino-bis-(3-ethyl-benzothiazoline-6-sulfonic acid) (ABTS), 1,1-diphenyl-2-picrylhydrazyl (DPPH), and vitamin C were purchased from Sigma (St. Louis, MO, USA).
ACPs extraction and preliminary purification
ACPs were extracted by
Chemical composition of ACPs
As shown in Table 1, the yields of the EA-ACPs (13.91%), EUA-ACPs (13.60%) and UA-ACPs (11.91%) were significantly higher than those of the HW-ACPs (8.42%) (P < 0.05), which suggested that the UA, EA, and EUA techniques could significantly improve the extraction yields of ACPs. Protein was not detected in the four ACPs. Different extraction techniques had no significant effects on the polysaccharides and moisture contents of ACPs (P > 0.05). However, the uronic acid contents of ACPs were
Conclusions
The effects of four extraction methods on the extraction yields, physicochemical characteristics and antioxidant activities of ACPs were investigated to select the optimal extraction technology. The results showed that the EA-ACPs had the highest extraction yield, smallest molecular weight and best antioxidant activities. The highest antioxidant activities of the EA-ACPs among the four ACPs might be related to their lower molecular weight and higher uronic acid content. Therefore, the
Acknowledgements
This work was supported by the National Natural Science Foundation of China (31601972); the General Financial Grant from the China Postdoctoral Science Foundation (2017M621224).
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