## 2 Extraction and Purification Methods of Ginger Polysaccharides
### Extraction Methods of Ginger Polysaccharides
#### 2.1.2 Ultrasonic-Assisted Extraction Method  
As a non-thermal physical processing technology, UAE has significant advantages in extracting ginger polysaccharides. The research shows that the extraction of ginger polysaccharides (GSLPs) by UAE takes 50 min, while the hot water extraction (HWE) takes 5 h. Compared with the HWE method, UAE technology can greatly reduce time consumption and improve the GSLP's preparation efficiency. The total sugar content of GSLP extracted by UAE was 44.65 \(\pm\) 0.95%, while that extracted by HWE was 37.85 \(\pm\) 0.55%. This may be related to the fact that the cavitation effect of ultrasound is more effective in destroying the cell wall and cell membrane of plants and enhancing the contact between solvents and polysaccharides, thus promoting the dissolution and release of polysaccharides in cells [35; 36]. The low total sugar content of HWE is due to the relatively high extraction temperature, which will also promote the dissolution of some non-polysaccharide components in the process of accelerating the dissolution and diffusion of polysaccharides into aqueous solution, thus reducing the total sugar content. Furthermore, the yield of GSLP extracted by HWE and UAE was 6.83 \(\pm\) 0.54% and 8.29 \(\pm\) 0.31%, respectively. It can be seen that UAE has a higher extraction rate than HWE. The yield of polysaccharides extracted from plant raw materials is largely affected by the cell wall and polysaccharide dissolution efficiency. HWE only slightly breaks the ginger cell wall, but the UAE method makes the structure of the cell wall and cell membrane of ginger stem and leaf more damaged and hydrolyzed, thus promoting the dissolution of polysaccharides. Therefore, from the perspective of extraction rate, the UAE method is a better choice to extract ginger stem and leaf polysaccharides [32].  
Because the UAE of ginger polysaccharides has the advantages of high efficiency and low energy consumption, researchers have paid more attention to the UAE of polysaccharides in recent years [37]. Ultrasonic technology has developed to a great extent, and multi-frequency ultrasonic technology has gradually entered the vision of researchers. Multi-frequency UAE is based on the combination of multiple frequencies. Compared with traditional single-frequency ultrasound, multi-frequency ultrasound has low energy consumption, low noise, and obvious cavitation effect. It is reported that the efficiency of multi-frequency UAE of polysaccharides is higher than that of single-frequency ultrasound. It was found that the yield of polysaccharides extracted by tri-frequency ultrasound and dual-frequency ultrasound was 10.50 \(\pm\) 0.20% and 9.74 \(\pm\) 0.30%, respectively, higher than that extracted by single-frequency ultrasound (8.29 \(\pm\) 0.31%) [32; 38]. This may be due to the uniform energy distribution and easy resonance of extracted raw materials in dual-frequency and triple-frequency ultrasonic treatment, while single-frequency ultrasonic treatment has a directional effect. It can be seen that multi-frequency ultrasound can significantly improve the extraction rate of polysaccharides, which is more conducive to scientific research and industrial production.