## 3 Results and Discussion
### Physicochemical Properties of SDF
#### 3.3.3 Effect of Three Extraction Methods on the Microstructures of SDF  
The microstructures of the SDF obtained using different extraction methods are shown in Figure 5. Under 500\(\times\) scanning magnification, it was found that the SDF extracted using acid and enzymatic methods consisted of lumpy particles of different sizes and shapes, and the particle size of the SDF extracted using the enzyme method was smaller. The SDF extracted using the alkali method was in the form of flocculent aggregates. Under 20,000\(\times\) scanning magnification, the SDF extracted using the acid method was in the form of dense lumps that formed a fault structure with a dense structure and no fluffy texture. The structure of the SDF particles extracted using the alkali and enzymatic methods became fluffy, both of which showed a honeycomb shape with a large number of pores. The surface of the particles extracted using the alkali method was not as homogeneous as that extracted using the enzymatic method. [PERSON] et al. [34] extracted SSR from pomeranate peel using different methods, finding that the SDF extracted using the enzymatic method had a porous surface with more pores, which were tighter and more regular than those of chemical methods. Also, the structure was much looser, which was similar to the results of this study. The decrease in pores of SDF-acid and SDF-alkali was attributed to the destruction of the SDF pore structure by chemical reagents [48].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**SDF** & **WHC/(g\(\cdot\)g\({}^{-1}\))** & **OHC/(g\(\cdot\)g\({}^{-1}\))** & **SC/(mL\(\cdot\)g\({}^{-1}\))** & **Solubility/(\%)** \\ \hline SDF-acid & 5.7 \(\pm\) 0.21 & 1.7 \(\pm\) 0.04 & 2.1 \(\pm\) 0.13 & 79.2 \(\pm\) 3.22 \\ SDF-alkali & 4.8 \(\pm\) 0.13 & 1.6 \(\pm\) 0.05 & 2.3 \(\pm\) 0.07 & 47.3 \(\pm\) 1.34 \\ SDF-enzyme & 6.2 \(\pm\) 0.24 & 2.4 \(\pm\) 0.13 & 3.2 \(\pm\) 0.14 & 97.4 \(\pm\) 4.75 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Physicochemical properties of soluble dietary fiber (SDF) extracted using three methods.  
The surface structure of the SDF obtained using ultrasound-assisted enzymatic extraction was uniform and loose with a large number of gaps and pores, indicating that there might be a large specific surface area [49]. It was likely that the enzyme and ultrasound altered the dense structure of DFs, the glycosidic bonds and hydrogen bonds of which were broken by enzymes, resulting in a looser structure. Some of the hydrophilic groups in the structure of SDF were exposed, which also led to a looser texture of the SDF surface, increasing the solubility of the SDF. The decrease in the polymerization of SDF and the exposure of more polar and nonpolar groups helped to improve the hydration and adsorption properties of the SDF. Ultrasonication increased the specific surface area of the SDF, which improved its functional properties such as solubility and WHC. Therefore, different extraction methods changed the structure of the SDF and thus affected its functional properties. The SDF obtained by enzymatic treatment had higher WHC, SC, solubility, and OHC, which were consistent with the results in Table 4.