3.2.1. Scanning Electron Micrograph (SEM)

The microstructure of HPIDF/F-HPIDF at 300 times and 3000 times was shown in Figure 2. It can be observed that whether at low magnification or high, F-HPIDF showed a few differences compared to HPIDF. At low magnification (Figure 2a,c), the state of a single dietary fiber particle of F-HPIDF was richer in spatial structure and less smooth than that of HPIDF. In addition, the edge of F-HPIDF's particle was smoother and the tearing feeling was lower than that of HPIDF. At high magnification (Figure 2b,d), the flatness of HPIDF was much higher than that of F-HPIDF, whose structure was chaotic and uneven. This might indicate that F-HPIDF had a higher specific surface area than HPIDF and may be attributed to the removal of some saccharide structure (monosaccharide, etc.,) and the fracture of fiber structure of HPIDF during fermentation. Therefore, we can speculate that some adsorption properties of F-HPIDF should be better than HPIDF because its specific surface area increased (Premise: no functional groups with adsorption properties were removed). This assumption needs to be verified by the following experiments.

**Figure 1.** The gut microbiota structure in the feces from C57BL/6 mice. (**a**: Phylum Level; **b**: Genus Level).

**Figure 2.** Scanning electron micrograph (SEM) of HPIDF/F-HPIDF. (**a**: HPIDF at high magnification; **b**: HPIDF at low magnification; **c**: F-HPIDF at high magnification; **d**: F-HPIDF at low magnification).

3.2.2. X-ray Diffraction (XRD)

The XRD analysis of HPIDF/F-HPIDF is shown in Figure 3. It could be seen from the XRD spectra that, whether HPIDF or F-HPIDF, there was just one obvious crystal diffraction peak at 2θ = 22◦, which was a characteristic XRD curve of hemicellulose [25]. Meanwhile, there was little difference between HPIDF and F-HPIDF, which proved our previous speculation that the cellulose content in HPIDF is less [16]. XRD spectra showed the same results. In short, the fermentation process did not change the crystal structure of HPIDF.

**Figure 3.** X-ray diffraction (XRD) of HPIDF/F-HPIDF. (**a**: HPIDF; **b**: F-HPIDF).
