3.4.2. HMAC of HPIDF/F-HPIDF

The adsorption capacity of HPIDF/F-HPIDF for metal ions is shown in Figure 7. On the whole, the adsorption capacities of F-HPIDF to four kinds of heavy metal were higher than that of HPIDF. The adsorption capacity of F-HPIDF for Cd2+ (1.82 μmol/g) and Pb2+ (1.91 μmol/g) was similar; meanwhile, the adsorption capacity of Cu2+ (0.68 μmol/g) was still the lowest. The HMAC of dietary fiber was directly related to its particle size and specific surface area [43], which led to the phenomenon in this study. This result was consistent with the structural analysis results.

**Figure 7.** Heavy metal-adsorption capacity of HPIDF/F-HPIDF. (\*\*: *p* < 0.01).

The excessive intake of heavy metals was not only harmful to intestinal health but also might damage multiple organs [44,45]. With the progress of fermentation, the HMAC of HPIDF gradually became stronger, which was a very good situation for the digestive system. Heavy metal ions were difficult to enter the blood and would be excreted via feces. In short, the fermentation process of HPIDF in the colon may prevent the body from ingesting excessive heavy metals.

#### 3.4.3. Potentially Harmful Substances-Adsorption Capacity of HPIDF/F-HPIDF

We used glucose-adsorption capacity, cholesterol-adsorption capacity, sodium cholateadsorption capacity, acrylamide-adsorption capacity, and nitrite-adsorption capacity to determine the adsorption function to potentially harmful substances of HPIDF and F-HPIDF. The results are shown in Figure 8.

**Figure 8.** Potentially harmful substances-adsorption capacity of HPIDF/F-HPIDF. (**a**: GAC; **b**: CAC; **c**: SCAC; **d**: AAC; \*: *p* < 0.05).

First, the adsorption capacity of HPIDF/F-HPIDF to nitrite was not shown in the results. According to previous research, HPIDF showed nitrite-adsorption capacity only in the gastric juice [16]. The same situation occurred in this study, we even did not find that F-HPIDF had a detectable nitrite-adsorption capacity. Therefore, the result was not shown in the figure. In short, whether it was fermented or not, HPIDF had no nitrite-adsorption capacity in the colon.

Normal amount of glucose is an important energy source for the body, but excessive glucose will cause a burden, which should be called a substance that may affect the health of the body. The GAC of F-HPIDF (Figure 8a) portrayed a better trend (0.23 g/g), suggesting that F-HPIDF may prevent glucose from being over absorbed in the intestine. The GAC of HPIDF is the result of multiple adsorption mechanisms [46]. Therefore, the higher available surface, smaller particle size, and more functional group exposure of F-HPIDF lead to better glucose adsorption than HPIDF. As one of the main places for the body to absorb glucose, the intestine could be prevented from absorbing due to the presence of F-HPIDF, especially the glucose produced by the decomposition of digestive juice.

As shown in Figure 8b, F-HPIDF (14.80 mg/g) exhibited a high cholesterol-adsorption capacity than HPIDF (5.58 mg/g). After F-HPIDF was mixed with cholesterol, the absorption in the colon of cholesterol could be controlled by reducing the solubility of cholesterol [47]. This mode of action was similar to GAC.

The SCAC of HPIDF/F-HPIDF is shown in Figure 8c. F-HPIDF (0.49 g/g) exhibited a small increase in SCAC than HPIDF (0.47 g/g). As a substance produced by bile acids, sodium cholate may lead to intestinal inflammation and even apoptosis [48,49]. A good sodium cholate-adsorption capacity may be a potentially beneficial effect on intestine health. Compared with our previous research [16], the sodium cholate-adsorption capacity in this experiment had a great improvement. We speculated that the reason may be the difference

between the adsorption time. In the early stage, when we carried out an adsorption test in the digestive solution, 4 h was used for adsorption, which was much shorter than 14 h. In this experiment, to distinguish the differences in adsorption capacity before and after fermentation, we selected a long time to simulate the retention of dietary fiber in the intestine [50]. This showed that the adsorption of sodium cholate was still going on after 4 h. Then, the cholate adsorption kinetics of F-HPIDF may be also an object worthy of further study.

The acrylamide-adsorption capacity of F-HPIDF (0.20 mmol/g) showed a little decrease than HPIDF (Figure 8d). We had emphasized that the AAC of okara-HPIDF did not change with the gastroenteric environments. Therefore, AAC should only be related to the structure of the dietary fiber. The acrylamide-adsorption capacity on plant DF is generally weak, meanwhile mainly depended on physical adsorption [51]. Then we speculated that the decrease of AAC of F-HPIDF still came from the degradation of hemicellulose, although the specific surface area of F-HPIDF increased with the progress of fermentation. Hemicellulose was used as an adsorption material or matrix of acrylamide in previous studies [52,53].

In summary, the adsorption capacity of HPIDF to potentially harmful substances changed significantly after fermentation by colonic flora. The differences of F-HPIDF and HPIDF in properties were due to the changes in their structure during fermentation in vivo. Therefore, this process depended on the changes caused by the specific metabolic mechanism of colonic microorganisms.

#### **4. Conclusions**

In this study, the changes in the structure (SEM, FT-IR, XRD, particle size, specific area, and monosaccharide composition) and the adsorption capacity (WHC, WSC, OHC, heavy metal-adsorption capacity, and harmful substances-adsorption capacity) were analyzed to measure the changes between F-HPIDF and HPIDF. The results suggested that after being fermented by colonic microorganisms, the structure and properties of okara-HPIDF changed greatly. These findings provide a more accurate analysis of HPIDF after entering the digestive system. Meanwhile, the excellent adsorption and physicochemical properties caused by structural changes are beneficial to colon health. The intake of HPIDF might increase the number of beneficial monosaccharides in the colon, which might improve the composition of SCFA and have a beneficial impact on the body. This study also speculated that HPIDF may be modified to enhance its biological activity after its fermentation mode had been identified. The above all proved that HPIDF may have more utilization value.

**Author Contributions:** B.L.: conceptualization, software, writing—original draft; Y.W.: visualization, software, writing—original draft; X.Z.: writing—review & editing; Y.C.: methodology; H.F.: investigation; T.L.: methodology; J.H.: visualization; Y.L.: investigation; H.Y.: funding acquisition, project administration; L.J.: funding acquisition, project administration. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by China Agriculture Research System of MOF and MARA, grant number CARS-04; The Excellent Youth Project of Natural Science Foundation of Heilongjiang Province, grant number YQ2021C023; Major Science and Technology Innovation Projects in Shandong Province, grant number 2019JZZY010722.

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Ethics Committee of Laboratory Animal Welfare and Ethics Committee of Northeast Agricultural University.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Raw data can be provided by the corresponding author on request.

**Acknowledgments:** The authors acknowledge Shanghai Personal Biotechnology Co., Ltd. for their support to the 16S rDNA Sequencing.

**Conflicts of Interest:** No conflict of interest.
