3.2.3. Analysis of Adsorption Properties Nitrite

Nitrite is a relatively toxic compound and can form carcinogenic compounds during human digestion [28]. Some studies have shown that nitro compounds with carcinogenicity to animals can also enter the fetus through the placenta and have teratogenic effects on the fetus [44,45]. As an important indicator, pH has a great influence on the ability of dietary fiber to adsorb nitrite ions. Therefore, the adsorption capacity of each pea fiber sample for nitrite ions under simulated gastric environment (pH = 2) and intestinal environment (pH = 7). Figure 6 shows that the nitrite adsorption capacity of pea fiber is affected by pH value and γ-irradiation dose. When pH = 2, the nitrite adsorption capacity was much higher than that of pH = 7. This may be because, in acidic conditions, NO2 <sup>−</sup> reacts with H<sup>+</sup> to produce HNO2 and then forms nitrogen oxides, including the strong electron affinity compound N2O3, which can combine with the negatively charged oxygen atoms of phenolic acid groups in dietary fiber and cause adsorption [46]. Another explanation is that the active groups such as uronic acid, amino acid, and especially phenolic acid contained in the structure of dietary fiber have a strong adsorption effect on nitroso groups under acidic conditions; this is why the nitrite ion adsorption capacity of pea fiber sample at pH = 2 is significantly higher than that at pH = 7 [47]. With the increase in irradiation dose, the nitrite adsorption capacity of pea fiber increases first and then decreases at two pH values. When the irradiation dose is 2 kGy, the nitrite adsorption capacity of pea fiber was the highest. The γ-irradiation changed the structure of pea fibers, the surface appeared loose and porous, and the specific surface area became larger, which exposed more adsorption sites for nitrite ions to a certain extent. The capillary effect formed by these pores accelerates the entry of nitrite ions into the interior of the fiber structure, so the adsorption capacity of pea fibers for nitrite ions after moderate irradiation treatment is improved in the gastrointestinal environment.

**Figure 6.** Effect of γ-irradiation on nitrite adsorption of pea fiber. Note: Different uppercase letters indicate significant difference in nitrite adsorption capacity under simulated gastric environment (pH = 2) (*p* < 0.05), and different lowercase letters indicate significant difference in nitrite adsorption capacity under simulated intestinal environment (pH = 7) (*p* < 0.05).

#### Cholesterol

The occurrence of coronary heart disease is directly related to the content of cholesterol in the blood. There is evidence that dietary fibers reduce the risk of cardiovascular disease and serum cholesterol levels through the absorption of cholesterol [48,49]. Thus, it is of great significance to improve the cholesterol adsorption capacity of dietary fiber. According to Figure 7, the cholesterol adsorption capacity of pea fiber was affected by pH value and γ-irradiation dose. With the increase in γ-irradiation dose, the cholesterol adsorption capacity of pea fiber increased first and then decreased at both pH values. When the γ-irradiation dose was 2 kGy, the cholesterol adsorption capacity of pH = 2 and pH = 7 was the largest, and the cholesterol adsorption capacity of pH = 7 was greater than that of pH = 2. This may be due to the repulsion between H+ and some positive charges in dietary fiber and cholesterol under acidic conditions, which affects the binding of dietary fiber and cholesterol, thus reducing the adsorption amount of cholesterol so that the absorption of cholesterol by pea fiber in intestinal environment is more effective than that in stomach environment. When the irradiation dose was lower than 2 kGy, the γ-irradiation increases the specific surface area of the insoluble dietary fiber of pea dregs and changes the structure of hemicellulose and lignin to enhance the capillary action, thereby increasing the adsorption capacity of cholesterol [18,47,50]. However, when the irradiation dose is too high, the surface pores and surface area of pea fiber decrease, which weakens the effect of capillaries, thus weakening the adsorption effect of pea fiber on cholesterol and reducing the adsorption amount of cholesterol [51].

**Figure 7.** Effect of γ-irradiation on cholesterol adsorption of pea fiber. Note: Different uppercase letters indicate significant difference in cholesterol adsorption capacity under simulated gastric environment (pH = 2) (*p* < 0.05), and different lowercase letters indicate significant difference in cholesterol adsorption capacity under simulated intestinal environment (pH = 7) (*p* < 0.05).

#### Glucose

The absorption of glucose by dietary fiber can delay or reduce the digestion and absorption of glucose in the gastrointestinal tract, thus playing a role in reducing blood glucose, which is also an important functional characteristic of dietary fiber [12]. It can be seen from Figure 8 that the adsorption capacity of pea fiber on glucose is affected by glucose concentration and γ-irradiation dose. With the increase in γ-irradiation dose, the adsorption capacity of pea fiber to different concentrations of glucose first increased and then decreased. When the dose of γ-irradiation was 2 kGy, the adsorption capacity of pea fibers to different concentrations of glucose was the highest, which indicated that γ-irradiation could improve the glucose adsorption capacity of pea fibers, possibly due to the increase in specific surface area and porosity caused by moderate irradiation, making it easier for glucose to enter the interior of the fiber and bind more tightly to the interior of the dietary fiber. Studies have shown that the increased hydration of dietary fiber makes it easier for glucose to bond to the fiber's surface [52]; this is confirmed by the findings in Section 3.2.2 of this paper. Furthermore, the adsorption capacity positively correlated with glucose concentration. This may be due to the increased contact probability between glucose and fiber, which improves the adsorption capacity of pea fiber. However, when the irradiation dose was too high, the damage to polar and non-polar groups in pea fiber structure weakens the interaction with glucose molecules [53], thus reducing the adsorption amount of glucose.

**Figure 8.** Effect of γ-irradiation on glucose adsorption of pea fiber. Note: Different small letters at the same concentration indicate significant differences in glucose adsorption capacity (*p* < 0.05).

#### **4. Conclusions**

Pea fiber was irradiated with different doses (0, 0.5, 1, 2, 3, and 5 kGy) by γ-irradiation technology to investigate the effects of irradiation dose on the structure and functional characteristics of pea fiber. According to the structural characteristics of pea fiber, when the γ-irradiation dose was 2 kGy, the contents of cellulose, hemicellulose and lignin in pea fiber decreased by 1.34 ± 0.42%, 2.56 ± 0.03% and 2.02 ± 0.05%, respectively, and the crystallinity of pea fiber decreased by 7.65%. The pore and irregular particles appeared on the microstructure surface of pea fiber treated by γ-irradiation. The results of infrared spectroscopy showed that the hemicellulose and lignin in pea fiber were destroyed by γ-irradiation. The results of the functional characteristics of pea fiber showed that, when the γ-irradiation dose was 2 kGy, the highest oil-holding capacity, swelling capacity and waterholding capacity of pea fiber were 8.12 ± 0.12 g/g, 19.75 ± 0.37 mL/g and 8.35 ± 0.18 g/g, respectively. Additionally, the adsorption capacity of sodium nitre, cholesterol and glucose

were also the strongest in these conditions. These results suggest that the functional properties of pea fiber can be improved by γ-irradiation, changing the structural properties of pea fiber. In this study, γ-irradiation technology was used as pretreatment to provide a theoretical basis for the application of pea fiber in food processing.

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

**Funding:** This research was funded by Major industrial key projects for the transformation of new and old kinetic energy in Shandong Province; Key R&D plan of Shandong Province (major sci-entific and technological innovation project), grant number 2022CXGC010603; Major scientific and technological special projects of "millions" project in Heilongjiang Province, grant number 2021ZX12B02; Heilongjiang Postdoctoral Scientific Research Developmental Fund, grant number LBH-Q20008.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data is contained within the article.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

