*3.1. Analysis of the Effect of Electron Beam γ-Irradiation on the Physicochemical Properties of Pea Fiber* 3.1.1. Analysis of Content of Main Components

According to Table 1, compared with the untreated sample, the contents of cellulose, hemicellulose and lignin in pea fiber significantly decreased (*p* < 0.05) following γ-irradiation. The contents of cellulose, hemicellulose and lignin significantly decreased (*p* < 0.05) with the increase in γ-irradiation dose. This may be a lignin with strong covalent bonds, hydrogen bonds and hemicellulose to form a stable compound composed of cellulose and wrapped up, but it is difficult to predict for certain in outside conditions decomposition [22]. Additionally, γ-irradiation of strong energy rays can promote the depolymerization of hemicellulose and cellulose; lignin association cracking occurred, and the part of dietary fiber component was transformed into oligosaccharides; then, the composition of insoluble dietary fiber in pea dregs changed [23]. The energy absorbed in the irradiation process destroyed the monomeric units of hemicellulose and lignin, cracking into small molecular components or other free radicals, thereby reducing their content [11]. With the increase in γ-irradiation dose, the structure of pea fiber was opened, the exposed cellulose was destroyed and degraded, and the content of pea fiber decreased [24]. The results showed that the binding sites and structures of cellulose, hemicellulose and lignin in pea fiber were destroyed by γ-irradiation, which changed the content of pea fiber.

**Table 1.** Effects of γ-irradiation on the content of main components of pea fiber.


Note: Different lowercase letters in the same column indicate significant difference (*p* < 0.05)—same below.

#### 3.1.2. Analysis of Particle Size and Specific Surface Area

According to Table 2, compared with the untreated sample, with the increase in γirradiation dose, the average particle size of pea fiber volume decreased significantly, and the specific surface area increased significantly (*p* < 0.05). Combined with the results in Section 3.1.1, γ-irradiation causes the depolymerization of lignin and hemicellulose in pea fibers by breaking molecular chains, reducing the average particle size and enhancing the specific surface area. Guo [11] and Al-Sheraji et al. [25] also showed that irradiation would destroy the glycosidic bonds between cellulose and reduce cellulose, resulting in a decrease in the particle size of dietary fiber and an increase in the specific surface area of insoluble dietary fiber. This indicates that γ-irradiation can affect the structure and physicochemical properties of pea fibers by breaking glycosidic bonds and changing fiber components.

**Table 2.** Effects of γ-irradiation on particle size and specific surface area of okra pea fiber.


#### 3.1.3. Analysis of SEM

According to Figure 1, the surface microstructure of untreated samples shows fiber strip structure, and cracks and pores appear in samples treated by γ-irradiation. When the dose of γ-irradiation increased from 0 to 2 kGy, the pore size of the samples gradually increased. When the irradiation dose was 2 kGy, the largest pores appeared in the fiber structure of the sample. When the irradiation dose increased from 2 kGy to 5 kGy, the pore size of the sample showed a downward trend, and when the irradiation dose reached 5 kGy, the surface of pea fiber breaks and forms a lamellar microstructure. This is consistent with the specific surface area results in Section 3.1.2. This may be due to the fact that the increase in γ-irradiation intensity can break the glycosidic bond in the molecular chain, reduce the molecular weight and weaken the interaction between molecules, promoting the formation of carbonyl and double bonds, and leading to the relaxation of the pea fiber's structure, which demonstrated a honeycomb structure and had larger pores [26,27]. The decrease in particle size and changes in the microstructure of pea fibers may generate capillary action and form a larger specific surface area, which may be important for its absorption capacities with some other compounds [17]. Jiang et al. [28] also demonstrated that high-intensity energy field treatment can change the surface structure and affect the specific surface area by destroying the intermolecular crosslinking of fibers. When the irradiation dose is too high, layered substances appear on the surface of pea fiber. This may be due to the accumulation of residual protein on the surface of the fiber and degraded fiber fragments caused by excessive dose irradiation. The research shows that the hydration properties and glucose absorption capacity of dietary fiber are mainly related to the porosity of the fiber structure [29,30]. This indicates that the appropriate dose γ-irradiation treatment can improve the functional activity by adjusting the microstructure and porosity of pea fiber.

**Figure 1.** Scanning electron microscope (SEM) images of pea fiber after γ-irradiation. Note: (**1**) as the control, the irradiation doses of (**2**)–(**6**) were 0.5, 1, 2, 3, and 5 kGy, respectively, and the magnification was 6000×. Note: The red circles represent cracks, pores, and layered structures present in the γ-irradiation treatment samples.

#### 3.1.4. Analysis of Fourier Transform Infrared Spectroscopy

Fourier transform infrared spectroscopy is an infrared absorption spectrum formed according to the vibration of molecules at different wavelengths, which is used to detect changes in molecular groups and chemical bonds of pea fibers. The position and strength of the absorption peak are mainly affected by the types of chemical bonds or molecular groups, and the position of the absorption peak changes with induction, conjugation or steric hindrance [31]. According to Figure 2, all the pea fiber samples showed similar spectral curves while retaining characteristic bands specific to each procedure. The absorption bands of all pea fibers in the range of 3000–3700 cm−<sup>1</sup> are due to O-H bond tensile vibration, and these absorption bands also indicate the presence of pectin and hemicellulose in soybean residue fibers [32]. The absorption peaks at 2853 cm−<sup>1</sup> and 2925 cm−<sup>1</sup> were attributed to the asymmetric and symmetrical C-H vibrational bands in the polysaccharide compound methylene [33]. The absorption peak was observed at 1235 cm<sup>−</sup>1, indicating the presence of a crystalline region. With the increase in γ-irradiation dose, the intensity of these absorption bands weakened, probably due to the destruction of intramolecular hydrogen bonds in cellulose and hemicellulose compared with untreated samples [34]. The intensity of the absorption peak at 1725 cm−<sup>1</sup> weakened, indicating that the adsorption of water on the fiber matrix became weak [35]. The decrease in absorption bands near 1000 cm−<sup>1</sup> was due to the stretching of C=O and aromatic skeleton of aldehyde/ester groups of hemicellulose and lignin, indicating that the hemicellulose and lignin in pea fiber were destroyed by γ-irradiation [36]. The reactive groups play an important role in the physicochemical and functional properties of dietary fibers, such as hydration, adsorption, cation exchange capacity, and metal chelation [37]. This indicates that γ-irradiation treatment can modulate physicochemical and functional properties by altering the reactive groups of pea fibers.

**Figure 2.** Effect of γ-irradiation on infrared spectrum of pea fiber.

#### 3.1.5. Analysis of X-ray Diffraction

The effect of γ-irradiation on the crystalline properties of pea fiber was studied by X-ray diffraction. The overall peak shape represents the crystal type of the sample. The increase in diffraction peak intensity indicates that the crystallinity at the diffraction angle increases. It can be seen from Figure 3 that the irradiation treatment does not cause the peak shape to change, and each sample has obvious absorption peaks at the scanning angles (2θ) of 19.96◦ and 34.56◦ (except 2 kGy, which is 35.05◦). This indicates that the crystals of the six groups of fiber samples are all of the type I cellulose type, which consists of ordered crystalline cellulose regions and disordered cellulose and hemicellulose regions, and irradiation treatment does not change the cellulose type of the samples. According to Figures 2 and 3, the highest crystallinity of untreated sample is 34.22%. With the increase in irradiation dose, the crystallinity of pea fiber decreased first and then increased. When the irradiation dose was 2 kGy, the crystallinity was the lowest with a value of 26.57%, which might be because the crystalline form and the amorphous form of pea fiber were destroyed by γ-irradiation, and the destroyed crystalline form was decomposed into the amorphous form, thus reducing the crystallinity of pea fiber [9]. When the irradiation dose was greater than 2 kGy, the γ-irradiation destroyed the monomer units of hemicellulose and lignin in the amorphous region, but the structural damage degree of cellulose monomer units in the crystallization region is small, so the destruction degree of the amorphous region is greater than that of the crystallization region, resulting in the increase in crystallinity [38]. The results showed that γ-irradiation changed the crystal structure of pea fibers by changing the contents of cellulose, hemicellulose and lignin in pea fibers.

**Figure 3.** Effects of γ-irradiation on X-ray diffraction of pea fiber.
