2.4.3. Determination of Adsorption Properties Nitrite

According to Luo et al. [17], the adsorption capacity of pea fiber to NaNO2 was measured. A 0.1 g sample was added to a 5 mL 20 μg/mL NaNO2 solution; the environments of the small intestine and stomach were simulated at pH = 7.0 and pH = 2.0, respectively. Then, the obtained mixture was left to stand at room temperature for 2 h, centrifuged at 4800 rpm for 10 min, and then the 0.5 mL of supernatant was placed in a glass tube. According to Gan et al. [18], NaNO2 levels in the supernatant were determined using p-aminobenzene sulfonic acid and naphthalenediamide hydrochloride. Deionized water was added to the tube until the volume of the mixture was 2 mL; then, 2 mL p-aminobenzene sulfonic acid (4 μg/mL) and 1 mL naphthalene hydrochloride (2 μg/mL) were added to the mixture. The solution was left in the dark to react for 30 min, at 538 nm. This enabled us to measure the concentration of NaNO2 and obtain the standard curve value (y = 1.9832x + 0.0467, R2 = 0.9981, y is the absorbance value, x is the concentration of NaNO2). The adsorption capacity of pea fiber to NaNO2 (NIAC) was calculated as follows:

$$\text{NIAC}(\mu\text{g/g}) = \frac{\mathbb{C}\_1 - \mathbb{C}\_2}{W} \times \text{V} \tag{5}$$

where *C1* and *C2* are, respectively, the concentration of NaNO2 in the supernatant before and after adsorption (μg/L), W is the weight of pea fiber (g), and V is the volume of NaNO2 solution (mL).

### Cholesterol

Cholesterol was added using the method used by Benitez et al. [19] with slight modifications. Fresh egg yolks were diluted with 9 times the weight of distilled water, then beaten until completely emulsified. Next, 0.5 g of pea fiber was added to 25 mL of the egg yolk emulsion and stirred to combine; then, the solution was shaken at 37 ◦C for 2, 5, 10, 15, 25, 40, 60, 90 and 120 min, respectively. After centrifugation at 4000 rpm for 15 min, the cholesterol content in the supernatant was determined at a wavelength of 550 nm using the phthalaldehyde method and a UV-2700 spectrophotometer (Shimadzu Company, Kyoto, Japan) (Gan et al., 2020). The standard curve was obtained (y = 1.6578x + 0.0254, R2 = 0.9975, y is the absorbance value, x is the cholesterol concentration). The cholesterol adsorption capacity (CAC) of pea fiber was calculated using the following equation:

$$\text{CAC} \left( \text{mg}/\text{g} \right) = \frac{\text{C}\_1 - \text{C}\_2}{\text{W}} \tag{6}$$

where *C1* and *C2* are the weight of cholesterol before and after adsorption (mg), and W is the weight of pea fiber (g).

#### Glucose

According to the method of Ma et al. [20] and Chen et al. [21], the glucose adsorption capacity of pea fiber samples was determined. A 0.5 g sample of pea fiber and 100 mL glucose solutions with concentrations of 10, 50, 100 and 200 mmol/L were prepared; they were then shaken at 37 ◦C for 6 h, centrifuged at 4000 rpm for 15 min, and retained in the supernatant. The glucose content in the supernatant was determined by dinitro salicylate (DNS) chromogenic reagent [18]. Next, 0.5 mL of supernatant was added into a glass tube, wherein deionized water was added until the volume reached 3 mL. This was then mixed with 2 mL dinitrosalicylate (DNS) chromogenic reagent. The mixture was continuously shaken in a water bath at 100 ◦C for 6 min. After the solution was cooled to room temperature, the glucose concentration was measured at 520 nm to generate a standard curve (y = 14674x + 0.0543, R2 = 0.9983, y is the absorbance value, x is the glucose concentration). The calculation formula of glucose adsorption capacity (GAC) of pea fiber is as follows:

$$\text{GAC}(\text{mmol}/\text{g}) = \frac{G\_1 - G\_2}{W} \times V \tag{7}$$

where *G1* and *G2* are glucose concentration before and after adsorption, respectively (mmol/g), W is weight of pea fiber (g), and V is volume of supernatant (mL).

#### *2.5. Statistical Analysis*

Three pea fiber samples were prepared, and all samples were tested for three times in parallel. The data were analyzed by one-way ANOVA with SPSS 22.0 software. The data result was mean ± SD, and the difference was significant with *p* < 0.05. Origin 9.0 software was used for data analysis, fitting and standard curve drawing. The areas of crystalline and amorphous regions of X-ray diffraction were calculated by Peakfit version 4.12 software.

#### **3. Results and Discussion**
