*2.6. Foaming Properties and Foaming Stability*

With reference to the method of Fekria et al. [24], 1.0% protein solutions were prepared, and the pH was adjusted to 3.0, 5.0, 7.0, 9.0, and 11.0. The protein solutions were stirred at room temperature for 30 min, and specific volumes of the protein solution (V0) were collected and emulsified with the homogeneous shear machine at a rate of 10,000 r/min. The protein solutions were transferred immediately to the tube to measure the foam volume (*V*1). After resting for 30 min, the foam volume (*V*30) was again measured. The foaming capacity (FC) and foaming stability (FS) were as follows:

$$\text{FC}^{\circ} = \frac{V\_1 - V\_0}{V\_0} \times 100\% \,\text{ } \tag{2}$$

$$\text{FS} = \frac{V\_{30}}{V\_1} \times 100\% \,\text{ } \tag{3}$$

#### *2.7. Emulsion Preparation*

A 1.0% protein solution was prepared, and the pH was adjusted to 3.0, 5.0, 7.0, 9.0, and 11.0. The solutions were stirred and dissolved for 30 min and then mixed with soybean oil at a ratio of 9:1 (*v/v*). The mixed proteins were blended with a T18 homogeneous shear machine (ULTRA-TURRAX, IKA, GER) for 1 min at a rate of 9000 rpm, followed by additional emulsification for 1 min at a rate of 15,000 rpm. The emulsions were stored under 4 ◦C.

#### *2.8. Emulsifying Activity Index (EAI) and Emulsifying Stability Index (ESI)*

The EAI and ESI of the proteins were measured via turbidimetry [25]. A 0.5% protein solution was prepared, and the pH was adjusted to 3.0, 5.0, 7.0, 9.0, and 11.0. The solutions were stirred and dissolved for 30 min. The protein solutions (6 mL) were mixed with soybean oil (2 mL) and emulsified with the homogeneous shear machine for 1 min (12,000 rpm). Subsequently, 20 μL of each emulsion was collected from the bottom container and dispersed in 4 mL of 0.1% SDS. The absorbance (*A*0) was measured at a wavelength of 500 nm. After 10 min, 20 μL of the emulsion was again collected from the bottom container and was dispersed in 4 mL of 0.1% SDS. The absorbance (*A*10) was tested at a wavelength of 500 nm. The blank control was 0.1% SDS. The EAI and ESI were calculated as follows:

$$\text{EAI} = \frac{2 \times 2.303 \times A\_0 \times D}{\text{C} \times q \times 10^4} (\text{m}^2/\text{g}) \tag{4}$$

$$\text{ESI} = \frac{A\_0}{A\_0 - A\_{10}} \times \Delta t \text{ (min)}\tag{5}$$

where *D* is the dilution ratio (200), *C* denotes the protein concentration (5 mg/mL), ϕ represents the volume fraction of oil in the emulsion (0.25), L refers to the optical cuvette path length (1 cm), and Δt indicates the measurement interval (10 min).

#### *2.9. Particle Size and Zeta Potential*

The particle size distribution of the emulsion, volume-weighted average particle size (*d*4,3), surface-area-weighted average particle size (*d*3,2), and zeta potential under different pH values were measured with a Malvern Zetasizer Nano ZS (Malvern Panalytical, Malvern, UK). The relative refractive index of the emulsion was 1.107, while the refractive indices of soybean oil and water were 1.472 and 1.330, respectively. The average particle size distribution of the emulsification was characterized by *d*4,3. Further, *d*4,3 and *d*3,2 values were measured using deionized water as the dispersing agent. The values of *d*- 4,3 and *d*- 3,2 were measured using 1.0% SDS as the dispersing agent.
