3.4.5. Rheological Properties

The processing and storage stability of an emulsion can be predicted based on the apparent rheological properties of the emulsion [59]. The shearing rheological properties of the protein emulsions under different pH values were analyzed by testing the apparent viscosity (Figure 12). With an increase in the shearing rate, the viscosity of the emulsions declined partially, and obvious shearing dilution was observed. This demonstrates that these emulsions are non-Newtonian fluids, and the flocculent structures of the emulsion droplets were destroyed in the shearing process. When the pH was close to the PI, the emulsions exhibited the highest viscosity and shearing dilution behavior due to the high flocculation of droplets, which is consistent with previous results [60]. For spherical oil droplets wrapped by proteins, the electrostatic repulsion among oil droplets decreases as the pH approaches the PI of the adsorbed protein, thus resulting in droplet clustering [61]. According to Stokes' law, the degree of flocculation of an emulsion is positively related to the viscosity and degree of shearing dilution [62]. This is because the aggregate maintains some continuous phases in the structure, thus increasing the viscosity. Due to the external shearing effect, flocculation structural damage causes shearing dilution. Therefore, the viscosity of the Co emulsion was lower than that of PPI, CPI, and BL emulsions when the pH was higher than the PI (pH 7.0–11.0), indicating a low degree of flocculation of the droplets.

**Figure 12.** Apparent viscosity of protein emulsions under different pH values. (**a**) pH = 3.0; (**b**) pH = 5.0; (**c**) pH = 7.0; (**d**) pH = 9.0; (**e**) pH = 11.0.

#### **4. Conclusions**

The comprehensive analysis shows that co-precipitation changes the subunit composition and hydrophobicity of mixed proteins, and the functional properties of Co and BL under deviating isoelectric points were synergistic. Additionally, the Co prepared by the ISP method exhibited better solubility, foamability, and emulsification than BL comprising a direct blend of isolated proteins, thereby indicating the advantages of the co-precipitation method. Further studies will be performed to investigate the functional properties of Co and protein modification under different temperatures and ion concentrations. This study provides a theoretical basis for further improvement of mixed protein processing methodology.

**Author Contributions:** Conceptualization: C.Z. (Chaohua Zhang); Methodology: H.Z. and C.Z. (Chunxia Zhou); investigation: X.Z. and L.Z.; resources: C.Z. (Chaohua Zhang) and C.Z. (Chunxia Zhou); supervision: C.Z. (Chaohua Zhang); writing—original draft: X.Z.; writing—review and editing: W.C. and C.Z. (Chaohua Zhang) All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Guangdong Higher Education Institutions Processing and Utilization Innovation Team of High Value of Aquatic Products (GDOU2016030503), the Industrial Development Special Fund Project of Dapeng New District, Shenzhen, China (KY20180111), the Science and Technology Innovation Program of Hunan Province (2019TP1028, 2019SK2122, 2019NK4229, S2021GCZDYF0535), and the Open Project of Hunan Provincial Key Laboratory of Soybean Products Processing and Safety Control (DZPJG202009).

**Data Availability Statement:** Research data are not shared.

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

#### **References**

