## 3 Results and Discussion
### Foam Properties  
Foam properties of the whey protein after modification by using oxidized phenolic compound are shown in Figure 4. Interaction between the whey protein and different oxidized phenolic compounds affected the foam properties of the modified whey protein. For foam expansion (FE) (Figure 4a), the whey protein modified with OFA and 1.0-5.0% OTA had poorer FE than the control whey protein (\(p<0.05\)), whereas no difference in FE of the whey protein modified with OGA and 0.5% OTA was observed when compared with the control (\(p<0.05\)). Protein foaming ability is corelated to water solubility and ability to form film at the air-water interface. Generally, proteins that adsorb quickly at the newly formed air-liquid interface during bubbling and undergo molecular rearrangement at the interface have superior foaming ability to proteins that adsorb slowly and resist molecular rearrangement at the interface [23]. Therefore, a decrease in FE of the whey protein modified with OFA and 1.0-5.0% OTA caused a loss in its water solubility. The whey protein modified with OFA had higher surface hydrophobicity, but protein solubility and dispersion in water were still necessary for foam forming. The presence of excess surface hydrophobicity in the whey protein modified with OFA might have caused a loss in foaming ability. Similarly, [PERSON] et al. [24] revealed that incorporation of nonpolar polyphenols to protein increased surface hydrophobicity, but also reduced water solubility. For the whey protein modified with OTA, the foaming ability of the resulting whey protein decreased with increasing concentrations of OTA. Tannic acid with large molecules and high hydroxyl group content can increase the hydrophilicity of the modified whey protein or lead to protein precipitation. The interaction between polyphenol and protein may induce cross-linking of the protein, resulting in decreased water solubility [25]. The whey protein modified with OTA was not able to effectively disperse to the air-water interface.  
For foam stability (Figure 4b), the whey protein modified with oxidized phenolic compounds had increased foam stability when compared with the unmodified whey protein. Foam stability of the modified whey protein increased with an increase of the oxidized phenolic compounds used. At the same level of the phenolic compound used, the whey protein modified with OTA had the highest foam stability, followed by whey protein modified with OGA and OFA, respectively. This result might be associated with the cross-linking of the whey protein after modification. Modified whey protein can form cohesive film at the air-water interface, which resists deformation effectively. [PERSON] and [PERSON] [26] reported that polymerized whey protein had a higher viscosity when  
Figure 4: Foam expansion (**a**) and foam stability (**b**) at 1, 5, 10, and 30 min of whey protein modified with different oxidized phenolic compounds at various concentrations. Different letters in the same concentration of oxidized phenolic compounds indicate significant differences. Different capital letters in the same type of oxidized phenolic compounds and control indicate significant differences.  
compared with native whey protein, resulting in more stabilized foams due to the effective slowdown of drainage. [PERSON] et al. [27] reported that polymerization can improve the foaming properties of egg white protein due to the cross-linked structure enhancing the unfolding of the protein during foam forming and generating more elastic foam networks at the air-water interfaces.