In this study, egg white protein (EWP) was covalently coupled with rosmarinic acid (RA) by a free radical method, and the coupling effect was verified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and RA binding equivalent. Fourier transform infrared spectroscopy (FTIR), circular dichroism (CD) spectroscopy, fluorescence and ultraviolet absorption spectroscopy were used to analyze the structural changes. The antioxidant capacity, antigenicity, and functional properties of EWP-RA complexes were evaluated through 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical, 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical cation scavenging assays, enzyme-linked immunosorbent assay (ELISA), and emulsification performance measurements. The results indicated that EWP-RA complexes underwent significant structural changes compared with EWP, resulting in altered antioxidant capacity and emulsifying ability. The EWP-0.2% RA complex scavenged 58.59% of DPPH radical and 72.08% of ABTS radical cation. The binding affinity of EWP to immunoglobulin (Ig) G and IgE antibodies in egg allergic patients was significantly reduced after conjugation with RA, indicating a decreased potential for antigen binding. The emulsifying activity index and emulsion stability index of the EWP-0.2% RA complex were 1.21 m²/g and 64.20 min, respectively. RA modification effectively enhanced the emulsifying capacity and emulsion stability of EWP. This study provides a theoretical basis for the covalent modification of EWP with polyphenols using the free radical method.

The present study aimed to investigate the effect of covalent coupling with the dietary polyphenol rosmarinic acid (RA) on the structure and properties of β-lactoglobulin (β-LG). β-LG-RA complexes were prepared using the free radical and alkaline methods. The structural changes of β-LG after coupling with RA were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), Fourier transform infrared (FTIR) spectroscopy, ultraviolet (UV) absorption spectroscopy, fluorescence spectroscopy, and circular dichroism (CD) spectroscopy. The antioxidant activity of β-LG and its complexes with RA was assessed using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical and 2,2’-azinobis(3-ethylbenzothiazoline-6-sulfonic acid (ABTS) radical cation scavenging methods. Furthermore, their binding capacity to serum-specific immunoglobulin G (IgG) were evaluated using enzyme-linked immunosorbent assay (ELISA). The results revealed that the secondary structure of β-LG transformed from α-helix to β-sheet and random coil after RA covalent conjugation. The protein underwent changes in its secondary and tertiary structures. Additionally, compared to β-LG, the β-LG-RA covalent conjugates showed significantly enhanced antioxidant activity and reduced binding ability with serum specific IgG. In conclusion, covalent coupling with RA led to a significant improvement in the antioxidant activity of β-LG and a significant decrease in its binding capacity to serum-specific IgG by introducing phenolic hydroxyl groups.