Molecular structural modification of egg white protein by pH-shifting for improving emulsifying capacity and stability

Abstract

The effect of pH-shifting on the molecular structure and emulsifying properties of egg white protein (EWP) was investigated. EWP was subjected to extreme alkaline pH treatment (11, 11.5, 12, 12.5, and 13) and then adjusted to neutral (pH 7.0). The results showed that the emulsifying activity and emulsion stability of EWP were enhanced under pH-shifting, especially at pH 13. The surface hydrophobicity of EWP13 increased significantly from 306.94 to 11653.67 (p < 0.05), and the free sulfhydryl content of EWP13 increased significantly from 15.106 μmol/g to 121.86 μmol/g (p < 0.05). Also, the size and turbidity of EWP13 were minimal, producing EWP soluble aggregates with an average size of 52.90 nm (p < 0.05). Moreover, circular dichroism (CD) and intrinsic fluorescence spectra showed that the secondary and tertiary structures changed. At EWP12.5 and EWP13, it showed a decrease of α-helix and an increase of β-sheet. The intrinsic fluorescence spectra suggested that Phe, Tyr, and Trp were exposed to a polar environment due to EWP unfolding. In conclusion, the EWP structure was changed by pH-shifting, resulting in more exposed hydrophobic groups, thus increasing the emulsion stability. Correspondingly, the emulsion of EWP13 showed the best stability under the conditions of oxidation, heating, and salt than native EWP. This work may provide an effective method and theoretical basis for molecular modification to improve the functional properties of EWP by pH-shifting treatment.