pH-induced egg white protein foaming properties enhancement: Insight into protein structure and quantitative proteomic analysis at protein adsorption layer

The foaming properties of egg white protein (EWP) after pH-induced unfolding (pH 2, pH 4, pH 10 and pH 12) and refolding treatment (pH 2–7, pH 4–7, pH 10–7 and pH 12-7 (adjusting the above pH back to 7)) were studied by structure changes and quantitative proteomic analysis. It was found that all pH treatment improved foaming capacity (FC) and foaming stability (FS) of EWP, especially at pH 2 and pH 12, where EWP was in a molten globule state with unfolded structure (especially at pH 12), high surface hydrophobicity (except for pH 12-treated EWP), more free sulfhydryl groups and up-regulated adsorption proteins. However, adjusting the pH back to 7 limited the foaming properties due to protein refolding. Meanwhile the presence of down-regulated proteins was another reason for relatively poor FC at pH 2–7. Notably, the firmest foam and high FS at pH 12-7 were attributed to the generation of aggregates and the long-term stability of adsorbed proteins. The results of quantitative proteomics implied that lysozyme, ovalbumin and protein TENP were identified as key proteins associated with excellent FC at pH 2, pH 12 and pH 12-7, while β-ovomucin contributed the most to FC at pH 2–7. Moreover, protein TENP and ovomucoid were more relevant to maintaining and reducing FS. Overall, this work elucidated the mechanism of pH treatment to enhance the foaming properties of EWP, which would provide the necessary theoretical support for the quality improvement and product development of EWP powder.