Combining surface dilatational rheology and quantitative proteomics as a tool for understanding microstructures of air/water interfaces stabilized by sodium caseinate/tannic acid complex

A combination method of surface dilatational rheology and quantitative proteomics can be used to investigate the adsorbed proteins on the air/water interface which stabilized by sodium caseinate (SC) or sodium caseinate/tannic acid (SC/TA) complex in this work. First, we explored the change in foam volume with time. Results showed that foam stability (FS) increased with the increasing of bulk concentration of TA. The number of actual aggregates in foam lamella which accounts for improved foam stability was well quantified using optical microscopy and CLSM. Besides, the surface dilatational rheological properties indicated that the formation of the SC/TA complex increased the elastic modulus (E-d) of the air/water interface, which is useful for inhibiting collapse. Finally, adsorbed proteins were obtained by freeze-drying method and proteins were labeled and subjected to reversed-phase high-performance liquid chromatography coupled to tandem mass spectrometry (RPLC-ESI-MS/MS) for protein identification and quantification. The results showed that the significant improvement in the foam stability of the system is due to the addition of TA to increase the amount of x-casein adsorbed on the interface, resulting in a more elastic air/water interface. The results presented in this study should provide detailed quantitative information of the interfacial layer, and will facilitate the application of the formed protein/polyphenol complex as functional ingredients in food and non-food systems.