Tunable self-assemblies of whey protein isolate fibrils for pickering emulsions structure regulation

Protein fibrils are potential emulsifiers with a highly pH-dependent structure. Therefore, the influence of pH on the self-assemblies of whey protein isolate (WPI) fibrils was investigated through turbidity, transmission electron microscopy, and small-angle X-ray scattering tests. Next, the effects of fibril structure on the interfacial profiles and physical properties of stabilized Pickering emulsions were studied. With an increase in pH from 2.0 to 5.0, the cross-sectional radius of WPI fibrils increased from 1.87 ± 0.12 nm to 7.75 ± 0.33 nm, while with a further increase in pH to 7.0, the fibrils decreased to single strand. For all pH conditions, the WPI fibrils effectively absorbed at the oil-water interface. At pH 5.0, the contact angle of fibrils was the maximum (50.6°), and the diffusion rate was the fastest. The d_4,3 values of the WPI fibril-stabilized Pickering emulsions were around 70–80 μm with no significant differences. Moreover, the Pickering emulsions stabilized by fibrils assembled at pH 5.0 showed a predominantly elastic gel-like behavior and high stability. This work clarifies the relationship between the structure of assembled WPI fibrils and their emulsifying properties, and it provides a theoretical basis for developing emulsion-based delivery systems.