Development of pH-responsive emulsions stabilized by whey protein fibrils

Globular proteins can form fibrillar structures, for example, by heating them above their thermal denaturation temperature under suitable pH (usually highly acidic) and ionic strength (usually relatively low) conditions. These fibrils can then be used as functional ingredients in foods. In this study, we investigated the potential of whey protein isolate fibrils (WPIFs) to act as stabilizers to create pH-responsive emulsions. Initially, changes in the properties of the WPIFs were measured when the system was changed from pH 2.5 to 10.0 and then back to pH 2.5. WPIF-based emulsions were then prepared and subjected to the same pH regime. For comparison, WPI-emulsions stabilized by the molecular form of native whey protein were subjected to the same treatment. The pH-dependence of the properties of the WPIF-emulsions was different from that of the WPI-ones. Insights into the molecular origin of these effects were obtained by measuring the interfacial properties, circular dichroism spectra, and surface hydrophobicities of the WPIFs and WPI. These results indicated the relative importance of electrostatic, hydrogen bonding, and hydrophobic interactions at the oil-water interface and/or continuous phase in stabilizing the emulsions. WPIF-emulsions were more resistant to coalescence than WPI-emulsions, which was attributed to the formation of a more robust interfacial coating around the oil droplets and a strong viscoelastic network in the aqueous phase. This research shows that the functional performance of food proteins can be extended by creating fibril structures.