Protein unfolding at fluid interfaces and its effect on proteolysis in the stomach

Rational design of food emulsions with potential health benefits requires a fundamental understanding of the physicochemical changes that occur to the emulsion as it passes through the gastrointestinal tract. This study focuses on the effects of in vitro gastric digestion of emulsions prepared from different oils (tetradecane and olive oil) and stabilised with the milk protein beta-lactoglobulin. The results show that the nature of the oil phase determined the degree of degradation of the protein in the stomach. The lower polarity (higher interfacial tension) of tetradecane allowed an increased orientation of the hydrophobic residues towards this phase, enhancing the interaction with the oil and reducing access of pepsin. The influence of unfolding of the interfacial proteins on the extent of gastric digestion was further demonstrated by modifying the interfacial protein network through addition of a surfactant (Tween 20). Novel experimental approaches have been used to quantify protein digestion from complementary perspectives. Physicochemical characterisation was used to measure the breakdown of the emulsions as they experienced simulated stomach conditions. Interfacial rheology has been used to quantify the equivalent degradation of the interfacial layer. Finally, SDS gel electrophoresis has been used to examine the peptides generated during the in vitro gastric digestion process for each system. This combination of techniques has provided a unique characterisation of the degradation process of the protein-stabilised emulsions, due to pepsin digestion under in vitro gastric conditions. The results show that the conformation of the protein at the interface determines the protein hydrolysis profile and may offer a route to more detailed analysis of the conformation changes on interfacial adsorption. These data suggest a possible route to rational control of the digestibility of proteins and rational interfacial engineering of the mechanical properties of the networks. The findings may have important implications in the rational design of emulsions to enhance health benefits, through modifying lipolysis to control fat intake, improving the digestibility of proteins to reduce allergenicity, or in the generation of novel bioactive peptides.