In vitro digestion of interfacial protein structures

It is important to understand how interfacial composition influences the digestion of coated interfaces in order to rationally design emulsion based food products with specific digestion profiles. This study has been designed to investigate the effects of gastrointestinal digestion on protein covered interfaces. In this work, we have used a new apparatus fully designed and assembled at the University of Granada: the OCTOPUS. This new device enables the design of a customised static sequential in vitro digestion process in a single droplet. Physiological conditions of each compartment/step of the digestion process are met through subphase exchange of artificial digestive media, hence mimicking the transit through the gastrointestinal tract. We can measure in situ the evolution of the interfacial tension throughout the whole simulated gastrointestinal transit and the mechanical properties of the interfacial layer (interfacial dilatational modulus) after each digestion stage (mouth, stomach, small intestines). The in vitro digestion model used here focuses on pepsinolysis and lipolysis of two dairy proteins: beta-lactoglobulin (BLG) and beta-casein (BCS) adsorbed at the olive oil-water interface. The results show different susceptibilities of interfacial layers of BLG and BCS to pepsinolysis; while pepsinolysis of adsorbed BLG weakens the interfacial network, pepsinolysis of adsorbed BCS strengthens it as measured by the dilatational moduli. These numbers provide an interfacial scenario for previous findings on emulsification of these proteins, which was found to improve BLG pepsinolysis but somehow protected BCS from pepsinolysis in the stomach. The desorption profiles provide quantification of the extent of lipid digestion in subsequent simulated intestinal fluid containing lipase. The extent of lipid hydrolysis was found to be similar in BLG and BCS covered interfaces and comparable to that in the absence of coverage (pure oil-water interface) indicating that proteins do not comprise a barrier to lipolysis. This similar susceptibility is attributed to the similar interfacial properties of the interfaces reaching the duodenum despite the structural differences between native BCS and BLG, thus demonstrating the impact of the transit through the gut on lipolysis. This research allows identification of the interfacial mechanisms affecting enzymatic hydrolysis of proteins and lipolysis. The results can be exploited in tailoring novel food matrices with improved functional properties such as decreased digestibility, controlled energy intake and low allergenicity.