Multiphase food systems consist of complex interfacial layers where surface active molecules complete compete for adsorption and interact with one another affecting the interfacial properties and the behavior of the food systems involved. The present work focuses on the examination of interactions between proteins playing an which play an important role in milk-rich food systems (namely κ-casein and β-lactoglobulin) and oleic acid (an anionic surfactant abundant in food systems) at the oil/water interface. An interesting feature of this system is that the molecules interacting at the interface originate from different phases and do not transfer/dissolve significantly to the other phase. The systems were examined using Electron Paramagnetic Resonance (EPR) spectroscopy and Dynamic Interfacial Tension measurements (dynamic interfacial tension and dynamic interfacial dilatational rheology). This combination of experimental methods provided the dynamics of adsorption at the interface and the mechanical properties of the interface allowing valuable insight on the interactions of the different molecules. The above information was coupled with direct information on fatty acid mobility in the oil bulk phase and indirect information on the degree of relative fatty acid/protein adsorption at the interface by EPR. Overall a synergistic effect of the protein and fatty acid on decreasing interfacial tension of the oil/water interface was evidenced. The fatty acid interacted differently with the random coil κ-casein and globular β-lactoglobulin in terms of relative adsorption and in terms on its effect on mechanical properties. Thermal denaturation of β-lactoglobulin affected the protein's interaction with the fatty acid due to conformational changes and exposure of non-polar sites.
Keywords: Dynamic interfacial tension; EPR; Oil/water interface; Oleic acid; β-lactoglobulin; κ-casein.
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