Importance of water flux through protein interfacial films as revealed by neutron reflectometry on lysozyme and ovalbumin layers
Résumé
In food technology, proteins are often used to stabilize dispersed systems through the formation of dense interfacial films (foaming, emulsification, encapsulation). Those films are of particular importance in many products (bread, beer, ice-cream, dressings...) as they partly determine their functionality. Egg white proteins are known for their ability to form interfacial films at air-water interfaces. Among them, lysozyme and ovalbumin were studied by ellipsometry at pH 7, 40mM ionic strength and showed very different behaviors [1]. Indeed, ovalbumin, which was negatively charged (-10e), formed monolayers at the interface, whereas lysozyme, which was positively charged (+8e), formed multilayers. Further studies by ellipsometry on ovalbumin at pH 3.6 showed that in these conditions of positive charge (+31e), this protein formed multilayers. Neutron reflectometry was performed to get information about the concentration profiles inside the interfacial layers generated by lysozyme and ovalbumin, at pH 7 and 3.6, and different ionic strengths. Surprisingly, the results did not show any formation of multilayers for lysozyme at pH 7 or ovalbumin at pH 3.6. The main difference between our ellipsometry and neutrons reflectivity experiments being the presence and absence, respectively, of an evaporation flux, we propose that the first layer is created by hydrophobic interactions with air, but that the supplementary layers require water transfer through the interface as a driving force. Then, after being carried by advection to the first monolayer, the nature of the interactions (repulsive/attractive) between the protein monomers determines the creation –or not– of a multilayer. As the concentrations in interfacial films of proteins can be very high (between 300 and 700 g/L ; i.e., 0.2–0.5 volume fraction), our results suggest that studies, in bulk, of interactions in highly crowded protein solutions is a necessary complement to fully understand the interfacial behaviour.