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How the physicochemical and structural properties of globular proteins affect their behavior at the air-solution interface and their foaming properties
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We have been studying the behavior of several globular food proteins at the air-solution interface and their capacity to stabilize aqueous foams. Our aim is to understand the relation of foaming ability to the properties of the adsorbed protein layer, and to understand the relation of the properties of the adsorbed protein layer to the structural and physicochemical characteristics of the adsorbed protein(s). We used a combination of techniques providing insight in the adsorption kinetics (ellipsometry, surface tension), in the interfacial shear rheology and in the molecular conformational rearrangements of adsorbed proteins (polarization-modulation infrared reflection-absorption spectroscopy, PM-IRRAS). We compared proteins with distinctive molecular features, native and chemically or physically modified forms of one protein, and studied the interfacial behavior of proteins in binary solutions. We found that depending on the physicochemical adsorption conditions, different proteins form qualitatively different interfacial layer, as regards monolayer or multilayer adsorption, or the interfacial shear rheology. Strikingly, very minor structural modifications prior to adsorption to the air-solution interface could lead to dramatic changes in the interfacial behavior, in parallel to dramatic changes in the foaming ability. We also showed that in the case of binary solutions of oppositely charged proteins, a clear co-adsorption occurs, leading to very high surface concentrations and to deeply modified interfacial film properties, which cannot be extrapolated from the addition of individual behaviors. In addition, preliminary results about the osmotic pressure of bulk, very highly concentrated protein solutions suggest that in interfacial studies, due to the protein crowding close to the interface, intermolecular interactions could be of prominent importance in the understanding of interfacial properties.
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