Résumé:

Challenges of public health and sustainable development require replacing in food producs animal proteins by plant proteins. In this optics, it is crucial to understand the structure and kinetic of formation of a film of plant proteins film in order to improve the control of emulsions and foams stabilized by these proteins. In this talk I will present experimental results on the interfacial properties of wheat gluten and sunflower proteins. Thanks to a combination of tensiometry, viscoelasticity and ellipsometry, a consistent and rational physical picture of the dynamics of the interfacial properties is achieved. For gluten proteins, a time-concentration superposition of the data is evidenced whatever the subphase concentration, which reveals that protein adsorption at the interface is dominated by bulk diffusion. We propose a consistent physical picture of the multistep diffusion-controlled irreversible adsorption of the gliadin proteins at an air/water interface [1]. Our data shows clearly three different regimes for the film formation (Figure 1). In the first regime, the film elasticity and surface protein concentration increase concomitantly due to the progressive coverage of the interface by proteins. In the second regime, an increase of the dissipative viscoelastic properties is associated to an anomalous evolution of the optical profile that we attribute to conformational changes of the proteins at the interface induced by surface pressure. In the last stage, at even higher surface pressure, the optical profile is not modified while the elasticity of the interfacial layer dramatically increases presumably due to the film gelation as the result of the formation of intermolecular bonds. Overall all our experimental results indicate that wheat gluten displays behaviour typical of soft proteins due to their structural flexibility. Sunflower proteins, by contrast, can be considered as hard proteins, and as such, do not reorganize once adsorbed at an interface and display a simpler film formation dynamics. In addition the failure at high concentration of the time-concentration superposition of the tensiometry and viscoelastic data strongly suggest an aggregation process.