Amélie Banc

The study delves into the adsorption of sunflower proteins at the air/water interface using specular X-ray reflection. The research involved fitting models of the protein films to the reflectivity data, resulting in detailed images of the X-ray scattering length density profiles perpendicular to the air/water interface. The sunflower protein isolate that is examined consists of multiple components, and the study proposes a transition from a 1-slab model to a 4-slab model to represent the changing layer structure over time. This transition is significant as it reflects the increasing complexity of the protein film as more proteins adsorb at the interface. Initially, sunflower proteins form a monolayer at the air/water boundary, consisting of a protein-rich, hydrophobic portion closest to the interface and a more diffuse, hydrophilic portion extending into the bulk aqueous phase. The structural changes at the interface over time depend on the bulk protein concentration in the solution. For solutions at relatively low concentrations (C <= 0.5 g/L), a lower amount of adsorption results in a larger, more extensive interface area for each species and a thinner protein adsorption layer. The overall thickness of a saturated monolayer is approximately 100 & Aring;, which is close to the maximum dimension of sunflower globulins, with the thickness of the corresponding hydrophobic portion being about 20 & Aring;. For solutions at relatively high concentrations (C >= 1.0 g/L), even after forming a saturated monolayer, structural evolution continues within the experimental time frame, occurring on both hydrophilic and hydrophobic sides. Additional proteins from the bulk diffuse toward the interface, forming an extra layer in the water phase and causing an increase in the overall thickness. Furthermore, a distinct sublayer develops next to the air phase, indicating a further structuration of the hydrophobic portion.

We study model near-critical polymer gelling systems made of gluten protein dispersions stabilized at different distances from the gel point. We impose different shear rates and follow the time evolution of the stress. For sufficiently large shear rates, an intermediate stress overshoot is measured before reaching the steady state. We evidence self-similarity of the stress overshoot as a function of the applied shear rate for samples with various distances from the gel point, which is related to the elastic energy stored by the samples, as for dense systems close to the jamming transition. In concordance with the findings for glassy and jammed systems, we also measure that the stress after flow cessation decreases as a power law with time, with a characteristic relaxation time that depends on the shear rate previously imposed. These features revealed in nonlinear rheology could be the signature of a mesoscopic dynamics, which would depend on the extent of gelation.

We study the efficiency of several asymmetrical flow field-flow fractionation (AF4) techniques to investigate self-associating wheat gluten proteins. We compare the use of a denaturing buffer including sodium dodecyl sulfate (SDS) and a mild chaotropic solvent, water/ethanol, as the eluent, on a model gluten sample. Through a thorough analysis of the data obtained from coupled light scattering detectors and with the identification of molecular composition of the eluted protein, we evidence coelution events in several conditions. We show that the focus step used in conventional AF4 with the SDS buffer leads to the formation of aggregates that coelute with monomeric proteins. By contrast, a frit-inlet device enables the fractionation of individual wheat proteins in the SDS buffer. Interestingly conventional AF4, using water/ethanol as eluent, is an effective method for fractionating gluten proteins and their complex dynamic assemblies, which involve weak forces and are composed of both monomeric and polymeric proteins.

Understanding the origin of the unique rheological properties of wheat gluten, the protein fraction of wheat grain, is crucial in bread-making processes and has raised questions of scientists for decades. Gluten is a complex mixture of two families of proteins, monomeric gliadins and polymeric glutenins. To better understand the respective role of the different classes of proteins in the supramolecular structure of gluten and its link to the material properties, we investigate here concentrated dispersions of gluten proteins in water with a fixed total protein concentration but variable composition in gliadin and glutenin. Linear viscoelasticity measurements show a gradual increase in the viscosity of the samples as the glutenin mass content increases from 7 to 66%. While the gliadin-rich samples are microphase-separated viscous fluids, homogeneous and transparent pre-gel and gels are obtained with the replacement of gliadin by glutenin. To unravel the flow properties of the gluten samples, we perform shear startup experiments at different shear-rates. In accordance with the linear viscoelastic signature, three classes of behavior are evidenced depending on the protein composition. As samples get depleted in gliadin and enriched in glutenin, distinctive features are measured: (i) viscosity undershoot suggesting droplet elongation for microphase-separated dispersions, (ii) stress overshoot and partial structural relaxation for near-critical pre-gels, and (iii) strain hardening and flow instabilities of gels. We discuss the experimental results by analogy with the behavior of model systems, including viscoelastic emulsions, branched polymer melts, and critical gels, and provide a consistent physical picture of the supramolecular features of the three classes of protein dispersions.

The binary phase diagram of γ-gliadin, a wheat storage protein, in water was explored thanks to the microevaporator, an original PDMS microfluidic device. This protein, usually qualified as insoluble in aqueous environments, displayed a partial solubility in water. Two liquid phases, a very dilute and a dense phase, were identified after a few hours of accumulation time in the microevaporator. This liquid–liquid phase separation (LLPS) was further characterized through in situ micro-Raman spectroscopy of the dilute and dense protein phases. Micro-Raman spectroscopy showed a specific orientation of phenylalanine residues perpendicular to the PDMS surfaces only for the diluted phase. This orientation was ascribed to the protein adsorption at interfaces, which would act as nuclei for the growth of dense phase in bulk. This study, thanks to the use of both aqueous solvent and a microevaporator, would provide some evidence for a possible physicochemical origin of the gliadin assembly in the endoplasmic reticulum of albumen cells, leading to the formation of dense phases called protein bodies. The microfluidic tool could be used also in food science to probe protein–protein interactions in order to build up phase diagrams.

The adsorption of a sunflower protein extract at two air–water and oil–water interfaces is investigated using tensiometry, dilational viscoelasticity, and ellipsometry. For both interfaces, a three step mechanism was evidenced thanks to master curve representations of the data taken at different aging times and protein concentrations. At short times, a diffusion limited adsorption of proteins at interfaces is demonstrated. First, a two-dimensional protein film is formed with a partition of the polypeptide chains in the two phases that depends strongly on the nature of the hydrophobic phase: most of the film is in the aqueous phase at the air–water interface, while it is mostly in the organic phase at the oil–water interface. Then a three-dimensional saturated monolayer of proteins is formed. At short times, adsorption mechanisms are analogous to those found with typical globular proteins, while strong divergences are observed at longer adsorption times. Following the saturation step, a thick layer expands in the aqueous phase and appears associated with the release of large objects in the bulk. The kinetic evolution of this second layer is compatible with a diffusion limited adsorption of the minor population of polymeric complexes with hydrodynamic radius R_H ∼ 80 nm, evidenced in equilibrium with hexameric globulins (R_H ∼ 6 nm) in solution. These complexes could result from the presence of residual polyphenols in the extract and raise the question of the role of these compounds in the interfacial properties of plant protein extracts.

We investigate the structure of gluten polymer-like gels in a binary mixture of water/ethanol, 50/50 v/v, a good solvent for gluten proteins. Gluten comprises two main families of proteins, monomeric gliadins and polymer glutenins. In the semi-dilute regime, scattering experiments highlight two classes of behavior, akin to standard polymer solution and polymer gel, depending on the protein composition. We demonstrate that these two classes are encoded in the structural features of the proteins in very dilute solution, and are correlated with the presence of proteins assemblies of typical size tens of nanometers. The assemblies only exist when the protein mixture is sufficiently enriched in glutenins. They are found directly associated to the presence in the gel of domains enriched in non-exchangeable H-bonds and of size comparable to that of the protein assemblies. The domains are probed in neutron scattering experiments thanks to their unique contrast. We show that the sample visco-elasticity is also directly correlated to the quantity of domains enriched in H-bonds, showing the key role of H-bonds in ruling the visco-elasticity of polymer gluten gels.

We investigate by time-resolved synchrotron ultra-small X-ray scattering the dynamics of liquid–liquid phase-separation (LLPS) of gluten protein suspensions following a temperature quench. Samples at a fixed concentration (237 mg ml⁻¹) but with different protein compositions are investigated. In our experimental conditions, we show that fluid viscoelastic samples depleted in polymeric glutenin phase-separate following a spinodal decomposition process. We quantitatively probe the late stage coarsening that results from a competition between thermodynamics that speeds up the coarsening rate as the quench depth increases and transport that slows down the rate. For even deeper quenches, the even higher viscoelasticity of the continuous phase leads to a “quasi” arrested phase separation. Anomalous phase-separation dynamics is by contrast measured for a gel sample rich in glutenin, due to elastic constraints. This work illustrates the role of viscoelasticity in the dynamics of LLPS in protein dispersions.