Hard and soft proteins at fluid interfaces

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.

Natural protein sources often display a huge complexity, being composed of a blend of polypeptides of various molecular weight, pHi and charge density. Gluten, the proteins extracted from wheat flour is one of such. Gluten is widely used for its viscoelastic properties as an improver of cereal products (bread, pastry, etc.). It is composed of two classes of proteins, named gliadin and glutenin, similar in their amounts in glutamine (30%) and proline (10%). The more than 25 different polypeptides belonging to the gliadin class are hard to fractionate into individual components because of high redundancy in the primary sequences. Glutenin are in the form of polymers made from several distinct polypeptides concatenated through inter-chain disulfide bonds. Their molecular weights are evenly distributed from 100 kg/mol to 7,000 kg/mol.While it is well established that gliadin confers viscosity to gluten whereas glutenin polymers are at the origin of its elastic resistance, the interactions existing between both classes of gluten protein remain unknown. We previously showed by SLS and multi-angle DLS that gluten proteins suspended in ethanol/water (50/50, v/v), a theta solvent, includes large proteins assemblies (26,000 kg/mol, Rh 100nm) displaying an internal dynamic1. To get a better insight of the composition of those assemblies, we combined biochemical and physicochemical approaches. On the one hand, gluten proteins suspended in ethanol/water were fractionated by Asymmetrical- Flow-Field-Flow Fractionation (A4F) coupled to UV, SLS and QELS detectors. On the other hand, gluten proteins were partitioned by liquid-phase decomposition in respect with temperature. Protein composition of partitioned phases and eluting fractions recovered from A4F were characterized by size-exclusion chromatography. A specific interaction between omega-gliadin and glutenin polymers was evidenced.This result strengthens a very recent study that demonstrates a significant positive correlation between glutenin polymeric proteins, the ω‐gliadin fraction, and dough properties.

Tackling the question of specific interactions in a complex blend of Proteins. Edible Soft Matter – a SoftComp Topical Workshop

Challenges of public health and sustainable development require replacing in food products 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 in order to improve the control of emulsions and foams stabilized by these proteins.In this talk we will present experimental results on the behaviour interfacial properties of wheat gluten, sunflower and rapeseed proteins at liquid interfaces. Thanks to a combination of tensiometry, dilatational rheology and ellipsometry, rational physical pictures of the dynamics of the interfacial properties are achieved, for the various proteins and at both air/water and oil/water interfaces. For gluten proteins, a time-concentration superposition of the data is evidenced whatever the subphase concentration, which reveals that the kinetics of 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, and evidence surface-induced conformational changes of the proteins followed by film gelation [1]. Sunflower and rapeseed proteins by contrast do not reorganize once adsorbed at an interface and display a simpler dynamics of film formation. In addition the failure at high concentration of the time-concentration superposition of the tensiometry and viscoelastic data strongly suggest a surface-induced aggregation process, which we confirm with turbidity measurements. By quantitatively comparing the surface pressure dependence viscoelasticity of the various interfaces, we hightlight the crucial role on the behavior of plant proteins at liquid interfaces of the solvent quality and of the protein softness, that is discussed in regard to the protein structure.

We present a new light scattering setup coupled to a commercial rheometer operated in the plate-plate geometry. The apparatus allows the microscopic dynamics to be measured, discriminating between the contribution due to the affine deformation and additional mechanisms, such as plasticity. Light backscattered by the sample is collected using an imaging optical layout, thereby allowing the average flow velocity and the microscopic dynamics to be probed with both spatial and temporal resolution. We successfully test the setup by measuring the Brownian diffusion and flow velocity of diluted colloidal suspensions, both at rest and under shear. The potentiality of the apparatus are explored in the startup shear of a biogel. For small shear deformations, $\gamma \le 2\%$, the rheological response of the gel is linear. However, striking deviations from affine flow are seen from the very onset of deformation, due to temporally and spatially heterogeneous rearrangements bearing intriguing similarities with a stick-slip process.