The origin of the unique rheological properties of gluten, the water-insoluble protein fraction of wheat, is crucial in bread-making processes and questions scientists since decades. Gluten is a complex mixture of monomeric and polymeric proteins. To better understand the supramolecular structure of gluten and its link to the material properties, we develop and characterize model gluten using a combination of rheology, biochemistry and scattering techniques. In this framework, we investigate the linear and non-linear viscoelastic properties of samples produced by the dispersion of gluten proteins in a solvent. We vary the quality of the solvent (various water/ethanol mixtures), the protein concentration, and the protein composition, which we finely control thanks to a novel protocol based on a liquid-liquid phase separation. We show that the complex viscoelasticity of the gels exhibits concentration/aging time/solvent composition superposition principles, demonstrating the self-similarity of the gels produced in different conditions. All gels can be regarded as near critical gels with characteristic rheological parameters, elastic plateau, and characteristic relaxation time, which are related to one another, as a consequence of self-similarity, and span several orders of magnitude when changing the parameters. Structural features probed by X-ray, neutron and light scattering experiments provide a quantitatively consistent physical picture and of near-criticality and provide crucial molecular clues of the role of intramolecular H-bonds in the gelation process. Non-linear rheology, as probed by shear start-up experiments, is also intimately related to the sample structural features.

Food transition requires the replacement in human diet of animal-based proteins by alternative sources of proteins including plant-based proteins. This calls for a detailed knowledge of the functional properties of plant-based proteins, including their surface activity. In this framework, we provide here a comparative study of the interfacial properties of two plant proteins, extracted respectively from wheat and sunflower. We combine time- and concentration-dependent measurements of the surface tension and the surface rheology, as measured with a pendant-drop set-up, and of the surface excess concentration, as measured by ellipsometry, of plant protein interfacial films. We demonstrate a time-concentration superposition principle for the surface pressure and surface excess concentration, showing that the kinetics for the building of the interfacial films is essentially governed by the diffusion of the proteins from the bulk to the interface. We find that the rheological and structural properties of the interfacial protein films show markedly different behaviors for the two classes of protein, which is encoded in the structural features of the individual proteins: wheat proteins are more surface active than sunflower proteins, are keen to compress and re-arrange at an air-water interface, whereas sunflower proteins do not. This work provides qualitative and quantitative analysis of the comparative interfacial behavior of flexible and rigid plant proteins extracted respectively from wheat and sunflower, and demonstrates that a combination of several experimental techniques is necessary to obtain insightful information on the interfacial properties of any species.

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.