Wheat gluten proteins are among the most complex protein networks in nature, due in particular to their poor solubility in water and to their viscoelastic behavior. Gluten networks are often considered as transient networks comprising extensible biopolymer segments of flexible or semiflexible chains between junction points. However, the exact structure of the network, the nature of the junction points, its mechanical properties and the way it gets structured under shear remain to be clarified.
Here we report the visco-elastic behavior of model systems composed of gluten proteins near gelation. We build model systems by dispersing in ethanol-water mixtures two major protein groups, gliadins and glutenins that we have purified from gluten. Rheological properties show a slow evolution over time scales of the order of days of the linear frequency dependence complex modulus of the samples, with a concentration-dependent liquid to solid transition. Interestingly, we find that all data acquired at different protein concentrations and different times after sample preparation (different sample ages) can be scaled onto a master curve, showing a cross-over from a soft solid behavior at low frequency to a visco-elastic fluid at higher frequency, and revealing the self-similarity of the gels.
Rheological data are completed by scattering experiments in order to elucidate the complex structure of the materials. For gel samples, the scattering profiles display at small length scales features typical of semi-dilute polymer solutions. At larger length scale a fractal structure is measured, that we interpret as being due to the highly disordered spatial organization of the junction points, at the origin of the solid behavior.