Stress relaxation of particulate whey protein hydrogels

The relaxation mechanics of hydrogels depend on how stresses are dissipated: through the polymeric network, through the flow of the entrapped solvent, or through both mechanisms. Particulate protein hydrogels, prepared from whey proteins at pH 7 and 0.1 M NaCl, were investigated using micro-relaxation tests to investigate the role of the microstructure. When deforming gels at high velocities (~400 μm/s), there is little relaxation during loading and the subsequent extensive relaxation is identified to be poroelastic by using different indenter sizes. The effective diffusivity and solvent permeability are estimated to be ~6×10⁻⁹ m²/s and 5×10⁻¹⁷ m² respectively, which are one magnitude order larger than in protein hydrogels with a stranded microstructure. When indenting gels at low speeds (~20 μm/s), the poroelastic relaxation due to solvent flow occurs mostly during the loading step, and the subsequent relaxation is mostly viscoelastic in nature. Particulate protein gels, therefore, present two well identified and separated relaxation regimes.