Poroelastic relaxation indentation of whey protein hydrogels

Many solid food matrices contain high amounts of solvent, typically water, yet the time dependent mechanical characterization of foods has typically been analyzed following a viscoelastic approach, omitting the effect of the solvent. In solvent rich solids the solvent can flow internally, for example as it is compressed, which is typically understood using poroelastic theory. A poroelastic approach allows the determination of hydrogel parameters, such as the Darcy's diffusivity or the intrinsic permeability, that have a physical meaning. Recently, it has been proposed for polymeric hydrogels a novel experimental methodology, based on relaxation after indentation, that greatly simplifies the poroelastic analysis. This methodology is applied and tested to heat induced whey protein hydrogels. Finite element analysis was performed to simulate the particularities of the indentation experiments. Relaxation data suggest, after a pure poroelastic analysis, that there is also a small fast viscoelastic relaxation in whey protein hydrogels. Poroviscoelastic relaxation was simulated with finite elements modeling, and a forward regression methodology was implemented. Results show that the water diffusivity in the protein hydrogels is larger that the self-diffusivity of water, as expected in a matrix with large pores where solvent flow occurs by convection. Whey protein hydrogels swollen to different degrees using different salt concentrations relaxed to different proportions. This result was unexpected as it leads, in poroelastic theory, to different drained Poisson's ratio, with even negative values for the less swollen gels.