Rheological fingerprinting and tribological assessment of high internal phase emulsions stabilized by whey protein isolate: Effects of protein concentration and pH

The effects of protein concentration and pH on the rheological and lubrication behavior of high internal phase emulsions (HIPEs), stabilized by whey protein isolate (WPI), were investigated by combining large amplitude oscillatory shear (LAOS) and tribology characterizations. All HIPEs showed predominantly solid-like behavior with greater stiffness at higher protein concentration. Emulsions at lower protein concentrations experienced extensive intracycle shear thinning with increased energy dissipation, indicating less resistance to large deformations and ready alignment in the shear flow. Increasing pH from 3 to 7 resulted in greater fragility with easier yielding to nonlinear behavior and faster transition to Newtonian fluid dissipation. This is likely due to the greater interfacial elasticity of oil droplets at neutral pH. In addition, tribological testing showed that increasing protein concentration resulted in a transition to the hydrodynamic regime. Emulsions at pH 3 had better mixed lubrication than those at pH 7, which can be attributed to easier entrainment of more deformable oil droplets. The findings of this study provide insights into the design potential of HIPEs with tunable textural properties and better prediction of structural transformations during food processing.