Lubrication by plant-based emulsions: Linking oil-water protein-stabilized interfacial mechanical properties to oil droplet lubrication properties

This work investigates how the viscoelasticity of the protein layer at the oil-water interface of emulsion droplets governs the emulsion lubrication behavior. Commercially-available (PPIC) and lab-produced (PPIL) pea protein isolate, and soy protein isolate (SPI), were used to stabilize the emulsions. Whey protein isolate (WPI) served as a reference system. We found that WPI formed stiff, solid-like interfacial layers, and PPIL formed an interface that exhibits high deformability. Both interfaces were strong enough to resist mechanical stresses. In contrast, PPIC and SPI were heavily aggregated in bulk solution, forming much weaker oil-water interfaces, which were disrupted at higher stresses. The emulsion droplets stabilized by WPI or PPIL remained stable under mechanical stress, and the oil droplets were hypothesized to act as particles that limited contact between the interacting surfaces, thereby providing lubrication via a rolling/sliding mechanism. In contrast, the PPIC- and SPI-stabilized emulsions exhibited more effective friction reduction, which was hypothesized to result from oil droplet coalescence and the subsequent formation of a lubricating film. These lubrication behaviors showed a high correlation with the mechanical properties of oil-water interfaces stabilized by the proteins, i.e. elastic dilatational moduli (Ed' and Ed'') and viscous dissipation of the odd (U_dτ2) and even (U_dτ3) harmonics. These results show that protein oil-water interfacial properties, especially the mobility and resistance against density change of adsorbed proteins, are strongly correlated with lubrication properties, indicating that by structuring the oil-water interface with certain proteins, lubrication properties can be achieved, offering a strategy to tailor mouthfeel.