Oral tribology, adsorption and rheology of alternative food proteins

Mechanistic knowledge using tribology and adsorption may help to screen various proteins with better lubrication; aiding the fast tracking of new ingredient formulations for use in low-fat/high protein food development. The aim of this study was to compare the lubrication, adsorption and physicochemical properties of alternative proteins (pea, potato, lupin and insect proteins) with whey protein isolate (WPI) as the control. Soluble fractions (1–10 wt%) of pea protein concentrate (PPC_sol), insect protein concentrate (IPC_sol), potato protein isolate (PoPI_sol) and lupin protein isolate (LPI_sol) were chosen as the alternative proteins. All proteins were negatively-charged at neutral pH and showed various degrees of aggregation (hydrodynamic diameters ranging from 25 nm for PoPI_sol to 244 nm for PPC_sol). The boundary friction coefficient (μ) at 5 wt% protein followed the trend as PPC_sol > LPI_sol > IPC_sol > PoPI_sol > WPI_sol, highlighting excellent lubrication performances of PoPI_sol, IPC_sol and WPI_sol. At higher protein concentrations (10 wt%), μ significantly increased for LPI_sol, PoPI_sol and IPC_sol, while decreasing for WPI_sol. Quartz crystal microbalance with dissipation monitoring (QCM-D) results revealed formation of rigid elastic films on hydrophobic surfaces by PoPI_sol and WPI_sol giving rise to low μ while more viscous films by PPC_sol led to high μ. PPC_sol had the highest hydrated mass (11.0 mg m⁻²) as compared to WPI_sol (8.0 mg m⁻²) with lower values reported for other proteins (5.0–5.4 mg m⁻²). Strong correlations existed between μ scaled to viscosity, size and hydrated mass and viscoelasticity of films in alternative proteins, validating the surface-linked phenomena in frictional response.