Comparing frictional behaviour of plant and dairy proteins: Case study on high protein concentration

The aim of this study was to understand the frictional behaviour of non-purified plant proteins (i.e. pea protein concentrate (PPc) and soy protein isolate (SPI)) compared to dairy proteins (whey protein isolate (WPI) and sodium caseinate (NaCas)). The comparison was conducted for aqueous dispersions of the proteins at 10 and 20 wt% at pH 6.8. Owing to protein-protein aggregation and lower solubilities, plant proteins showed prominent shear thinning behaviour, unlike dairy proteins, which showed less shear dependence. Addition of proteins reduced the boundary friction coefficients (μ) with NaCas showing the lowest boundary μ (p < 0.05). In general, the dairy proteins showed larger hydrodynamic size and better lubricity whilst the plant proteins increased the mixed μ with twice as high a calculated fluid film thickness (hmin) required for onset of the elastohydrodynamic regime as compared with that of dairy proteins. Such low μ values in dairy proteins might be attributed to increased adsorption (∼1.5–5× more) of the elastic films as compared to plant proteins. Findings suggest that product development towards more sustainable formulations, requires innovative strategies to reduce poor lubrication and aggregation when using higher concentrations of plant proteins.