Lubrication and delubrication behaviour of micron-sized whey protein microgels

This study aims to understand the tribological properties of whey protein microgels (WPM) by varying their sizes in the micron-scale and deformability. To decipher the lubrication mechanisms of WPM dispersions (3-48 vol %), we exploited two tribological systems using conventional smooth substrate and state of the art biomimetic tongue-like surfaces. We fabricated relatively hard WPM (G’ ≈ 350 kPa) of three distinct sizes (D4,3 ≈ 1-50 μm) and a soft WPM (G’ ≈ 85 kPa, D4,3 ≈ 50 μm) using water-in-oil emulsion templating. On smooth surfaces, dispersions of soft WPM delivered good lubrication in boundary and mixed regimes across the studied volume fractions (3-48 vol %), which was attributed to deformation-induced entrapment of WPM particles and the resulting surface separation. In contrast, hard WPM appeared to be entrapped between the contacting surfaces when their size is comparable to the theoretically derived film thickness (e.g. 1 μm). The incorporation of 1 μm hard WPM caused delubrication, which can be attributed to increased proportion of surface asperities which disrupt the aqueous lubrication. In a simulated tongue-palate interface (biomimetic tongue), microgel participation through the collision between WPM and papillated structure was found to be largely delubricating irrespective of size in the micron-scale which we postulate can be caused by jamming of WPM particles and/or even deformation of the papillae-like features by WPM. Our findings demonstrate that hard, micron-sized WPM exhibit delubrication when relatively soft tongue-like surfaces are used. These insights can inspire development of food products where an interplay between size and deformability of semi-solid particles is key to modulate mouthfeel.