Film forming and lubrication mechanisms of whey proteins and mucin

The sensory perception of whey protein is closely related to its film formation process on the oral surface and the structure of that film. Mucins, key components of human saliva, play a significant role in this film-forming process and in oral lubrication. Direct exploration of their film-forming behavior in the oral environment is crucial for understanding the mechanisms of mouthfeel. Sequential adsorption experiments revealed that Whey Protein Isolate (WPI) and mucin molecules could form hydrated layers immediately on gold or Polydimethylsiloxane (PDMS) surfaces. Moreover, mucin exhibited a stronger adsorption capacity, displacing WPI molecules adsorbed on the surfaces. Conversely, pre-formed mucin layers facilitated the formation of mucin-WPI hybrid layer. Film thickness and the quantity of adsorbed mass increased over time, following pseudo-second-order (PSO) model, indicating a strong chemical bond between the molecules and the substrate surface. The lubrication properties of the hybrid layer were confirmed using an optical interference approach. Under Hertzian contact at a speed of 500 mm/s and a load of 10 N, a thin film lubrication (TFL) state was observed, with the highest film thickness reaching approximately 30nm. In contrast, under a compliant contact with a 10mN load, the lubrication state transitioned from TFL to soft-elastohydrodynamic lubrication (soft-EHL) at an entrainment speed of just 2.5 mm/s, with film thickness reaching nearly 350nm. The presence of mucin enhanced the consistency of WPI adsorption, leading to a thicker and more reliable film. Further investigation revealed that the addition of mucin reduced friction by 30% to 50% in both boundary and TFL regimes. The superior lubrication performance of the mucin/WPI emulsion was attributed to the formation of the hybrid layer and the mucoadhesive properties of mucin. This study provides guidance for utilizing mucin to enhance the smoothness of WPI and alleviate its mouth-drying effects.