Film forming and lubrication mechanisms of whey proteins and mucin

Abstract

The sensory perception of whey protein is closely related to its ability to form films on the oral surface and the structure of the resulting 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. Subsequent 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, preformed mucin layers facilitated the formation of a mucin-WPI hybrid layer. The film thickness and quantity of adsorbed mass increased over time, following the pseudo-second-order (PSO) model, indicating strong chemical bonding between the molecules and the substrate surface. The lubrication properties of the hybrid layer were confirmed via 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 30 nm. In contrast, under compliant contact with a 10 mN load, the lubrication state transitioned from TFL to soft-EHL at an entrainment speed of just 2.5 mm/s, with the film thickness reaching nearly 350 nm. 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%–50% in both the 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 the use of mucin to increase the smoothness of WPIs and alleviate their mouth-drying effects.