Influence of plant-based compounds on the structural stability of mucous boundary layers in tribological contact

This study presents a new methodology for understanding the molecular interactions of plant-based compounds on lubricating boundary layers in oral contacts. The methodology was used to investigate the impact of a plant protein (fava bean protein isolate) and a phenolic compound (tannic acid) on the structure and lubricating property of mucous boundary layers. Tribological experiments, coupled with quartz crystal microbalance with dissipation monitoring measurements and fluorescence microscopy imaging, were used to elucidate the mechanism of boundary film disruption and lubrication losses. It was found that even at concentrations up to 5 % w/v, fava bean proteins bind to mucins and hydrophobic polydimethylsiloxane (PDMS) surfaces, forming a hydrated, hydrophilic layer that sustains lubrication. Conversely, even at concentrations lower than 0.5 % w/v, tannic acid interacts strongly with mucins via hydrophobic interactions and/or hydrogen bonding, leading to protein aggregation, interrupting mucin binding onto PDMS, and increasing friction due to the disruption of the lubricating boundary layer. Fluorescent microscopy images revealed that the tannic acid-bovine submaxillary mucin interaction impairs the stability of the mucous boundary film, facilitating mucous protein detachment during a single sliding experiment of a PDMS probe. Results from these investigations provide critical insights into the molecular-level mechanisms influencing oral lubrication, particularly for the development of polyphenol-rich foods, and emphasize the importance of tribological assessment as a research tool in the development of plant-based protein alternatives.