Smoothness as a tactile percept: Correlating 'oral' tribology with sensory measurements

The objective of the work was to carry out a comprehensive study to investigate the tribological response of the oil-in-water (o/w) emulsion systems between polydimethylsiloxane disc and steel ball surface using a ball-on-disc tribology assembly (3 point-contact). Simultaneously, oral and finger tactile sensory measurements were recorded to predict 'smoothness', in order to establish a reliable correlation between instrumental characterization and human perception. We hypothesized that the lubrication behavior of emulsions (constant oil droplet size) with varying oil mass fractions stabilized by different emulsifiers influences smoothness perception. Different emulsifiers studied were whey protein isolate (pH 6.7 & pH 3.5), modified starch and lysozyme while oil mass fractions of 30, 20, 10, 5 and 1 w% were examined. In-vitro studies with simulated saliva were also performed on the tribometer. All the emulsions showed typical Stribeck curve patterns with clear boundary and the mixed regime, while, the hydrodynamic regime was observed for high oil mass fractions. Friction coefficient (CoF) for all the emulsions was significantly different at most of the oil mass fractions (P < 0.05) and was compared with the smoothness scores (sensory analysis) along with discussion on plausible physical mechanisms for perception. Correlation and regression analyses provide empirical evidence that the o/w emulsions can be differentiated based on tribology studies and tactile perception. Smoothness is significantly correlated with the friction force on the tongue especially at sliding speeds between 0.1 and 30 mm/s constituting the mixed regime and hydrodynamic regime. Also, oral tactile smoothness scores correlated highly with CoF values than finger tactile.