On relating rheology and oral tribology to sensory properties in hydrogels

The aim of this study was to understand the relationship between rheological, tribological and sensory properties (n = 11 panellists) of hydrogels differing in hydrocolloid type, concentration and degree of inhomogeneity. Fracture properties of hydrogels containing different ratios of kappa-carrageenan (kappa C) and/or locust bean gum (LBG), sodium alginate (NaA), 300/1000 mu m calcium alginate beads (CaA) at 1-4 wt% concentration were determined. Viscosity and friction coefficients (mu) of the hydrogel-boli after simulated oral processing were characterized. Tribology measurements were conducted in a polydimethylsiloxane ball/disc set-up with pre-adsorbed artificial salivary film at 37 degrees C. 'Scaling' with boli viscosity showed good agreement of observed data with the Stribeck master curve, however only in the mixed regime i.e. at intermediate values of the product of velocity and lubricant viscosity (U eta). Low mu values of gel boli in the boundary regime were largely driven by the formation of a viscous layer of bolus fragments between opposing surfaces. Fracture properties of hydrogels and boli viscosity were correlated with all chewing-related texture attributes i.e. 'firm', 'elastic', 'chewy' and 'cohesive' and inversely correlated with lubrication-related attributes 'melting' and 'pasty' (p < 0.05). On the other hand, mu of the bolus filtrate at orally relevant speeds (50 mm/s) was inversely correlated with lubrication-related attributes 'pasty' and positively with 'slippery' (p < 0.05). The lack of correlations with 'smooth' could be explained due to sample inhomogeneity and the absence of 'ball-bearing'-ability of the gel beads. A combination of initial fracture properties, boli viscosity and tribology of bolus filtrates (mixed regime) impacted the lubrication-related attribute 'salivating' (p < 0.05).