Adapting tribology for use in sensory studies on hard food: The case of texture perception in apples

Application of tribology in food systems has primarily focused on liquids and semi-solids. The present study examined texture perception in a solid food, using apples as the proof-of-concept. The study aimed to assess a) the ability of tribological measurements to predict a multicomponent sensory property (mealiness) in hard food, and b) the impact of two common motion patterns (rotational and linear reciprocating) on tribological measurements and mathematical correlations with sensory texture of a hard food. The textures of ten apple varieties were evaluated by a trained sensory panel while friction and wear behavior were measured instrumentally. Spearman correlations indicated that texture attributes (crisp, juicy, mealy, and rate of melt) significantly correlated with total penetration depth (p < 0.05) and with friction coefficients during the transient (non equilibrium) phase (p < 0.05), but not with friction coefficients from the steady-state phase in both rotational and linear tribology (p > 0.05). This was an important finding as the steady-state phase is predominantly used in food tribology research, yet our findings showed poor correlations with steady-state data, while showing strong correlations with sensory perception in the transient phase. The strong mathematical correlations found in the transient phase suggest that test conditions that provoke a dynamic friction response from the sample may more closely resemble the conditions under which humans perceive friction during oral processing.