Tribological properties of real foods using extended Stribeck curves and their relationship with nutritional and rheological parameters

The tribological properties of 19 commercial food products, grouped into six categories (yogurt, dressings, spreads, porridges, emulsified sauces, and syrups) were investigated in relation to their rheological (dynamic oscillatory shear test) and nutritional properties (fat, carbohydrate, and protein). A tribological system (a glass ball and three polydimethylsiloxane pins) generated the extended Stribeck curve, monitoring friction factors (f) over an extended range of sliding speed (v) (10^-8 to 10⁰ m/s). Tribological parameters (f, v) at four inflection points dividing the frictional regimes (X₁, breakaway point between the static and kinetic regimes; X₁-X₂, boundary; X₂-X₃, mixed; X₃-X₄, hydrodynamic regimes) and the slope between X₃ and X₄ (s) were subjected to principal component analysis and hierarchical clustering on principal components, using rheological and nutritional parameters as quantitative supplementary variables. Tribological patterns were predominantly influenced by viscosity, viscoelasticity, yield stress, fat content, and the presence of particles (e.g., sugar, proteins, and fibers) and pasting materials (e.g., starches and modified starches). The 19 tribological patterns were classified into 3 clusters: low f and s for fat- and/or viscoelastic-dominant foods (Cluster 1), low f and high s for food emulsions and/or those with low extent of shear-thinning (Cluster 2), and high f at the boundary regime either for the most viscous foods or for those in the presence of particulates (Cluster 3). These results suggest that the compositional and rheological properties have a more profound impact on the classification of complex tribological patterns than the categories of food products.