Milk matrix-driven structural reorganization modulates oral friction and sensory perception in coffee-milk systems

Coffee-milk beverages are valued for their characteristic flavor and smooth mouthfeel, yet the role of milk matrix in regulating structure, oral friction, and sensory perception remains insufficiently understood. This study prepared four coffee–milk systems using ultra-high-temperature and pasteurized whole or skim milk to evaluate the effects of milk matrix characteristics on physicochemical, tribological, and sensory properties. Milk addition induced structural reorganization, as demonstrated by increased pH, lightness, viscosity, particle size, fluorescence changes, and FTIR spectral variations, while electron microscopy confirmed aggregate formation. Molecular docking revealed that coffee components interacted with milk proteins mainly through hydrogen bonding and π–π stacking interactions, providing molecular insights into complex stabilization. These structural changes enhanced interparticle interactions and reduced interfacial lubrication, increasing friction coefficients. Conversely, fat disrupted aggregate structures and reduced friction, as confirmed by fat add-back experiments. This study provides insight into optimizing mouthfeel and sensory quality of reduced-fat coffee-milk beverages.