Lubrication studies of β-lactoglobulin-stabilized emulsions mixed with bovine submaxillary mucin

Understanding the oral processing of β-lactoglobulin (BLG)-stabilized emulsions is crucial for optimizing the physical and sensory attributes of colloidal food systems. This study investigates the tribological properties of BLG-stabilized emulsions and their mixtures with bovine submaxillary mucin (BSM) under various contact surfaces and testing conditions. Emphasis was placed on harmonizing tribological surfaces, including hydrophilic, hydrophobic, and roughened materials, to establish mechanistically supported generalized relationships under aligned testing conditions. Wettability measurements showed superior lubrication on hydrophilic steel surfaces, whereas hydrophobic materials (HDPE, PDMS, and rough PDMS) exhibited lower wettability and higher friction. Droplet size measurements using laser diffraction and optical microscopy revealed increased flocculation and the formation of larger droplet populations (∼90 μm) upon BSM addition, indicating reduced emulsion stability. Pin-on-disk tribometry indicated increased coefficients of friction (COFs) in BSM-containing emulsions at low- and mid-speed regimes, likely due to hindered droplet entrainment and inter-droplet aggregation. At high speeds, COF differences diminished due to effective fluid entrainment. Investigations with various tribopairs revealed that hydrophobic and rough surfaces hindered fluid film formation, while hydrophilic–hydrophobic interfaces, such as steel–PDMS, enhanced lubrication even with mucin present. Mini-traction machine (MTM) tests demonstrated that BSM-containing emulsions maintained lubricity at high speeds under rolling/sliding conditions. However, insufficient viscosity and low contact pressure delayed the transition to elastohydrodynamic lubrication (EHL). These findings highlight the complex interplay between emulsion droplet size, surface characteristics, and mucin's amphiphilic and flocculation-promoting behavior in boundary lubrication. The study provides mechanistic insights into food tribology, aiding the design of emulsion-based food systems with improved mouthfeel and texture.