Modulating Casein-Stabilized Emulsions Through the Incorporation of Different Emulsifiers: Impacts on Microstructure and Oral Tribology

This study investigated the combined effects of oil concentration, emulsifier type, and saliva on the lubrication behavior of casein-based oil-in-water emulsions to support the design of milk-based foods with optimized mouthfeel. Emulsions stabilized with Tween 20, whey protein isolate (WPI), or sucrose ester were prepared at oil concentrations ranging from 0.01% to 3%, and their viscosity, microstructure, tribological properties, and ζ-potential were systematically characterized, with human saliva incorporated to simulate oral conditions. Oil concentration did not significantly alter viscosity, although droplet aggregation increased with higher oil levels. Lubrication performance was governed primarily by emulsifier type: Tween 20 generated an oil film at approximately 0.2% oil, WPI exhibited progressively enhanced lubricity with increasing oil concentration, and sucrose ester produced consistently poor lubrication due to its rigid interfacial layers. Saliva addition improved lubrication across all systems and reduced oil precipitation by promoting the formation of smaller, more stable structures. These findings demonstrate that emulsifier selection is central to modulating oil–protein–saliva interactions, with WPI at moderate oil levels yielding favorable lubrication with controlled oil release, thereby providing a mechanistic basis for developing healthier, palatable milk-based foods.