Colloidal properties of protein-stabilized emulsions in the presence of saliva: Impact on oral lubrication and sensory perception.

The colloidal behavior of protein-stabilized oil-in-water emulsions in the presence of saliva was investigated, as well as the impact of oil droplet-saliva interactions on the physicochemical and sensory properties of these emulsions. The emulsions were stabilized by three amphiphilic food proteins: sodium caseinate (NaCas), whey protein isolate (WPI), or lysozyme. Saliva incorporation decreased the negative charge and increased the particle size of all the emulsions, indicating a reduction in their stability. The NaCas emulsions exhibited the least structural changes in the presence of saliva. Saliva significantly increased the shear viscosity of the WPI and NaCas emulsions, which was mainly attributed to its ability to promote droplet flocculation. In contrast, saliva caused little change in the shear viscosity of the lysozyme-emulsions, suggesting that mucin had less impact on droplet flocculation in this system. The potential oral behavior of the emulsions was examined by measuring lipid droplet retention on a porcine tongue. The lysozyme-emulsions exhibited the highest retention, whereas the whey protein ones exhibited the lowest. In terms of lubrication properties, the incorporation of saliva caused a significant decrease in friction on a model porcine tongue surface for the WPI and NaCas emulsions but minimal changes for the lysozyme-emulsions. Sensory evaluation of the different emulsions showed clear textural differences during and after oral processing. The whey protein-emulsions had a smooth and creamy texture. The sensory attributes of the lysozyme emulsions were dominated by "thickness" during oral processing and by a persistent "mouth coating" sensation after oral processing. In contrast, the NaCas emulsions displayed a transition from "greasiness" to "thickness" and then to a "mouth coating" sensation. These findings offer valuable insights into the relationships between emulsion structure, oral tribology, and sensory perception.