In vitro stability study of saliva emulsions: The impact of time, calcium ion and pH

Human saliva is a complex colloidal system that offers multiple functions during food oral processing. Most interestingly, saliva was recently proved to act as an effective natural emulsifier during the oral consumption of oil/fat, leading to speculations about the mechanisms of oral sensation of oil/fat. For this reason, this project was designed to investigate the stability of saliva emulsions, as influenced by storage time (0–240 min), Ca2+ concentrations (0–45 mM), and pH (4–8). A total of 12 healthy young subjects were recruited, and saliva emulsions were prepared in vitro under gentle homogenization conditions. Emulsion stability was monitored by ζ-potential, particle size distribution, microstructure, and apparent viscosity. The results showed that emulsifying capability of saliva and the stability of emulsion was significantly different among subjects (p < 0.05). During the observation period, d32 of saliva emulsions only increased slightly with the extension of storage time. The stability of saliva emulsions was affected by Ca2+ concentrations or pH values to varying degrees. The lowest stability was observed at 45 mM Ca2+ or pH 4 (close to pI). Also, all saliva emulsions were exhibited shear-thinning behavior. Despite the results are still preliminary, it is evident that oral emulsifying capability and the stability of saliva emulsions are highly individualized and are significantly affected by complex factors. Confirmation of oral emulsification reminds us that very different thinking is needed in explaining oral mechanisms of oil/fat sensation and perception.