When emulsions meet saliva : a physical-chemical, biochemical and sensory study

Keywords: Emulsion, flocculation, bridging, saliva, salivary protein, salivary peptides, lysozyme, -lactoglobulin, complex formation, LC-MS, SELDI-TOF-MS, proteomics.

Upon consumption food emulsions undergo various structural and compositional changes in the mouth. One of these changes is that mixing of an emulsion with saliva induces droplet flocculation
In the study described in this thesis we investigated the influence of saliva on emulsions properties, the mechanism of flocculation and the role in sensory perception. Firstly, we started with evaluating the effect of parameters related to emulsions on flocculation (i.e. differently charged surfactants and proteins such as -lactoglobulin and lysozyme used as emulsifiers and oil-volume fraction). Among the obtained results, we observed that the sign and the density of the charge on the surface of the droplets determine the (ir-)reversibility of flocculation upon dilution with water and shearing. Secondly, the effect of saliva-related parameters was analyzed. Among other aspects, it appeared that an increase in salivary protein concentration increased emulsion flocculation, and that extensive flocculation is typically found for unstimulated saliva. This approach shows that both emulsion and saliva properties affect the flocculation behavior of emulsions/saliva mixtures.
To investigate the nature of the flocculation, we characterized the salivary protein composition in both the continuous phase of the emulsion/saliva mixture and on the emulsion droplets. Different physical-chemical and biochemical techniques were used. For this approach, we focused on -lactoglobulin and lysozyme stabilized emulsions, which flocculated reversibly and irreversibly, respectively, upon mixing with saliva. A large number of salivary proteins and peptides in the molecular mass (Mr) range between 0.8 kDa and 100 kDa and the salivary mucins MUC5B and MUC7 (Mr > 200 kDa) associated with emulsion droplets of the emulsions. The results also indicate that the emulsifying protein at the oil-water interface determines which salivary components associate with the droplets in the flocs. A hypothesis is formulated that emulsion flocculation is mainly driven by a complex formation involving specific interactions and electrostatic attraction between salivary peptides/proteins and the emulsifying proteins at the droplets surface.
The importance of the saliva-induced droplet flocculation was demonstrated with a sensory paneling study. Emulsions stabilized by whey protein isolate, (predominantly composed of -lactoglobulin) showed reversible flocculation and were perceived as creamy. In contrast, emulsions stabilised by lysozyme showed irreversible flocculation and were perceived as dry, rough and astringent.
To conclude, this thesis shows that saliva-induced emulsion flocculation is driven mainly by association of salivary peptides and proteins to the droplets surface. Because of this, flocculation is determined by the composition of the droplet interface as well as the composition of the saliva, and can be controlled by variation of emulsion parameters (charge, pH, ionic strength). This interaction between emulsions and saliva may help to improve our understanding an control the sensory perception of emulsions.