Identification of salivary proteins at oil–water interfaces stabilized by lysozyme and β-lactoglobulin

In this research, we investigated the interaction occurring between oil-in-water emulsion droplets, stabilized by different emulsifiers, i.e. lysozyme and β-lactoglobulin (β-lg), and salivary proteins (SPs) with a molecular mass (Mr) above about 10 kDa. Different techniques, i.e. infrared spectroscopy, Western blotting, PAS staining and SDS-PAGE coupled to MS, were employed for this purpose. This study demonstrated the interaction between several salivary proteins and the emulsifiers at the oil–water interfaces. In particular, results show that the high Mr mucin MUC5B was strongly bound to lysozyme stabilized emulsions, whereas β-lg stabilized emulsions associated with MUC7 and, moderately, with MUC5B. Furthermore, we observed that salivary proteins in the range Mr 10–100 kDa associated differently with emulsion droplets. A large majority of SPs was found to interact with lysozyme stabilized emulsion droplets whilst in case of β-lg stabilized emulsions, the SPs distribute more evenly between the fraction associated and non-associated with the droplets. A clear example is α-amylase (Mr ∼ 55 kDa) which predominantly associates with lysozyme stabilized emulsion droplets, but not with β-lg emulsion droplets. To conclude, our findings indicate that adsorption/association of salivary protein components onto the emulsion droplets is related to the type of emulsifying proteins at the oil–water interfaces and it is probably driven by the overall net charge at the droplet's oil–water interfaces, i.e. positive for lysozyme stabilized emulsions and negative for β-lactoglobulin stabilized emulsion at neutral pH.