Colloidal stability and interactions of milk-protein-stabilized emulsions in an artificial saliva

Oil-in-water emulsions (20 wt% soy oil) with lactoferrin or β-lactoglobulin (β-lg) as the interfacial layer were prepared using a two-stage valve homogenizer. At pH 6.8, lactoferrin produces a stable cationic emulsion whereas β-lg forms an anionic emulsion. These emulsions were mixed with an artificial saliva that contained a range of mucin concentrations and salts. Negatively charged mucin was shown to interact readily with the positively charged lactoferrin-stabilized emulsion droplets to provide a mucin coverage of approximately 1 mg/m2. As expected, the negatively charged β-lg-stabilized emulsion droplets had lower mucin coverage (0.6 mg/m2 surface load) under the same conditions. The β-lg-stabilized emulsions were stable but showed depletion flocculation at higher mucin levels (≥1.0 wt%). In contrast, lactoferrin-stabilized emulsion droplets showed considerable aggregation in the presence of salts but in the absence of mucin. This salt-induced aggregation was reduced in the presence of mucin, possibly because of its binding to the positively charged lactoferrin-stabilized emulsion droplets and thus a reduction in the positive charge at the lactoferrin-coated droplet surface. However, at higher mucin concentration (≥2.0 wt%), lactoferrin-stabilized emulsions also showed droplet aggregation.