Interactions of milk protein-stabilized oil-in-water emulsions with bile salts in a simulated upper intestinal model

The behaviour of milk protein-stabilized emulsions (1.0 wt% protein) as influenced by the addition of bile salts was studied in simulated intestinal conditions (37 °C; pH 7.5; 39 mM K2HPO4, 150 mM NaCl, 30 mM CaCl2; with continuous agitation at ∼95 rpm for 2 h). Oil-in-water emulsions (20.0 wt% soy oil) stabilized by lactoferrin or β-lactoglobulin (β-lg) were prepared at pH 6.8 to produce cationic or anionic interfaces respectively. Varying physiological concentrations of bile salts (0.0–25.0 mg/ml) were added to each emulsion. The changes in droplet size, ζ-potential and confocal microstructures were monitored as a function of incubation time. Pre-heat treatment of simulated intestinal buffer containing bile salts was performed to eliminate any residual enzymatic activities.For β-lg-stabilized droplets, ζ-potential significantly changed from −63.1 ± 0.5 mV to −37.2 ± 0.3 mV in presence of bile salts due to competitive interfacial displacement of β-lg by bile salts as characterized by SDS-PAGE analysis of the continuous phase. On the other hand, lactoferrin-stabilized emulsion droplets showed considerable aggregation in presence of intestinal electrolytes alone at pH 7.5. The ζ-potential values of lactoferrin emulsion decreased gradually from +53.5 ± 0.6 mV to −12.2 ± 0.2 mV in presence of bile salts due to certain electrostatic effects (e.g. pH shift towards the isoelectric point, binding of anionic bile salts to cationic interfacial lactoferrin layer) in comparison to the behaviour of β-lg-stabilized droplets.

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