Adsorption behaviour of lactoferrin in oil-in-water emulsions as influenced by interactions with β-lactoglobulin

Oil-in-water emulsions (pH 7.0 or pH 3.0) containing 30 wt% soya oil and various concentrations of lactoferrin were made in a two-stage valve homogenizer. The average droplet size (d32d32), the surface protein coverage (mg/m2) and composition, and the ζ  -potential of the emulsions were determined. The value of d32d32 decreased with increasing lactoferrin concentration up to 1%, and then was almost independent of lactoferrin concentration beyond 1% at both pH 7.0 and pH 3.0. The surface protein coverage of the emulsions made at pH 7.0 increased almost linearly with increasing lactoferrin concentration from 0.3 to 3%, but increased only slightly in emulsions made at pH 3.0 at lactoferrin concentrations >1%. The surface protein coverage of the emulsions made at pH 3.0 was lower than that of the emulsions made at pH 7.0 at a given protein concentration. The emulsion droplets had a strong positive charge at both pH 7.0 and pH 3.0, indicating that stable cationic emulsion droplets could be formed by lactoferrin alone. When emulsions were formed with a mixture of lactoferrin and β-lactoglobulin (β-lg) (1:1 by weight), the charge of the emulsion droplets was neutralized at pH 7.0 suggesting the formation of electrostatic complexes between the two proteins. The composition of the droplet surface layer showed that both proteins were adsorbed, presumably as complexes, from the aqueous phase at pH 7.0 in equal proportions, whereas competitive adsorption occurred between lactoferrin and β-lg at pH 3.0. At this pH, β-lg was adsorbed in preference to lactoferrin at low protein concentrations (⩽1%), whereas lactoferrin appeared to be adsorbed in preference to β-lg at high protein concentrations.

Graphical abstractFormation of electrostatic complexes between β-lactoglobulin and lactoferrin affects their adsorption behavior in oil–water emulsions which in turn determines the ζ-potential of oil droplets. The ζ-potential of emulsion droplets made with 30 wt% soya oil and a mixture of lactoferrin and β-lg (1:1 by weight) at pH 7.0 (●) and 3.0 (○), as well as β-lg alone (▴) at pH 7.0, as a function of the total protein concentration.Figure optionsDownload full-size imageDownload as PowerPoint slide