Adsorption of κ-casein onto native milk fat globule, latex particle, and emulsion surfaces

The adsorption of κ-casein onto three surfaces was examined: (1) native milk fat globules, (2) low charge density sulfated polystyrene latex particles coated with milk fat globule membrane (MFGM) material, and (3) MFGM-coated soy oil emulsions. Particle hydrodynamic diameter was measured using dynamic light scattering before and after addition of κ-casein, and also after subsequent addition of Mucor miehei rennet protease or trypsin. The thickness of MFGM coating on latex particles was 25 nm at pH <5.8 reducing to 10 nm at pH >6.4. There was no increase in diameter after adsorption of κ-casein onto MFGM-coated latex particles or soy oil globules. The radius of native fat globules decreased by 15–20 nm after κ-casein adsorption. Addition of rennet or trypsin to the latex or emulsion+MFGM+κ-casein system caused aggregation, providing indirect evidence of κ-casein adsorption. In most cases the aggregate size reached a plateau value within 12 min, except at pH 5.5 after addition of trypsin where aggregation continued for at least 25 min. A reduction in radius of around 10–20 nm occurred initially after the addition of rennet or trypsin to MFGM-coated latex particles at pH 6.7 and native fat globules at pH 5.5 and 6.0, both with an adsorbed secondary layer of κ-casein, providing evidence of steric stabilization by a κ-casein ‘hairy layer’. This did not occur for MFGM-coated soy oil globules with a κ-casein adsorbed layer.