The interfacial properties of milk fat globules govern their interactions with proteins: role of high shear stress and pH probed by AFM based force spectroscopy

The interfacial properties of colloidal particles influence their interaction with each other and with their environment. This, in turn, governs the physical and functional characteristics of colloidal systems such as emulsions found in foods, drugs or cosmetics. Milk has been an important part of the human diet for thousands of years. It is a natural oil-in-water emulsion in which the lipids are dispersed as ~0.1-10 μm colloidal assemblies called the milk fat globules (MFG). MFG are composed of a core droplet of triacylglycerols (TAG) surrounded by a biological membrane. This membrane, mainly composed of polar lipids, cholesterol and glycoproteins, acts as the interface with the surrounding bulk of milk proteins (ie. casein micelles and whey proteins) or with enzymes of the gastrointestinal tract during digestion. The physicochemical properties of MFG and their membrane composition are altered by technological treatments during the manufacture of dairy products (Lopez et al., 2015, Dairy Science & Technology). Shear stress induces the adsorption of caseins from the aqueous phase of milk to the MFG surface and heat-treatment causes the co-adsorption of highly-reactive denatured whey proteins. All these proteins precipitate at acid pH, which is used for milk gelation in yoghurt-making. Here, atomic force spectroscopy was used to measure the adhesion forces, adhesion work and rupture distances between individual casein micelles, attached to the AFM probe, and MFG taken at different stages of processing: native MFG, then processed MFG, prior to and after acidification. Results showed that native MFG present low affinity for casein micelles independently of the pH, due to the steric effect of the glycocalyx surrounding their native membrane. When casein were adsorbed to the MFG’s surface, the adhesion forces increased, particularly at acid pH due to a decrease in electrostatic repulsion. The presence of denatured whey proteins further enhanced adhesion between the MFG and individual casein micelles. These results evidenced a clear correlation between adhesion forces at the nanoscale and the bulk properties of the milk gels. This showed that the interfacial properties of MFG are important to drive the connectivity of the acid gel. This study open perspectives for the closer evaluation of individual colloid interface properties in relationship with the bulk physical and functional properties of colloid systems.