Homogenized milks, casein micelles interfaces

The structure of the interfacial layers in food colloids can be quite complex as these are usually comprised of mixtures of a variety of surfactants and all are probably at least partly adsorbed at interfaces which even individually, can form complex adsorption layers. The layers can be viscoelastic. Phospholipids form multi-lamellar structures at the interface and proteins, such as casein, can adsorb in a variety of conformations [78]. Lecithins not only adsorb also at interfaces, but can affect the conformations of adsorbed casein. The situation in food emulsions can be complicated further by the additional presence of solid particles. For example, the fat droplets in homogenized milk are surrounded by a membrane that contains phospholipid, protein and semi-solid casein micelles [78,816], Similarly, the oil droplets in mayonnaise are partly coated with granular particles formed from the phospho and lipo-protein components of egg yolk [78]. Finally, the phospholipids can also interact with proteins and lecithins to form independent vesicles [78], thus creating an additional dispersed phase. 

After homogenization, the milk proteins readily adsorb to the bare surface of the fat droplets. The proteins are mostly adsorbed on the aqueous side of fat-matrix interface, with hydrophobic parts at the interface. Free casein, casein micelles and whey have different surface activities, so they adsorb differently onto the fat droplets for example, casein adsorbs more than whey. Proteins are very good at stabilizing oil-in-water emulsions against coalescence because they provide a strong, thick membrane around the fat droplet. Interactions between the proteins on the outside of the droplets make it harder for the droplets to come into close contact. This is known as steric stabilization. 

Two forms of the casein proteins are used to stabilize oil/Water emulsions. Most siirply, a mixture of the proteins (as in sodium caseinate), or the individual proteins themselves, allows the formation thin layers, if not monolayers, of protein at fat/Water interfaces (1). By contrast, in preparations such as homogenized milk, the entities which bind to and stabilize the fat/Water interface are much larger and more complex, and may be considered as intact or semi-intact casein micelles (2). These particles are highly aggregated complexes of the four caseins ... 

This brief review conprises three subject areas (i) the structure and properties of the K-casein surface layer in casein micelles (ii) the properties of the protein fraction in homogenized milks (i.e. basically intact casein micelles adsorbed at fat-water interfaces) (iii) the properties of caseinate and individual caseins adsorbed at the interfaces. In this, we are at present less... 

Effective models of casein micelles might in principle be constructed by binding caseins to a fat/water interface in an emulsion or to a polystyrene latex matrix. This is in fact not the case. It is questionable whether simple systems such as these will explain the behaviour of either casein micelles or homogenized milk, and we have sought an explanation for this anomaly. 

---