Homogenized milks, casein micelles

The effects of homogenization on milk components have been summarized by Walstra and Jenness (1984) and Harper (1976). Homogenization disrupts fat globules and results in an increase in fat surface area (about 4-10 times). Casein micelles adsorb on the fat surface and constitute part of the fat globule membrane. The curd tension of milk is thus lowered. Walstra and Jenness (1984) have described the effect of homogenization on rennet coagulation. 

The concentrated milk is homogenized at 140 to 210 kg/cm2 (2000 to 3000 lb/in2) at about 48°C (Hall and Hedrick 1966). This process is essential to provide adequate physical stability to the milk fat emulsion system to withstand prolonged storage at room temperature (Brunner 1974). However, homogenization lowers the heat stability of concentrated milk products (Parry 1974), which may be due to increased adsorption of casein micelles onto the newly created milk fat globule surfaces, thus making them more sensitive to heat-induced aggregation. 

Several compositional and processing variables affect the physical stability of the casein micelles in frozen milk concentrates. These factors include pH, mineral composition, total solids content, forewarming treatment, homogenization and fat content, freezing rate, storage temperature, and fluctation of storage temperature (Keeney and Kroger 1974 Webb 1970). 

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. 

Casein or egg-yolk proteins are used as emulsifiers in a number of food products, such as O/W food emulsions (Table 13.1) [78,824]. A key difference here is that in caseinate-stabilized oil emulsions, the casein forms essentially monolayers and there are no casein micelles nor any calcium phosphate. Such emulsions are thought to be stabilized more by electrostatic repulsive forces and less by steric stabilization, in contrast to the situation in homogenized milk products [824]. 

These studies demonstrate that treatments that have been reported to modify the heat stability of milk also may change the distribution of tracer milk proteins between the physical phases of milk. Analysis of these interactions may provide useful information about the mechanisms of these effects. In unpublished experiments, we have extended this approach for studying homogenized milk-based systems, also containing a lipid phase, to investigate lipid-protein interactions. Dual-label experiments, for example in milk containing M-/3-L and 3H-methyl-/c-casein, could be applied to the isolation and characterization of protein complexes in milk. Labeled caseins could prove valuable as probes for elucidating micelle structure. 

Figure 5.3. Schematic illustration of the relative effects of heating and homogenization on fat globules in milk. MFG = milk fat globule, CM = casein micelle, WP = whey protein, dWP = denatured whey protein.

Sandra, S. and Dalgleish, D. G. (2005). Effects of ultra-high-pressure homogenization and heating on structural properties of casein micelles in reconstituted skim milk powder. Int. Dairy ]. 15,1095-1104. 

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... 

Of the caseins, only K-casein is capable of exerting a stabilizing effect in systems vdiich contain calcium ions in appreciable quantities. This is the case in milk, where the other caseins are effectively rendered insoluble by their binding to calcium phosphate, and vdiere there are also appreciable pools of calcium ions. When the calcium is removed, all of the caseins act as surfactants, and can stabilize emulsions. Even in milk, the casein micelles can bind to and stabilize unprotected fat surfaces, as in homogenized milk. 

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