Steric stabilization, casein micelles

As discussed in Chapter 4, the casein micelles are stabilized by tc-casein, which represents 12-15% of the total casein and is located mainly on the surface of the micelles such that its hydrophobic N-terminal region reacts hydrophobically with the calcium-sensitive asl-, as2- and j8-caseins while its hydrophilic C-terminal region protrudes into the surrounding aqueous environment, stabilizing the micelles by a negative surface charge and steric stabilization. 

Van der Waals forces There has been some success in relating these forces to micellar stability. However, the steric stabilization has been found to be also of some importance. Especially, the hairy layer interferes with the interparticle approach. There are several factors that will affect the stability of the casein micelle system ... 

Since the micelles are closely packed, intermicellar collisions are frequent however, the micelles do not normally remain together after collisions. The micelles are stabilized by two principal factors (1) a surface (zeta) potential of c. —20 mV at pH 6.7, which, alone, is probably too small for colloidal stability, and (2) steric stabilization due to the protruding K-casein hairs. 

Many food colloids are stabilized from proteins from milk or eggs [817]. Milk and cream, for example, are stabilized by milk proteins, such as casein micelles, which form a membrane around the oil (fat) droplets [817]. Mayonnaise, hollandaise, and bearnaise, for example, are O/W emulsions mainly stabilized by egg-yolk protein, which is a mixture of lipids (including lecithin), proteins, and lipoproteins [811,817]. The protein-covered oil (fat) droplets are stabilized by a combination of electrostatic and steric stabilization [817]. Alcohols may also be added, such as glycerol, propylene glycol, sorbitol, or sucrose sometimes these are modified by esterification or by... 

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]. 

Historically, ideas of casein micelle structure and stability have evolved in tandem. In the earlier literature, discussions of micellar stability drew on the classical ideas of the stability of hydrophobic colloids. More recently, the hairy micelle model has focused attention more on the hydrophilic nature of the micelle and steric stabilization mechanisms. According to the hairy micelle model, the C-terminal macropeptides of some of the K-casein project from the surface of the micelle to form a hydrophilic and negatively charged diffuse outer layer, which causes the micelles to repel one another on close approach. Aggregation of micelles can only occur when the hairs are removed enzymatically, e.g., by chymosin (EC 3.4.23.4) in the renneting of milk, or when the micelle structure is so disrupted that the hairy layer is destroyed, e.g., by heating or acidification, or when the dispersion medium becomes a poor solvent for the hairs, e.g., by addition of ethanol. 

FIGURE 7.2 Calcium phosphate nanocluster model of a casein micelle. Substructure arises from the calcium phosphate nanocluster-like particles in the micelles (dark spheres). There is a smooth transition from the core to the diffuse outer hairy layer that confers steric stability on the micelle. (Courtesy of Holt and Roginski, 2001.)... 

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. 

The micelles on the fat surfaces cannot be coirpletely intact, because the original hydrophilic K-casein surface of the micelle is unlikely to bind to the fat surface. Homogenization must cause partial disruption of the micelles (H), allowing hydrophobic points of contact with the freshly exposed fat surfaces. There is no evidence that casein micelles interact with polystyrene latices to form a model system, for exanple (Dalgleish, unpublished results). Thus, although the micelles v ich bind to the fat in homogenized milk appear to be intact, their surfaces must have suffered some distortion, particularly of the sterically stabilizing K-casein molecules diich are near to the point of interaction of the micelle and the fat surface. 

Milk is a natural colloidal dispersion that contains casein micelles, self-assembled protein associates with a diameter of about 200 nm [20]. The casein micelles are protected against flocculation by an assembly of dense hairs (often called a brush ) at their surfaces. Polymer brushes can thus provide steric stabilization of colloids. For millennia, man used the fact that milk flocculates and gels when it is acidified, as in yogurt production. Below pH = 5 macroscopic flocculation of the casein micelles in milk is observed [21]. This means that the interactions between casein micelles change from repulsive to attractive. The explanation is that acidification leads to collapse of the casein brushes [22]. In cheese-making the steric stabilization is removed by enzymes, which induce gelation into cheese curd. 

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