Casein micelle disruption

Roach, A. and Harte, F. (2008). Disruption and sedimentation of casein micelles and casein micelle isolates under high-pressure homogenization. Innovative Food Sci. Emerg. Technol. 9,1-8. 

The most abundant milk protein is casein, of which there are several different kinds, usually designated a-, (1-, and K-casein. The different caseins relate to small differences in their amino acid sequences. Casein micelles in milk have diameters less than 300 nm. Disruption of the casein micelles occurs during the preparation of cheese. Lactic acid increases the acidity of the milk until the micelles crosslink and a curd develops. The liquid portion, known as whey, containing water, lactose and some protein, is removed. Addition of the enzyme rennet (chymosin) speeds up the process by hydrolysing a specific peptide bond in K-casein. This opens up the casein and encourages further cross-linking. 

The weak physical forces that hold together self-assembled nanoparticles are, of course, susceptible to disruption under the influence of thermodynamic and/or mechanical stresses. Hence some workers have investigated ways to reinforce nanoscale structures via covalent bonding. For instance, improved stability of protein nanoparticles, in particular, casein micelles, can be achieved by enzymatic cross-linking with the enzyme transglutaminase, which forms bonds between protein-bound glutamine and lysine residues. By this means native casein micelles can be converted from semi-reversible association colloids into permanent nanogel particles (Huppertz and de Kruif, 2008). 

Since the micelles are of colloidal dimensions, they are capable of scattering light and the white colour of milk is due largely to light scattering by the casein micelles the white colour is lost if the micelles are disrupted, e.g. by removing colloidal calcium phosphate (by citrate, ethylene... 

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. 

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. 

The stomach environment is acidic as a result of HC1 secretion by the parietal cells. The acidic pH serves to denature many proteins, thus making them susceptible to proteolysis. The chief cells of the stomach produce pepsinogen, which is activated to pepsin by the HC1 (see Table 20.3). The optimum pH of peptic activity is around 2, and pepsin is inactivated at neutrality. Another stomach enzyme is rennin or chymosin, which is present in infants but not in adults. It removes a glycopeptide from milk-K-casein, disrupting the casein micelle and promoting milk protein coagulation and digestion. 

Avoidance of interference of other milk constituents with measurements is also of importance for example, dissociation of casein micelles by calcium-chelating agents, such as trisodium citrate or ethylenediamine tetra-acetic acid (EDTA), may used to avoid interference of the micelles in particle size measurement, while clusters of fat globules can be disrupted by adding a low level of sodium dodecyl sulphate (SDS). 

Polyphosphates improve the sensory quality of many food products. They prevent the separation of butter fat and aqueous phases in evaporated milk, and the formation of gel in concentrated milk sterilized by high-temperature short-time (HTST). They also stabilize the fat emulsion in processed cheese by disrupting the casein micelles and thus enhance the hydrophobic interactions between lipids and casein. Polyphosphates are also used in meat processing for increasing the WHC and improving the texture of many cooked products. The mechanisms involved in different applications depend on the properties of the phosphates and the commodities, as well as the parameters of processing. 

High pressure induces disruption of casein micelles and denaturation of whey proteins, increases pH of milk, reduces rennet coagulation time, and increases cheese yield, thereby possessing potential application in cheese making (O Reilly et al.,... 

Huppertz and Smiddy (2008) demonstrated that application of high pressure (250-300 MPa) led to an initial rapid micellar disruption this was followed by a partial reversal of the high-pressure-induced reassociation of micellar fragments. Partial internal cross-linking of casein micelles by transglutaminase prior to pressure treatment considerably slowed down both the disruption and reassociation processes. 

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. 

IX were present in the casein micelle film in a weight ratio of 0.64 0.36. From the close-packing area of Fraction I, and the specific volume for caseins reported by Schultz and Bloomfield ]), we have calculated the average diameter of complexes in this fraction. The diameter is 9.1 nm, which is in agreement with results published by Pepper and Farrell on the diameter of the submicelles ( ) The submicellar structure of caseins has also been observed in electron microscopy studies. It has been reported that such submicelles could adsorb at fat-serum interface without becoming disrupted (17,25). 

Huppertz, T., Vaia, B, Smiddy, M.A. Reformation of casein particles from alkaline-disrupted casein micelles. J. Dairy Res. 75, dd—17 (2008)... 

Madadlou, A., Mousavi, M.E., Emam-Djomeh, Z., Sheehan, D., Ehsani, M. Alkaline pH does not disrupt re-assembled casein micelles. Food Chem. 116, 929-932 (2009)... 

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