Milk micelles

Dev, B. C., Sood, S. M., DeWind, S., and Slattery, C. W. (1994). K-Gasein and P-cascins in human milk micelles Structural studies. Arch. Biochern. Bkrpfrys. 314,329-336. 

Figure 7. Solid sphere milk micelle model proposed by Waugh and Noble (174)...

Figure 9. Milk micelle model proposed by Parry and Carroll (176) showing...

Figure 10. Milk micelle model of Slattery and Evard (171) containing ca. 40 subunits each comprised of c-, a8-, and (3-caseins.

Rennet casein is somewhat different from acid-precipitated casein and it is a relatively minor industrial product. It is obtained from skim milk by the action of the enzyme rennin (chymosin) around pH 6.6. The enzyme splits up the casein component and this destabilises the milk micelles. This is followed by aggregation and precipitation of calcium caseinate along with the calcium phosphate which is present. 

A useful property of the red seaweed extracts is their abiUty to form gels with water and milk. Kappa-carrageenan reacts with milk protein micelles, particularly kappa-casein micelles. The thickening effect of kappa-carrageenan in milk is 5—10 times greater than it is in water at a concentration of 0.025% in milk, a weak thixotropic gel is formed. 

The butter fat is a coarse dispersion readily removable on standing or by a centrifuging operation. The casein will be present in the skimmed milk as colloidally dispersed micelles of diameter of the order of 10 cm, and is associated with calcium and phosphate ions. 

Destruction of the casein micelles in the milk with subsequent precipitation of the casein can be accomplished in a number of ways. The action of heat or the action of alcohols, acids, salts and the enzyme rennet all bring about precipitation. In commercial practise the two techniques used employ either acid coagulation or rennet coagulation mechanisms. 

Caseins are the major proteins in bovine milk and about 95% of the caseins exist as casein micelles. The structure and properties of casein micelles influence a wide range of technological uses of milk. Light microscopy, SEM, and TEM have been frequently used to study casein... 

Farrell, H. M., Jr., Malin, E. L., Brown, E. M., and Qi, P. X. (2006). Casein micelle structure What can be learned from milk synthesis and structural biology Curr. Opin. Colloid Interface Sci. 11,135-147. 

Horne, D. S. (2009). Casein micelles structure and stability. In "Milk Proteins From Expression to Food", (A. Thompson, M. Boland, and H. Singh, Eds), pp. 133-179. Academic Press, San Diego. 

The elastic modulus (G ) of MP, BCAS, and BLG5 rapidly rose to plateaus that corresponded to different G saturations (Gjat) (Table 2). MP and BCAS coagula showed the more important Gsat value (142 N/m ), meaning that the emulsions stabilized by skim milk proteins (mainly casein micelles) and 6-casein formed the coagula with the strongest protein network. 

Similarly, the protein in milk is very rich in colloidal chemistry. Most of the protein is bound within aggregates called casein micelles (see p. 512). The colloids in milk are essentially stable even at elevated temperatures, so a cup of milky tea, for... 

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 Daily Industiy. The first step in cheese manufacture is the coagulation of milk. Coagulation can be divided into two distinct phases, enzymatic and the non-enzymatic. In the primary enzymatic phase a proteol ic enzyme such as chymosin (rennet), or less effectively, pepsin, carries out an extremely specific and limited proteolysis, cleaving a phenylalanine-methionine bond of /c-casein, making the casein micelle metastabie. In the second, non-enzymatic phase, the... 

The following factors must be considered when assessing the stability of the casein micelle The role of Ca++ is very significant in milk. More than 90% of the calcium content of skim milk is associated in some way or another with the casein micelle. The removal of Ca++ leads to reversible dissociation of P-casein without micellular disintegration. The addition of Ca++ leads to aggregation. The same reaction occurs between the individual caseins in the micelle, but not as much because there is no secondary structure in casein proteins. 

Age gelation is an aggregation phenomenon that affects shelf-stable, sterilized dairy products, such as concentrated milk and UHT milk products. After weeks to months of storage of these products, there is a sudden sharp increase in viscosity, accompanied by visible gelation and irreversible aggregation of the micelles into long chains forming a three-dimensional network. The actual cause and mechanism is not yet clear however, some theories exist. 

The viscosity of milk and milk products is reported to be important in the rate of creaming. The viscosity of milk increases with decrease in temperature because the increased voluminosity of casein micelles temperatures above 65°C increases viscosity due to the denaturation of whey proteins pH an increase or decrease in the pH of milk also causes an increase in casein micelle voluminosity. Fat globules that have undergone cold agglutination may be dispersed due to agitation, causing a decrease in viscosity. 

Nature itself gives us a spectacular example of a biopolymer-based delivery system in the form of the native casein micelle of mammalian milk (Lemay et al, 2007). This is primarily a colloidal delivery system for calcium, where the micronutrient is in the form of calcium phosphate, which does not give a bitter taste, and which provides good bioavailability owing to its colloidal size, amorphous state and quick dissolution in gastric conditions (pH 1-2). Nevertheless, the casein micelle structure is unique there are no other readily available natural delivery systems for most nutraceuticals. Therefore some new designs are clearly required (Velikov and Pelan, 2008 McClements et al, 2008, 2009). 

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