Casein association

All the major caseins associate with themselves and with each other. In unreduced form, sr-casein is present largely as disulphide-linked polymers. ic-Casein also forms hydrogen and hydrophobic bonds with itself and other caseins but these secondary associations have not been studied in detail. 

The major caseins interact with each other and, in the presence of Ca , these associations lead to the formation of casein micelles. 

---

Rollema, H.S. (1992). Casein association and micelle formation. In Fox, P.F. (Ed.). Advanced Dairy Chemistry, Vol. 1 Proteins, London Elsevier Applied Science, pp. 111-140. 

Kolar, C. K. and Brunner, J. R. 1970. Proteose-peptone fraction of bovine milk Lacteal serum components 5- and 8-casein-associated glycoproteins. J. Dairy Sci. 53, 997-1008. 

However, temperature and pH strongly affect casein association and cause changes in micellular structure (cf. 10.1.2.1.2 and 10.1.2.1.3). An example of such a change is the pH-dependent heat coagulation of skim milk. The coagulation temperature drops with decreasing pH (Fig. 10.16 and 10.9). Salt concentration also has an influence, e. g., the heat stability of milk decreases with a rise in the content of free calcium. 

There are no universally accepted definitions of substitute dairy foods, which are referred to as imitations, simulates, substitutes, analogues, and mimics and are associated with terms such as filled, nondairy, vegetable nondairy, and artificial milk, cheese, etc. The term nondairy has been used indiscriminately to describe both imitation dairy products and products legally defined as not being imitation dairy products. Dairy substitutes can be divided into three types those in which an animal or vegetable fat has been substituted for milk fat those that contain a milk component, eg, casein [9000-71-9] or whey protein and those that contain no milk components (see Milk and milkproducts). The first two types make up most of the substitute dairy products. 

Soybean Protein Isolates. Soybean protein isolates, having a protein content of >90 wt%, are the only vegetable proteins that are widely used in imitation dairy products (1). Most isolates are derived from isoelectric precipitation, so that the soybean protein isolates have properties that are similar to those of casein. They are insoluble at thek isoelectric point, have a relatively high proportion of hydrophobic amino acid residues, and are calcium-sensitive. They differ from casein in that they are heat-denaturable and thus heat-labile. The proteins have relatively good nutritional properties and have been increasingly used as a principal source of protein. A main deterrent to use has been the beany flavor associated with the product. Use is expected to increase in part because of lower cost as compared to caseinates. There has been much research to develop improved soybean protein isolates. 

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. 

Casein may be considered to be a conjugated protein, that is the protein is associated in nature with certain non-protein matter known as prosthetic groups. In the case of casein the prosthetic group is phosphoric acid. The protein molecule is also associated in some way with calcium. The presence of these inorganic materials has an important bearing on the processability and subsequent use of casein polymers. 

Although acid caseins are employed for a number of purposes, rennet caseins in which the protein remains associated with calcium and phosphate are preferred for plastics applications. Rennet is the dried extract of rennin, obtained from the inner lining of the fourth stomach of calves, and is a very powerful coagulant. As little as 0.2 parts per million are said to be sufficient to coagulate slightly acidic milk. Its coagulating power is destroyed at 100°C. 

FIGURE 6.10 Three characteristic structures of pressure-treated casein micelles representative AFM images together with the associated size-histograms are shown. The solid lines are fit to Gauss distributions. (A) Intact micelles, P < SO MPa (B) compact reconstituted micelles, 120 MPa < P < 240 MPa (C) mini-micelles, P > 280 MPa. Reprinted with permission from Gebhardt et al. (2006). 

All the described properties of such a s-fraction of poly(NVCl-co-NVIAz) synthesized at the temperature above the PST of the reacting system allowed us to draw the conclusion that the chains of this type had the comonomer sequence, which at the temperatures above the conformation transition facilitated the formation of polymer particles, where H-blocks are in the interior shielded by the P-blocks against additional intermolecular association. Such a behaviour of this copolymer in aqueous media is close to that of oligomeric proteins similar to casein [46] possessing a rather hydrophobic core surrounded by the polar segments. 

Around the same time as bioinformatics revealed similarities between CSN, proteasome, and eIF3, Dubiel and coworkers, while trying to identify new components of the 26S proteasome, identified a novel protein complex that possessed protein kinase activity. They found that the new protein complex was capable of phosphor-ylating c-jun and IkKBq and the precursor of NFk-B was called pl05. Other protein kinases are found to interact with CSN as well. For example, inositol 1,3,4,-trisphosphate 5/6 kinase, casein kinase 2, and protein kinase D associate with CSN. " ... 

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. 

---