Definitions

casein-glue

Casein

[key-seen, -see-in, key-seen]
See Casein paint for information about casein usage in artistic painting.

Casein (from Latin caseus "cheese") is the predominant phosphoprotein (αS1, αS2, β, κ) that accounts for nearly 80% of proteins in milk and cheese. Milk-clotting proteases act on the soluble portion of the caseins, K-Casein, thus originating an unstable micellar state that results in clot formation. When coagulated with rennet, casein is sometimes called paracasein. Chymosin (EC 3.4.23.4) is an aspartic protease that specifically hydrolyzes the peptide bond in Phe105-Met106 of κ-casein and is considered to be the most efficient protease for the cheese-making industry (Rao et al., 1998). British terminology, on the other hand, uses the term caseinogen for the uncoagulated protein and casein for the coagulated protein. As it exists in milk, it is a salt of calcium. Casein is not coagulated by heat. It is precipitated by acids and by rennet enzymes, a proteolytic enzyme typically obtained from the stomachs of calves. The enzyme trypsin can hydrolyze off a phosphate-containing peptone.

Casein consists of a fairly high number of proline peptides, which do not interact. There are also no disulfide bridges. As a result, it has relatively little secondary structure or tertiary structure. Because of this, it cannot denature. It is relatively hydrophobic, making it poorly soluble in water. It is found in milk as a suspension of particles called casein micelles which show some resemblance with surfactant-type micellae in a sense that the hydrophilic parts reside at the surface. The caseins in the micelles are held together by calcium ions and hydrophobic interactions. There are several models that account for the special conformation of casein in the micelles (Dalgleish, 1998). One of them proposes that the micellar nucleus is formed by several submicelles, the periphery consisting of microvellosities of κ-casein (Walstra, 1979; Lucey, 2002). Another model suggests that the nucleus is formed by casein-interlinked fibrils (Holt, 1992). Finally, the most recent model (Horne, 1998) proposes a double link among the caseins for gelling to take place. All 3 models consider micelles as colloidal particles formed by casein aggregates wrapped up in soluble κ-casein molecules.

The isoelectric point of casein is 4.6. The purified protein is water insoluble. While it is also insoluble in neutral salt solutions, it is readily dispersible in dilute alkalis and in salt solutions such as sodium oxalate and sodium acetate.

Applications

In addition to being consumed in milk, casein is used in the manufacture of adhesives, binders, protective coatings, plastics (such as for knife handles and knitting needles), fabrics, food additives and many other products. It is commonly used by bodybuilders as a slow-digesting source of amino acids as opposed to the fast-digesting whey protein, and also as an extremely high source of glutamine (post-workout). Another reason it is used in bodybuilding is because of its anti-catabolic effect, meaning that casein consumption inhibits protein breakdown in the body. Casein is frequently found in otherwise nondairy cheese substitutes to improve consistency, especially when melted. An enzymatic hydrolysate of casein to its individual amino acids, called "NZ-Amine" is commonly used as a constituent of agar plates in molecular biology or to supplement the protein content of foods. Purified casein is also frequently used as a blocking agent in Western blotting and ELISA protocols.

Controversy

Opioid

Casein has been documented to break down to produce the peptide casomorphin, an opioid that appears to act primarily as a histamine releaser. Casomorphine is suspected by some sources to aggravate the symptoms of autism However, a 2006 review concluded that insufficient evidence existed to support the use of elimination diets (i.e., casein or gluten free) in the treatment of autism spectrum disorders. More importantly preliminary data from the first and only double-blind randomized control trial - reported in the Journal of autism and developmental disorders - of a gluten- and casein-free diet showed "no statistically significant findings even though several parents reported improvement in their children.

Casein-free diet

Casein has a molecular structure that is quite similar to that of gluten. Thus, most gluten-free diets are combined with casein-free diets and referred to as a gluten-free, casein-free diet. Casein may also be a trigger of migraines and other types of headaches. Casein may also be linked to the promotion of cancer and other diseases which was discovered in the 1980s by nutrition and health researcher, Dr. T. Colin Campbell, author of The China Study. Casein is often listed as sodium caseinate, calcium caseinate or milk protein. These are often found in energy bars, drinks as well as packaged goods.

Blocking positive effects of tea

A study of Charité Hospital in Berlin showed that adding milk to tea will block some of the normal, healthful effects that tea has in protecting against cardiovascular disease (Lorenz 2007). It does this because casein from the milk binds to the molecules in tea that cause the arteries to relax, especially a catechin molecule called EGCG. One of the researchers told New Scientist magazine that "[i]t probably also blocks tea's effect on other things, such as cancer. However a similar study by Reddy et. al. (2005) suggests that the addition of milk to tea does not alter the antioxidant activity in vivo and the cardiovascular effect remains controversial.

Notes

Sources

  • Dalgleish, D. G. 1998. Casein micelles as colloids. Surface structures and stabilities. J. Dairy Sci. 81:3013–3018.
  • Green, V., et al. 2006. "Internet Survey of Treatments Used by Parents of Children with Autism." Research in Developmental Disabilities. 27 (1):70-84
  • Holt, C. 1992. Structure and stability of bovine casein micelles. Adv.Protein Chem. 43:63–151.
  • Horne, D. S. 1998. Casein interactions: Casting light on the black boxes, the structure in dairy products. Int. Dairy J. 8:171–177.
  • Lucarelli, S., et al. 1995. "Food allergy and infantile autism." Panminerva Med. 37(3):137-141.
  • Lucey, J. A. 2002. ADSA Foundation Scholar Award Formation and Physical Properties of Milk Protein Gels. J. Dairy Sci. 85:281–294.
  • Lorenz, M., et al. 2007. "Addition of milk prevents vascular protective effects of tea." European Heart Journal (DOI: 10.1093/eurheartj/ehl442) (PMID: 17213230)
  • Rao, M. B., A. M. Tanksale, M. S. Ghatge, and V. V. Deshpande. 1998. Molecular and biotechnological aspects of microbial proteases. Microbiol. Mol. Biol. Rev. 62:597–635.
  • Walstra, P. 1979. The voluminosity of bovine casein micelles and some of its implications. J. Dairy Res. 46:317–322.

See also

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