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Casein (from Latin caseus "cheese") is the most predominant phosphoprotein found in milk and cheese. When coagulated with rennet, casein is sometimes called paracasein. 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 disulphide 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.
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.
Additional recommended knowledge
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.
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, in a recent review it was 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 changes in groups on an off the diet . At this point it is premature to suggest that casein- or gluten-free diets can help children with autism spectrum disorders.
Casein has a molecular structure that is extremely 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.
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.  
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Casein". A list of authors is available in Wikipedia.|