- See Casein paint for information about casein usage in artistic painting.
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.
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). Casein is frequently found in otherwise nondairy cheese substitutes to improve consistency, especially when melted.
Possible Cancer Link
Casein has been implicated very strongly as a carcinogenic compound according to The China Study by T. Colin Campbell. Also mentioned is the incidence of higher cancer rates in countries that consume more dairy products, specifically cheese, which has more than 10 times the casein density of milk. The book overviews many previous studies of the effects of animal fats, and builds a strong case for the possibility of a correlation between a diet containing excessive amounts of fat and the development of cancer. The authors admit freely that the book is controversial, but maintain that they are only presenting the conclusions of the studies.   Others have noted that the studies discussed in the The China Study do not conclude what Campbell claims and caution, "Campbell consistently presents only half the story -- at best -- through the duration of the book." 
Casein has been documented to break down in the stomach 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 .
A study found that children with Autism placed on a casein-free diet for eight weeks showed significant behavior improvements (Lucarelli 1995). In many cases, casein free diets are combined with gluten-free diets and are 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.1 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. The calcium in milk also binds with calcium oxylate molecules found in tea, and may work to prevent kidney stones caused by heavy tea drinking. One of the researchers told New Scientist magazine that "[i]t probably also blocks tea's effect on other things, such as cancer."2
- Green, V., et al. 2006. "Internet Survey of Treatments Used by Parents of Children with Autism." Research in Developmental Disabilities. 27 (1):70-84
- Lucarelli, S., et al. 1995. "Food allergy and infantile autism." Panminerva Med. 37(3):137-141.
- Lorenz, M., et al. 2007. "Addition of milk prevents vascular protective effects of tea." European Heart Journal (DOI: 10.1093/eurheartj/ehl442)
- Healing Thresholds summarizes scientific evidence on casein-free diets and other therapies for autism
- GFCF Diet Support Group
- Food Allergy and Anaphylaxis Network
- Caseins at the US National Library of Medicine Medical Subject Headings (MeSH)