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 cow 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 188.8.131.52) 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 accountfor the special conformation of casein in the micelles (Dalgleish, 1998). One of them proposes that the micellarnucleus 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 totake place. All 3 models consider micelles as colloidal particles formed by casein aggregates wrapped up insoluble 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.
Casein has been reported to reduce tooth decay  .
An attractive property of the casein micelle is its ability to form a gel or clot in the stomach . The ability to form this clot makes it very efficient in nutrient supply. The clot is able to provide a sustained slow release of amino acids into the blood stream, sometimes lasting for several hours . This provides better nitrogen retention and utilization by the body.
Casein has been documented to break down to produce the peptide casomorphin, an opioid that appears to act primarily as a histamine releaser. Casomorphine was once suspected by some sources to aggravate the symptoms of autism, however, a 2006 review concluded that there was no benefit from the use of elimination diets (e.g., 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." Because individuals suggested that the diet was beneficial while the study showed the opposite, this suggests that individuals are more likely to believe in medical hoaxes when they appear to be a change from the past.
Casein has a molecular structure that is quite similar to that of gluten. Thus, some gluten-free diets are combined with casein-free diets and referred to as a gluten-free, casein-free diet. 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.
A study of Charité Hospital in Berlin showed that adding milk to tea causes the casein to bind to the molecules in tea that cause the arteries to relax, especially a catechin molecule called EGCG. 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.