Casein (or, from Latin caseus, "cheese") is the name for a family of related phosphoproteins (αS1, αS2, β, κ). These proteins are commonly found in mammalian milk, making up 80% of the proteins in cow milk and between 20% and 45% of the proteins in human milk. Casein has a wide variety of uses, from being a major component of cheese, to use as a food additive, to a binder for safety matches. As a food source, casein supplies amino acids; carbohydrates; and two inorganic elements, calcium and phosphorus.
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 tertiary structure. 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. Several models account for the special conformation of casein in the micelles (Dalgleish, 1998). One of them proposes the micellar nucleus is formed by several submicelles, the periphery consisting of microvellosities of κ-casein (Walstra, 1979; Lucey, 2002). Another model suggests 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 three 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. Since milk's pH is 6.6, casein has a negative charge in milk. 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.
Human beta-casein is the major protein found in human milk. The protein binds to calcium at its phosphorylated regions, which in turn are highly conserved. The calcium then binds the caseins together and forms micelles, which better enable it to be ingested by infants. It also has an opiod type effects on newborn sleeping patterns. The protein, as a whole, is disordered and is characterized as a random coil protein. This structure can be modified to some extent with an increase of temperature.
Casein paint is a fast-drying, water-soluble medium used by artists. Casein paint has been used since ancient Egyptian times as a form of tempera paint, and was widely used by commercial illustrators as the material of choice until the late 1960s when, with the advent of acrylic paint, casein became less popular.
Cheese consists of proteins and fat from milk, usually the milk of cows, buffalo, goats, or sheep. It is produced by coagulation of casein. Typically, the milk is acidified and then coagulated by the addition of rennet, containing a proteolytic enzyme, typically obtained from the stomachs of calves. The solids are separated and pressed into final form.
Unlike many proteins, casein is not coagulated by heat. During the process of clotting, milk-clotting proteases act on the soluble portion of the caseins, κ-casein, thus originating an unstable micellar state that results in clot formation. When coagulated with chymosin, casein is sometimes called paracasein. Chymosin (EC 22.214.171.124) 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.
Some of the earliest plastics were based on casein. In particular, galalith was well known for use in buttons. Fiber can be made from extruded casein. Lanital, a fabric made from casein fiber (known as Aralac in the United States), was particularly popular in Italy during the 1930s. Recent innovations such as QMilch are offering a more refined use of the fiber for modern fabrics.
An attractive property of the casein molecule is its ability to form a gel or clot in the stomach, which 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 use by the body. Plasma immunoreactive IGF-1 concentration in rats given a casein diet was higher than that in rats given a soya-bean-protein or protein-free diet. Because of its slow digesting properties, casein is widely used between meals and before bed to help bodybuilders maintain muscle in an anabolic state.
Casein has been documented to break down to produce the peptide casomorphin, an opioid that appears to act primarily as a histamine releaser. Some research indicates that this casomorphine aggravates the symptoms of autism. A 2006 review of seven studies indicated that, although benefits were seen in all studies from the introduction of elimination diets (e.g., casein-free or gluten-free) in the treatment of autism spectrum disorders, none of the studies were performed in a manner to create an unbiased scientific opinion. Preliminary data from the first and only double-blind randomized control trial of a gluten- and casein-free diet "indicated no statistically significant findings even though several parents reported improvement in their children." Although research has shown high rates of use of complementary and alternative therapies (CAM) for children with autism, including gluten and/or casein exclusion diets, the evidence for efficacy of these diets is currently unsubstantiated.
Four casein proteins make up about 80% of the protein in cow's milk. One of the major caseins is beta-casein, of which there are several types, but "A1" and "A2" are the most common. According to Food Standards Australia New Zealand (FSANZ), certain breeds of cows, such as Friesians, produce mostly A1 milk, whereas other breeds, such as Guernseys, as well as sheep and goats, produce mostly A2 milk.
FSANZ also reports despite some hypotheses that consumption of A2 milk "provides levels of protection to consumers from autism in children as well as schizophrenia, diabetes and heart disease", the scientific evidence for such claims is "very limited". Additionally, the European Food Safety Authority carried out a literature review in 2009 concluding "a cause and effect relationship is not established between the dietary intake of BCM7, related peptides or their possible protein precursors and non-communicable diseases" (see A2 milk). Studies supporting these claims have had significant flaws, and the data are inadequate to guide autism treatment recommendations.
T. Colin Campbell's The China Study (2005), a book about one of the largest nutritional studies ever conducted, describes a direct correlation between casein administered to rats and the promotion of cancer cell growth when exposed to carcinogens. Aflatoxin (a potent carcinogen) was administered to these rats over a 2 week dosing period. The rats were given a 1 week post-dosing period before beginning the test (promotion period). During the promotion period, one group of rats was put on a 5% casein protein diet and another group on a 20% casein protein diet. None of the rats on 5% casein protein developed foci, precursors to cancerous cell growth, and every rat on 20% casein protein developed the pre-cancer foci. It should be noted that all test groups were fed a 20% casein diet for a total of 5 weeks (2-wk acclimation, 2-wk dosing, 1-wk post-dosing) prior to the 12 week promotion period in order to survive the initial aflatoxin B1 (AFB1) dosing, regardless of whether they were in the 5% or 20% test groups.
Other studies conducted by Dr. Campbell on humans confirmed this correlation between the amount of casein protein consumed and the promotion of cancerous cell growth. Dr. Campbell claims to have found a correlation between cancer growth and the amount of casein protein in diet. A 2001 study suggests another milk protein, whey protein, but not casein, may play a protective role against colon tumors in rats. According to a study from the Australian Dairy Council, casein has antimutagenic effects.
See main article: Gluten-free, casein-free diet. Casein has a molecular structure 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, and packaged goods. A small fraction of the population is allergic to casein.
A study of Charité Hospital in Berlin by Lorenzo et al., published in The European Heart Journal, showed 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, although a more recent study by Reddy et al. (2005) suggests the addition of milk to tea does not alter the antioxidant activity in vivo, and the cardiovascular effect remains controversial. A study published in the journal Free Radical Biology and Medicine indicates casein reduced the peak plasma levels of beneficial polyphenols after the consumption of blueberries and urination.
Caseinate salts have been shown to reduce the bioavailability of some drugs, including phenytoin. A patient on this drug should consult their doctor about altering their diet so that the efficacy of the drug is not diminished.