Casein

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Casein (/ˈks.ɪn/ or /ˈkˌsn/, 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.[1] 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.[2] As a food source, casein supplies amino acids, carbohydrates, and the two inorganic elements calcium and phosphorus.[3]

Composition[edit]

Casein contains a fairly high number of proline residues, 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 only limited resemblance with surfactant-type micellae in a sense that the hydrophilic parts reside at the surface and they are spherical. However, in sharp contrast to surfactant micelles, the interior of a casein micelle is highly hydrated. The caseins in the micelles are held together by calcium ions and hydrophobic interactions. Any of several molecular models could account for the special conformation of casein in the micelles.[4] One of them proposes the micellar nucleus is formed by several submicelles, the periphery consisting of microvellosities of κ-casein.[5][6] Another model suggests the nucleus is formed by casein-interlinked fibrils.[7] Finally, the most recent model[8] 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.

The enzyme trypsin can hydrolyze off a phosphate-containing peptone. It is used to form a type of organic adhesive.[9]

Uses[edit]

Paint[edit]

Casein preparation in an old etching operation in Müllheim

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.[10][11] It is still widely used by scene painters, although acrylic has made inroads in that field as well.[12]

Glue[edit]

Casein-based glues, formulated from casein, water, hydrated lime and sodium hydroxide were popular for woodworking, including for aircraft, as late as the de Havilland Albatross airliner.[13][14] Casein glue is also used in transformer manufacturing (specifically transformer board) due to its oil permeability.[15] While largely replaced by synthetic resins, casein-based glues still have a use in certain niche applications, such as laminating fireproof doors and the labeling of bottles.[13][16][17][18]

Cheesemaking[edit]

Cheesemaking

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.[19]

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 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.

Plastics and fiber/fibre[edit]

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.

Protein supplements[edit]

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.[20] Often casein is available as hydrolyzed casein, whereby it is hydrolysed by a protease such as trypsin. Hydrolysed forms are noted to taste bitter and such supplements are often refused by infants and lab animals in favour of intact casein.[21]

Medical and dental uses[edit]

Casein-derived compounds are used in tooth remineralization products to stabilize amorphous calcium phosphate (ACP) and release the ACP onto tooth surfaces, where it can facilitate remineralization.[22][23]

Controversies[edit]

Autism[edit]

Although research has shown high rates of use of complementary and alternative therapies for children with autism, including gluten and/or casein exclusion diets, as of 2008 there was a lack of evidence for the efficacy of these diets.[24]

A1/A2 beta caseins in milk[edit]

Main article: A2 milk

A1 and A2 beta-casein are genetic variants of the beta-casein milk protein that differ by one amino acid; a proline occurs at position 67 in the chain of amino acids that make up the A2 beta-casein, while in A1 beta-casein a histidine occurs at that position.[25][26] Due to the way that beta-casein interacts with enzymes found in the digestive system, A1 and A2 are processed differently by digestive enzymes, and a seven-amino peptide, beta-casomorphin-7, (BCM-7) can be released by digestion of A1-beta-casein.[25]

The A1 beta-casein type is the most common type found in cow's milk in Europe (excluding France), the USA, Australia and New Zealand.[27]

Interest in the distinction between A1 and A2 beta-casein proteins began in the early 1990s via epidemiological research and animal studies initially conducted by scientists in New Zealand, which found correlations between the prevalence of milk with A1 beta-casein proteins and various chronic diseases.[25] The research generated interest in the media, among some in the scientific community, and entrepreneurs.[25] A company, A2 Corporation, was founded in New Zealand in the early 2000s to commercialize the test and market "A2 Milk" as a premium milk that is healthier due to the lack of peptides from A1.[25] A2 Milk even petitioned the Food Standards Australia New Zealand regulatory authority to require a health warning on ordinary milk.[25]

Responding to public interest, the marketing of A2 milk, and the scientific evidence that had been published, the European Food Safety Authority (EFSA) reviewed the scientific literature and published a review in 2009 that found no relationship between chronic diseases and drinking milk with the A1 protein.[27] An independent review published in 2005 had found the same thing.[25] Both studies emphasized the dangers of drawing conclusions from correlations identified in epidemiological studies and the dangers of not reviewing all the evidence at hand.[25][27]

Casein allergy[edit]

A small fraction of the population is allergic to casein.[28][29]

The China Study on cancer[edit]

The China Study[30] was a large-scale human study done with the support of the Chinese government and provincial governments across China. It concluded that populations with greater than 10% of the protein in their diet from casein were at risk of cancer due to casein acting as a cancer promoter. Neal D. Barnard focuses on the addictive nature of milk products and opiate products associated with A1 milk in particular, and notes that "The problem with milk is not simply its casein—that's the part that produces the casomorphin opiates. The nutrient 'package' in milk—loads of sugar (lactose), animal protein, and fat—triggers the production of IGF-I in the body, and that may be the reason it is linked to certain forms of cancer."[31]

The book was criticized by blogger Denise Minger[32] in 2010, and due to the large response to her post, Minger received a response from Campbell soon after. Campbell concluded his counter-critique with: "I should conclude by noting the suggestion of the professional epidemiologist, cited above, who suggested that ultimately Denise may wish to publish her findings in a peer-reviewed journal but who presently felt strongly that the current version would not be accepted. I concur."[33]

See also[edit]

References[edit]

  1. ^ Kunz, C; Lonnerdal, B (1990). "Human-milk proteins: analysis of casein and casein subunits...". American Journal of Clinical Nutrition (The American Society for Clinical Nutrition) 51 (1): 37–46. PMID 1688683. Retrieved 14 January 2011. 
  2. ^ "Industrial Casein", National Casein Company
  3. ^ "casein". Sixth Edition. Columbia University. 2011  |chapter= ignored (help)
  4. ^ Dalgleish DG (1 November 1998). "Casein micelles as colloids. Surface structures and stabilities". Journal of Dairy Science 81 (11): 3013–8. doi:10.3168/jds.S0022-0302(98)75865-5. 
  5. ^ Walstra, Pieter (1979). "The voluminosity of bovine casein micelles and some of its implications". Journal of Dairy Research 46: 317–323. doi:10.1017/S0022029900017234. ISSN 1469-7629. PMID 469060. 
  6. ^ Lucey JA (1 February 2002). "Formation and Physical Properties of Milk Protein Gels". Journal of Dairy Science 85 (2): 281–94. doi:10.3168/jds.S0022-0302(02)74078-2. PMID 11913691. 
  7. ^ Holt C (1992). "Structure and stability of bovine casein micelles". Adv Protein Chem. 43: 63–151. doi:10.1016/S0065-3233(08)60554-9. PMID 1442324. 
  8. ^ Horne DS (March 1998). "Casein interactions: Casting light on the black boxes, the structure in dairy products". Int Dairy J. 8 (3): 171–7. doi:10.1016/S0958-6946(98)00040-5. 
  9. ^ "CCMR – Ask A Scientist!". Ccmr.cornell.edu. 1998-09-24. Retrieved 2011-09-29. 
  10. ^ Reader's Digest Crafts & Hobbies edited by Daniel Weiss, Susan Chace. 1979, page 223
  11. ^ The Grove Encyclopedia of Materials and Techniques in Art By Gerald W. R. Ward. Oxford University Press, 2008. page 2
  12. ^ Scenic Design And Lighting Techniques: A Basic Guide for Theatre By Chuck B. Gloman, Rob Napoli. Focal Press. 2006. pages 281–282
  13. ^ a b http://www.fpl.fs.fed.us/documnts/fplrn/fplrn158.pdf
  14. ^ http://www.soyinfocenter.com/HSS/if_laucks_and_soybean_glue.php
  15. ^ http://www.weidmann-electrical.com/en/markets-a-products/board/cellulose-based/laminated-board
  16. ^ Arthur A. Tracto. Coatings Materials And Surface Coatings. CRC Press. 2006. pages 19–7 to 19–11
  17. ^ Robert S. Forsyth Waterborne Adhesives for Bottle Labeling
  18. ^ Label Glues
  19. ^ Fankhauser, David B. (2007). "Fankhauser's Cheese Page". Retrieved 2007-09-23. 
  20. ^ Boirie Y, et al. Slow and fast dietary proteins differently modulate postprandial protein accretion" Proc Natl Acad Sci U S A 23;94(26) 14930-5.
  21. ^ Field KL, Kimball BA, Mennella JA, Beauchamp GK, Bachmanov AA. (2008). "Avoidance of hydrolyzed casein by mice". Physiol Behav 28 (93): 189–99. PMID 17900635. 
  22. ^ Louis Malcmacher. "Enamel Remineralization: The Medical Model of Practicing Dentistry". Dentistry Today. 
  23. ^ Glenn Walker, Fan Cai, Peiyan Shen, Coralie Reynolds, Brent Ward, Christopher Fone, Shuji Honda, Megumi Koganei, Munehiro Oda and Eric Reynolds (2006). "Increased remineralization of tooth enamel by milk containing added casein phosphopeptide-amorphous calcium phosphate". Journal of Dairy Research 73 (1): 74–78. doi:10.1017/S0022029905001482. PMID 16433964. 
  24. ^ Millward C, Ferriter M, Calver S, Connell-Jones G. Gluten- and casein-free diets for autistic spectrum disorder" Cochrane Database of Systematic Reviews 2008, Issue 2. Art. No.: CD003498. doi:10.1002/14651858.CD003498.pub3.
  25. ^ a b c d e f g h Truswell, A.S. (2005), "The A2 milk case: a critical review", European Journal of Clinical Nutrition 59: 623–631, doi:10.1038/sj.ejcn.1602104, retrieved 29 June 2014 
  26. ^ Truswell, AS (2006). "Reply: The A2 milk case: a critical review". European Journal of Clinical Nutrition 60: 924–925. doi:10.1038/sj.ejcn.1602454. Retrieved 30 July 2014. 
  27. ^ a b c 1 February 2009, EFSA review of scientific literature on A1 and A2 milk, Review of the potential health impact of β-casomorphins and related peptides
  28. ^ Solinas C et al. Cow's milk protein allergy. J Matern Fetal Neonatal Med. 2010 Oct;23 Suppl 3:76-9. doi: 10.3109/14767058.2010.512103. PMID 20836734
  29. ^ Mayo Clinic: Milk Allergy
  30. ^ Campbell, T.C. and Campbell, T.M. 2006. The China Study. Benbella Books
  31. ^ Dr. Neal Barnard 2004, Breaking the Food Seduction, St Martin's Griffin
  32. ^ The China Study: Fact or Fallacy?
  33. ^ Colin Campbell's Response to Critic Denise Minger

Further reading[edit]

External links[edit]