Composition
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. 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.
The enzyme trypsin can hydrolyze off a phosphate-containing peptone. It is used to form a type of organic adhesive.
Read more about this topic: Casein
Famous quotes containing the word composition:
“Give a scientist a problem and he will probably provide a solution; historians and sociologists, by contrast, can offer only opinions. Ask a dozen chemists the composition of an organic compound such as methane, and within a short time all twelve will have come up with the same solution of CH4. Ask, however, a dozen economists or sociologists to provide policies to reduce unemployment or the level of crime and twelve widely differing opinions are likely to be offered.”
—Derek Gjertsen, British scientist, author. Science and Philosophy: Past and Present, ch. 3, Penguin (1989)
“If I don’t write to empty my mind, I go mad. As to that regular, uninterrupted love of writing ... I do not understand it. I feel it as a torture, which I must get rid of, but never as a pleasure. On the contrary, I think composition a great pain.”
—George Gordon Noel Byron (1788–1824)
“Modern Western thought will pass into history and be incorporated in it, will have its influence and its place, just as our body will pass into the composition of grass, of sheep, of cutlets, and of men. We do not like that kind of immortality, but what is to be done about it?”
—Alexander Herzen (1812–1870)