General Properties of Casein
The four main types of casein comprise approximately 80% of the total protein in bovine milk. The remaining protein components are termed whey proteins;
β-lactoglobulin: ~ 10%
Serum albumin: ~ 1%
Immunoglobulins: ~ 2%
Other Proteins: ~ 2%
In milk, caseins exist as the calcium salt arranged in micellar particles surrounded by soluble kappa-casein. The casein component of milk is relatively heat-stable, capable of surviving pasturization at ~62-71 °C. Conversely the whey protein component is denatured at these temperatures.
In general, caseins have limited α-helix and β-sheet secondary structure. They tend to be high in proline content and have very few disulfide bonds.
Casein solubility is pH dependent and is also affected by ionic strength and composition. Addition of sodium chloride will affect solubility differently depending on when it is added to the solution during pH adjustment.
J. Dairy Sci., 77, 1218, 1994.
Four different types of bovine casein exist, each with several genetic variants.
||g protein/L in skim milk
|J. Dairy Sci., 67, 1599-1631, 1984, and from Table 1, J. Dairy Sci., 68, 2195-2205, 1985.
(NOTE: genetic variations and variable phosphorylation account for some of the ranges given.)
||α-s1 Casein is the most prevalent form of casein in bovine milk. It has been reported to exhibit antioxidant and radical scavenging properties.1 It has also been reported to be involved in the transport of and casein from the endoplasmic reticulum to the Golgi apparatus.2
||Proteolytic fragments of α-s2 Casein have been shown to exhibit antibacterial activity. Specifically the 39 amino acid casocidin-1 peptide fragment has been shown to inhibit E. coli and Staph. carnosis growth.3
||β-Casein and its fragments have been implicated in a number of biological functions. The casoparan peptide has been reported to activate macrophage phagocytosis and peroxide release. Casohypotensin and casoparan may be involved in bradykinin regulation. Casohypotensin has also been shown to be a strong inhibitor of endo-oligopeptidase A, a thiol-activated protease capable of degrading bradykinin and neurotensin, and hydrolyzing enkephalin-containing peptides to produce enkephalins. β-Caseins are also a source of casomorphin peptides which exhibit opioid activity binding to opioid receptors. Casomorphins may be the hydrolysis product of dipeptidyl peptidase IV.4
||κ-Casein's orientation on the surface of the casein micelle functions as an interface between the hydrophobic interior caseins and the aqueous environment. During clotting of milk, hydrolysis by chymosin or rennin releases the water soluble fragment, para-k-casein and the hydrophobic caseinomacropeptide.
Casoxins A, B and C have opioid antagonist activity. Casoxin C binds to the complement C3a receptors. Casoplatelin inhibits platelet aggregation.5
- Kitts, D. D., Trends in Food Science & Technology, 18, 549-554 (2005)
- Chanat, E., J. Cell Sci., 112, 3399-3412 (1999)
- Zucht, H. D., FEBS Lett., 371, 185-188 (1995)
- Miyamoto, Y., et al., Am. J. Physiol. Renal. Physiol., 252, 670-F677 (1987)
- Lawrence K., Creamer, L. K., et al., Journal of Dairy Science, 81, 3004-3012 (1998)
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