Structure: The Casein Micelle
Most, but not all, of the casein proteins exist in a colloidal particle known as the casein micelle. Its biological function is to carry large amounts of highly insoluble CaP to mammalian young in liquid form and to form a clot in the stomach for more efficient nutrition. Besides casein protein, calcium and phosphate, the micelle also contains citrate, minor ions, lipase and plasmin enzymes, and entrapped milk serum. These micelles are rather porous structures, occupying about 4 mL/g and 6-12% of the total volume fraction of milk. Size ranges from 50-250 nm in diameter.
Casein micelle image from Dalgleish, D. G., P. Spagnuolo and H. D. Goff. 2004. A possible structure of the casein micelle based on high-resolution field-emission scanning electron microscopy. International Dairy Journal. 14: 1025-1031. This micelle is 120 nm in diameter.
There have been many models developed over the years to explain the structure of the casein micelle, based on all of the information available about its composition and reactivity. The casein sub-micelle model was prominent for many years, but there is sufficient evidence now to conclude that there is not a defined sub-micellar structure to the micelle at all. More recent models suggest a more open structure comprised of aggregates of protein around calcium phosphate nanoclusters. Each of the casein proteins has unique abilitites to either bind with CaP or with other caseins, which gie rise to the aggregates. The nanoclusters provide regions of more or less density. The structure is sufficiently porous to hold a considrable amount of water, and for the surface, and even part of the interior, to be reactive to other substances. All models agree that the k-casein is mostly present as a stabilizng layer around the exterior of the micelle. Please see any of the following references for great detail about micelle structures and models.
Holt, C. & D. S. Horne. 1996. The hairy casein micelle: evolution of the concept and its implication for dairy technology. Neth. Milk Dairy J. 50: 85-111.
Horne, D. S. 1998. Casein interactions: casting light on the black boxes, the structure in dairy products. Internat. Dairy J. 8: 171-177.
Walstra, P. 1999. Casein sub-micelles: do they exist? Internat. Dairy J. 9: 189-192.
Horne, D. S. 2002. Casein structure, self-assembly and gelation. Current Opinion in Colloid and Interface Sci. 7: 456-461.
Dalgleish, D. G. 2011. On the structural models of bovine casein micelles - review and possible improvements. Soft Matter. 7: 2265-2272.
Dalgleish, Douglas G. and Milena Corredig. 2012. The Structure of the Casein Micelle of Milk and Its Changes During Processing. Annual Reviews Food Sci. Technol. 3:449–67.
de Kruif , Cornelis G., Thom Huppertz, Volker S. Urban and Andrei V. Petukhov. 2012. Casein micelles and their internal structure. Advances in Colloid and Interface Science 171–172: 36–52.