The first representation of casein micelle structure shown here is that of the "casein sub-micelle" model. It has evolved from several earlier models. It is very difficult to imagine exactly what the micelle looks like. However, scientists do know a great deal about the micelle from its behaviour, and any model needs to account for all of the known facts. This model, which nows dates to about 20 years old, is based on the inclusion of many known facts about casein micelle behaviour. See Walstra and Jenness for discussion of this model.
There is not universal acceptance of the above model among dairy scientists, in fact there is mounting evidence that well-defined casein submicelles do not exist, rather the structure is more open and fluid, perhaps a "bowl-of-spaghetti" type of model. The big problem with the earlier model was the distribution of calcium phosphate, and it is certainly apparent now that calcium phosphate exists more evenly distributed throughout the micelle, so based on the above model, both within and outside the submicelles.
The next model shown here has evolved recently, from work especially by Carl Holt and co-workers at Hannah Research Institute, Scotland. It shows a more or less spherical, highly hydrated, and fairly open particle. Polypeptide chains in the core are partly cross-linked by nanometer sized clusters of Ca phosphate; the internal structure gives rise to an external region of lower segment density known as the hairy layer, which confers steric and/or charge stability to native casein particles. This figure also shows equilibria between the micelle and the milk serum with acidification and with heating. (Thanks to Dr. Carole Tranchant, Ph.D. Disertation, Univ. of Guelph, 1999, for use of this figure). It may be many years yet before out visualization techniques (e.g., electron microscopy) allow us to know the true structure of the casein micelle.
Selected References
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.
![[Dairy Home Page]](dairyicon3.gif)