Casein, Caseinates, and Milk Protein Concentrates 163the caseins and calcium phosphate as the
framework of the casein micelle structure. In
the latter model calcium phosphate is in the
form of nanoclusters, which interacts with
the caseins via the phosphoseryl clusters of
the calcium - sensitive caseins (Holt 1992 ,
Holt et al. 2003 ). More recently, an inter-
locked lattice model has been proposed in
which polymeric casein aggregates and
casein - calcium phosphate aggregates con-
tribute to casein micelle integrity (McMahon
and Oommen 2008 ). The polymeric casein
aggregates interact via hydrophobic and elec-
trostatic interactions and calcium bridging.
These aggregates are believed to be inter-
locked within the casein - stabilized calcium
phosphate nanoclusters.
Although the structural details of the
casein micelle still remain elusive, it is gener-
ally accepted that κ - casein is located primar-
ily, if not completely, on the micellar surface
(Holt and Horne 1996 ). The hydrophilic
region of κ - casein protrudes into the sur-
rounding serum to give a “ hairy ” layer (5 to
10 nm thick) that provides steric and electro-
static stabilization (de Kruif and Zhulina
1996 ). Removal of the hairy layer by rennet
or its collapse by acidifi cation causes desta-
bilization of the casein micelle (de Kruif
1999 , Holt and Horne 1996 ).
The casein micelles are largely responsi-
ble for many of the technologically important
physicochemical properties of traditional and
new dairy products. Dried, frozen, and heated
products, for example, rely on the ability of
the casein micelles to retain their structural
identity during concentration (e.g., ultracen-
trifugation, evaporation), freezing, homoge-
nization, heat, and most other standard dairy
processing operations (Fox and Kelly 2004 ).
However, the casein micelle is also condu-
cive to controlled destabilization routes (e.g.,
high pressure, acidifi cation to pH 4.6, hydro-
lysis of κ - casein, alcohol, anionic detergents)
(Fox and Kelly 2004 ), which are exploited
in the manufacture of cheese and fermented
dairy products.A typical casein micelle contains approxi-
mately 10^4 individual casein molecules (Fox
and Kelly 2004 ). The calcium - binding prop-
erties of the caseins and their organization
into casein micelles facilitates a high concen-
tration of calcium phosphate to be carried by
the caseins, which would otherwise precipi-
tate and block the mammary glands. It is not
surprising then that the main physiological
function of the casein micelle system is to
prevent pathological calcifi cation of the
mammary gland (de Kruif and Holt 2003 ).
The casein micelles are extensively
hydrated, with approximately 3.4 g water/g
dry matter (Morris et al. 2000 ). Approximately
15% of the water is bound to the protein (de
Kruif and Holt 2003 , Farrell et al. 2003 ) and
the remaining water is occluded within the
casein micelle. On a dry basis the micelles
comprise 94% protein and 6% colloidal
calcium phosphate (CCP), which is essen-
tially a mixture of calcium and phosphate
with small quantities of magnesium, citrate,
and trace metals (e.g., zinc) (McGann et al.
1983 ). Removal of CCP by acidifi cation or
calcium chelating agents disperses the
micelles into smaller particles; hence, CCP
plays an integral role in the structure of the
casein micelles (de Kruif and Holt 2003 ).
Protein - protein interactions including elec-
trostatic and hydrophobic interactions and
hydrogen bonding also appear to be involved
in maintaining casein micelle integrity (de
Kruif and Holt 2003 , Horne 1998 ).
Various models for the casein micelle
structure have been suggested and refi ned
over the last 50 years. Recent critical reviews
of the various models are available in the
literature (for example, see Fox 2003 , Fox
and Kelly 2004 , Horne 2006 , Phadungath
2004 , Qi 2007 ). Selected models are described
here.
The dual binding model emphasizes the
role of hydrophobic interactions between
casein molecules in maintaining the integrity
of the casein micelle (Horne 1998 ). An alter-
nate model attributes interactions between