Emulsions, Microemulsions, and Lyotropic Liquid Crystals - Surface and Colloid Chemistry: Principles and Applications - page 206

temperature dependent; and will form a large polymer at 20°C but not at 4°C. Less
sensitive to calcium precipitation.
9.5.3.4 the casein micelle
It was mentioned earlier that some specific molecules, such as soaps or detergents,
aggregate to form micelles. Most, but not all, of the casein proteins exist in a col-
loidal 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. The “casein
submicelle” model has been prominent for the past several years and is illustrated
and described with the following link, but there is no universal acceptance of this
model and mounting research evidence to suggest that there is no defined submicel-
lar structure to the micelle at all. In the submicelle model, it is thought that there
are small aggregates of whole casein, containing 10 to 100 casein molecules called
submicelles. It is thought that there are two different kinds of submicelles: with and
without kappa-casein. These submicelles contain a hydrophobic core and are cov-
ered by a hydrophilic coat that is at least partly composed of the polar moieties of
kappa-casein. The hydrophilic CMP of the kappa-casein exists as a flexible hair.
The open model also suggests there are more dense and less dense regions within
the micelle, but there is less of a well-defined structure. In this model, calcium phos-
phate nanoclusters bind caseins and provide for the differences in density within the
casein micelle.
Colloidal calcium phosphate (CCP) acts as a cement between the hundreds or
even thousands of submicelles that form the casein micelle. Binding may be covalent
or electrostatic. The casein micelles are not static; there are three dynamic equilibria
between the micelle and its surroundings:
The free casein molecules and submicelles
The free submicelles and micelles
The dissolved colloidal calcium and phosphate
The following factors must be considered when assessing the stability of the casein
micelle: The role of Ca ++ is very significant in milk. More than 90% of the calcium
content of skim milk is associated in some way or another with the casein micelle.
The removal of Ca ++ leads to reversible dissociation of β-casein without micellular
disintegration. The addition of Ca ++ leads to aggregation. The same reaction occurs
between the individual caseins in the micelle, but not as much because there is no
secondary structure in casein proteins.
No cysteine residues are found for alpha(s1) and β-caseins do. If any S-S bonds
occur within the micelle, they are not the driving force for stabilization. Caseins are
among the most hydrophobic proteins, and there is some evidence to suggest that
they play a role in the stability of the micelle. It must be remembered that hydropho-
bic interactions are very temperature sensitive.