38 Chapter 2
casein and α (^) s1 - casein, which contain no cys-
teine residues, α (^) S2 - casein contains two
cysteine residues. It exists as a dimer in milk.
β - casein consists of 209 amino acids and
is approximately 24 kDa. It is the most hydro-
phobic casein molecule. There are six known
genetic variants that contain between zero
and fi ve phosphoserine units. Similar to α (^) s1 -
casein, β - casein has few secondary structures
due to the presence of 35 proline residues.
γ - casein is derived by hydrolysis of β -
casein by the enzyme plasmin. Three variants
have been identifi ed near the C - terminal end
of the β - casein molecule: which are γ 1 f(29 –
209), γ 2 f(106 – 209/11) and γ 3 f(108 – 209).
Casein micelles consist of a large portion
(approximately 95%) of casein proteins that
interact with each other and calcium. They
vary in size from 80 to 1,000 nm with an
average diameter of 150 nm in bovine milk.
The micelle sizes of sheep and goat milk are
different, and the caprine micelles are less
heat stable than bovine milk micelles (Park
et al., 2007 ).
Bovine casein micelles contain water,
protein (about 94%), and salts (about 6%),
including calcium, phosphorus, magnesium,
citrate (these four ions are known as colloidal
calcium phosphate [CCP]), and other traces
of metals, enzymes (lipases, esterases, prote-
ases), and milk serum.
Several models have been proposed for
the structure of the casein micelles, including
the coat - core model and sub - structure model,
as well as the proposal that proteins exist as
a porous network. The sub - structure model is
the most evolved. It suggests that casein
micelles contain submicelles that range from
12 to 15 nm in diameter and contain approxi-
mately 20 to 25 casein molecules and water
(2 to 5 g/protein) with some submicelles con-
taining K - casein.
Whey Proteins
Whey proteins are hydrophobic, globular,
highly ordered proteins that contain disulfi de
linkages. Unlike casein proteins, whey pro-
36%, and 13% of whole casein, respectively.
Overall, 21 variants of casein have been iden-
tifi ed that occur by genetic mutations such as
the deletion or the substitution of amino
acids.
Caseins have distinct disordered molecu-
lar structures that lack disulfi de bridges, and
therefore they are very heat stable, withstand-
ing temperatures above 140 ° C before disso-
ciating. Their hydrophobicity is due to the
high ratios of apolar amino acids including
valine, leucine, isoleucine, phenylalanine,
and proline (between 35% and 45%).
However, this is counteracted by the high
phosphate content and low concentrations of
sulphur - containing amino acids such as
methionine and cysteine that allow the
caseins to be reasonably water soluble.
Furthermore, their susceptibility to proteoly-
sis is due to the lack of secondary and tertiary
structures, particularly the proteins α (^) s1 - casein
and β - casein.
κ - casein consists of 169 amino acids and
is approximately 19 kDa, and it contains both
glycosylated and phosphorylated residues. It
can exist as a dimer up to a decamer with the
subunits held together by disulfi de linkages.
Unlike the other caseins, it is not sensitive to
calcium and surrounds the micelles, keeping
them intact. It usually contains one phospho-
serine unit; however, genetic variants con-
taining two or three phosphoserine units have
been identifi ed. Further, nine variants have
been identifi ed that demonstrate different
degrees of glycosylation.
α (^) s1 - casein consists of 199 amino acids and
is approximately 23.6 kDa. It has the highest
charge of all the casein molecules. It consists
of at least eight phosphoserine units; however,
the genetic variant α (^) s0 - casein contains nine.
It has 17 proline residues that ultimately
disrupt the formation of secondary structures,
such as α - helices and β - sheets.
α (^) S2 - casein consists of 207 amino acids and
is approximately 25.4 kDa. It is the least
hydrophobic casein molecule. There are
several genetic variants that contain between
10 and 13 phosphoserine units. Unlike β -