162 Chapter 7
and Kelly 2004 ). κ - Casein, with just one
phosphoserine, is relatively calcium -
insensitive and when combined with the
calcium - sensitive caseins can stabilize and
protect the calcium - sensitive caseins from
precipitation via formation of colloidal par-
ticles, termed casein micelles.
Caseins are rheomorphic (Holt and Sawyer
1993 ) due to a high content of proline resi-
dues (Farrell et al. 2004 ) and readily self -associate in solution. For example, α (^) s1 - casein
can form polymers via end - to - end associa-
tion of its hydrophobic region (Thurn et al.
1987 ). β - Casein forms spherical particles in
the absence of calcium (Rollema 1992 ),
and κ - casein polymerizes via hydropho-
bic and intermolecular disulphide bonds (Fox
and Kelly 2004 ). Their lack of intrinsic struc-
ture affords stability to the caseins against
denaturants such as heat or urea (Fox and
Kelly 2004 ), and also renders the caseins as
ideal substrates for enzyme action. For
example, caseins are readily hydrolyzed by
proteases, which is important for their digest-
ibility and in cheese ripening (Sousa et al.
2001 ). Hydrolysis of caseins also results in
peptides with various bioactive properties
(Hartmann and Meisel 2007 ).
The Casein Micelle
Casein micelles contain approximately 85%
to 90% of the caseins in milk (Fox and Kelly
2004 ). They exist as porous, spherical, micel-
lar aggregates of 50 to 600 nm in diameter
(average 100 nm) (de Kruif and Holt 2003 ).provided in Table 7.1 , and a select number of
these are discussed in further detail below.
β - Casein is the most hydrophobic casein
and contains a hydrophobic region that encom-
passes two - thirds of the protein in the C -
terminal end (Farrell et al. 2004 ). α (^) s2 - Casein,
the most hydrophilic casein, contains three
distinct hydrophobic domains that are dis-
tributed along the polypeptide (Farrell et al.
2004 ). κ - Casein is the only glycosylated casein
with complex oligosaccharides, comprising
galactose, sialic acid ( N - acetylneuraminic
acid), and galactosamine, which are attached
to Thr residues in the C - terminal region. The
attachment of the oligosaccharides increases
its level of hydrophilicity (Farrell et al. 2004 ).
A unique feature common to all of the
caseins is the presence of phosphate groups.
The phosphate groups occur esterifi ed to Ser
residues, and to a minor extent, to Thr resi-
dues (Fox and Kelly 2004 ). α (^) s2 - Casein is the
most phosphorylated, with 10 to 13 phospho-
serine residues (Farrell et al. 2004 ). α (^) s1 - Casein
and β - casein are extensively phosphorylated,
with eight to ten, and four to fi ve phosphoser-
ine residues, respectively (Farrell et al. 2004 ).
κ - Casein is the least phosphorylated with
only one phosphoserine (Farrell et al. 2004 ).
The phosphate groups bind calcium and tend
to occur in clusters of two to four residues
(Holt 1992 , Swaisgood 2003 ).
Owing to their high levels of phosphoryla-
tion, α (^) s1 - , α (^) s2 - , and β - casein are precipitated
by calcium concentrations greater than
approximately 6 mM at 20 ° C (68 ° F), and are
referred to as calcium - sensitive caseins (Fox
Table 7.1. Properties of the major caseins in bovine milk. Adapted from Eigel et al. (1984) , Fox and
Mulvihill (1983).
Protein % Total
casein
Molecular
weight
Phosphate
residues
(mol/mol
protein)
Proline
residues
(mol/mol
protein)
Hydrophobic regions Sulfhydryl
groups
α (^) s1 - casein 44 – 46 22,068 – 23,724 8 – 10 17 1 – 44, 90 – 113, 132 – 199 –
α (^) s2 - casein 12 25,230 10 – 13 10 90 – 120, 160 – 207 2
β - casein 32 – 35 23,944 – 24,092 4 – 5 35 2/3 of C terminal end –
κ - casein 8 – 12 19,007 – 19,039 1 20 5 – 65, 105 – 115 2