Food Biochemistry and Food Processing (2 edition)

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24 Chemistry and Biochemistry of Milk Constituents 453

blood serum albumin (BSA), which represent about 40%, 20%,

10% and 10%, respectively, of the total whey protein in mature

milk. The remaining 10% is mainly non-protein nitrogen and

trace amounts of several proteins, includingca. 60 indigenous.

Sixty indigenous enzymes. About 1% of total milk protein is

part of the MFGM, including many enzymes.β-Lg andα-La

are synthesised in the mammary gland and are milk-specific;

they exhibit genetic polymorphism (in fact, the genetic poly-

morphism of milk proteins was first demonstrated forβ-Lg in

1956). BSA, most of the Ig (i.e., IgG) and most of the minor

proteins are transferred from the blood.

Methods for the isolation of the individual proteins were de-

veloped and gradually improved during the period 1950–1970,

so that by around 1970 all the principal milk proteins had been

purified to homogeneity.

The concentration of total protein in milk is affected by most

of the factors that affect the concentration of fat, that is breed,

individuality, nutritional status, health and stage of lactation but,

with the exception of the last, the magnitude of most effects is

less than in the case of milk fat. The concentration of protein

in milk decreases very markedly during the first few dayspost-

partum, mainly due to the decrease in Ig from approximately

10% in the first colostrum to 0.1% within about 1 week. The

concentration of total protein continues to decline more slowly

thereafter, to a minimum after about 4 weeks and then increases

until the end of lactation. Data on variations in the groups of pro-

teins throughout lactation have been published (see Mehra et al.

1999) but there are few data on variations in the concentrations

of the individual principal proteins.

Molecular Properties of Milk Proteins

The six principal milk-specific proteins have been isolated and

are very well characterised at the molecular level; their chemical

composition is summarised in Table 24.2. The most notable

features of the principal milk-specific proteins are discussed

later.

They are quite small molecules, approximately 15–25 kDa.

All the caseins are phosphorylated but to different and vari-

able degrees, 1–13 mol P/mol protein; the phosphate groups are

esterified as monoesters of serine residues. The primary struc-

tures of the caseins have a rather uneven distribution of polar

and non-polar residues along their sequences. Clustering of the

phosphoseryl residues is particularly marked, resulting in strong

anionic patches inαs1-,αs2-andβ-caseins.κ-Casein does not

have a phosphoseryl cluster but theN-terminal two-thirds of the

molecule is quite hydrophobic while itsC-terminal is relatively

hydrophilic – it contains no aromatic and no cationic residues.

These features give the caseins an amphipathic structure, making

them very surface-active, with good emulsifying and foaming

properties. The amphipathic structure ofκ-casein is particularly

significant and is largely responsible for its micelle-stabilising

properties. The distribution of amino acids inβ-Lg andα-La is

quite random.

The two principal caseins,αs1-andβ-caseins, are devoid

of cysteine or cystine residues; the two minor caseins,αs2-

andκ-caseins, contain two inter-molecular disulphide bonds.

β-Lg contains two intra-molecular disulphide bonds and one

sulphydryl group that is buried and unreactive in the native pro-

tein but becomes exposed and reactive when the molecule is

denatured; it reactsviasulphydryl-disulphide interactions with

other proteins, especiallyκ-casein, with major consequences

for many important properties of the milk protein system, espe-

cially heat stability and cheese making properties.α-La has four

intra-molecular disulphide bonds.

All the caseins, especiallyβ-casein, contain a high level of

proline (inβ-casein, 17 of the 209 residues are proline), which

disruptsα-andβ-structures; consequently, the caseins are rather

unstructured molecules and are readily susceptible to proteol-

ysis, which is as would be expected for proteins the putative

function of which is as a source of amino acids for the neonate.

However, theoretical calculations suggest that the caseins may

have a considerable level of secondary and tertiary structure;

to explain the differences between the experimental and theo-

retical indices of higher structures, it has been suggested that

the caseins are very mobile, flexible, rheomorphic molecules

(i.e., the casein molecules, in solution, are sufficiently flexible

to adopt structures that are dictated by their environment; Holt

and Sawyer 1993). In contrast, the whey proteins are highly com-

pact and structured, with high levels ofα-helices,β-sheets and

β-turns. Inβ-Lg, theβ-sheets are in an anti-parallel arrangement

and form aβ-barrel calyx.

Table 24.2.Characteristics of the Principal Proteins in Bovine Milk

Amino Acid Residues

Protein

Molecular

Weight Total Proline Cysteine

Phosphate

Groups

Concentration

(g/L) Genetic Variants

αs1-casein 23,164 199 17 0 8 10.0 A, B, C, D, E, F, G, H

αs2-casein 25,388 207 10 2 10–13 2.6 A, B, C, D

β-casein 23,983 209 35 0 5 9.3 A^1 ,A^2 .A^3 ,B,C,D,E,F,G

κ-casein 19,038 169 20 2 1 3.3 A, B, C, D, E, FS,FI,GS,H,I,J

β-lactoglobulin 18,277 162 8 5 0 3.2 A, B, C, D, E, F, G, H, I, J

α-lactalbumin 14,175 123 2 8 0 1.2 A, B, C

Source: Adapted from Fox 2003.