Food Biochemistry and Food Processing (2 edition)

BLBS102-c24 BLBS102-Simpson March 21, 2012 13:47 Trim: 276mm X 219mm Printer Name: Yet to Come

454 Part 4: Milk

The caseins are often regarded as very hydrophobic proteins

but they are not particularly so; however, they do have a high

surface hydrophobicity, owing to their open structures. Because

of their hydrophobic patches they have a high propensity to yield

bitter hydrolysates, even in cheese that undergoes relatively little

proteolysis. In contrast, the highly structured whey proteins are

very resistant to proteolysis in the native state and may traverse

the intestinal tract of the neonate intact.

About 60% ofκ-casein molecules are glycosylated. The

sugar moieties present are galactose, galactosamine andN-

acetylneuraminic acid (sialic acid), which occur as trisaccha-

rides or tetrasaccharides. The oligosaccharides are attached to

the polypeptideviathreonine residues in theC-terminal region

of the molecule and vary from 0 to 4 mol/mol of protein.

Probably because of their rather open structures, the caseins

are extremely heat stable, for example sodium caseinate can be

heated at 140◦C for 1 hour without obvious physical effects. The

more highly structured whey proteins are comparatively heat-

labile, although compared to many other globular proteins, they

are quite heat-stable; they are completely denatured on heating

at 90◦C for 10 minutes.

Under the ionic conditions in milk,α-La exists as monomers

of molecular weight (MW) approximately 14.7 kDa.β-Lg exists

as dimers (MW∼36 kDa) in the pH range 5.5–7.5; at pH values

<3.5 or>7.5, it exists as monomers, while at pH 3.5–5.5, it

exists as octamers. The caseins have a very strong tendency to

associate. Even in sodium caseinate, the most soluble form of

whole casein, the proteins exist mainly as decamers or larger ag-

gregates at 20◦C. At low concentrations, for example<0.6%, at

4 ◦C,β-casein exists as monomers but associates to form micelles

if the concentration or temperature is increased. In the presence

of Ca, for example, calcium caseinate, casein forms large ag-

gregates of several million Daltons. In milk, the caseins exist as

very complex structures, known as casein micelles, which are

described in Section ‘Casein Micelles’.

The function of the caseins appears to be to supply amino

acids to the neonate. They have no biological functionstricto

sensubut their Ca-binding properties enable a high concentra-

tion of calcium phosphate to be carried in milk in a ‘soluble’

form; the concentration of calcium phosphate far exceeds its sol-

ubility and, without the ‘solubilising’ influence of casein, would

precipitate in the ducts of the mammary gland and cause atopic

milk stones.

β-Lg has a hydrophobic pocket within which it can bind

small hydrophobic molecules. It binds and protects retinolin

vitroand perhaps functions as a retinol carrierin vivo.Inthe

intestine, it exchanges retinol with a retinol-binding protein. It

also binds fatty acids and thereby stimulates lipase; this is per-

haps its principal biological function.β-Lg is a member of the

lipocalin family, which now includes 14 proteins (for review see

Akerstrom et al. 2000); all members of the lipocalin family have

some form of binding function.

α-La is a metalloprotein; it binds one Ca atom per molecule in

a peptide loop containing four aspartic acid residues. The apo-

protein is quite heat-labile but the metalloprotein is rather heat-

stable; when studied by differential scanning calorimetry,α-La

is in fact observed to be quite heat-labile. The metallo-protein

renatures on cooling, whereas the apo-protein does not. The dif-

ference in heat stability between the halo- and apo-protein may

be exploited in the isolation ofα-La on a large (i.e., potentially

industrial) scale.

As discussed in Section ‘Lactose’,α-La is an enzyme mod-

ifier protein in the lactose synthesis pathway; it makes UDP-

galactosyl transferase highly specific for glucose as an acceptor

for galactose, resulting in the synthesis of lactose.

Interspecies Comparison of Milk Proteins

The milk of all species that have been studies contain caseins

and whey proteins, but the protein systems differ in detail:

 The total concentration of protein ranges from approxi-

mately 1% in human milk and that of the other great apes to

approximately 20% for lagomorphs; the protein content is

directly related to the growth rate of the neonate.


 The ratio of casein to whey proteins ranges from approxi-

mately 80:20 in bovine, buffalo, ovine and caprine milk to

approximately 30:70 for human milk.


 The types and proportions of the caseins and whey proteins

show considerable inter-species differences. The milk of

all species studied seems to containα-,β- andκ- caseins

but human milk and that of some other primates contains

very littleα-casein and equine milk contains littleκ-casein.

The milk of humans and that of many others, but not all,

primates and rodents lacksβ-Lg. The principal protein in

human milk isα-La but the milk of some seals lacks this

protein. The milk of some species contains a protein called

acid whey protein, which is absent from the milk of many

species, including cow, buffalo, sheep and goat. There are

particularly large inter-species differences with respect to

many minor proteins, including enzymes.


 All individual milk proteins show inter-species differences

with respect to amino acid composition and perhaps other

features. An interspecies comparison of the principal lacto-

proteins of several species has been made by Martin et al.

(2003, 2011).

Casein Micelles

Three of the caseins,αs1-,αs2-andβ-, which together represent

approximately 85% of total casein, are precipitated by calcium

at concentrations>6 mM at temperatures> 20 ◦C. Since milk

contains approximately 30 mmol/L Ca, it would be expected

that most of the caseins would precipitate in milk. However,κ-

casein is soluble at high concentrations of Ca and it reacts with

and stabilises the Ca-sensitive caseins through the formation of

casein micelles.

Since the stability, or perhaps instability, of the casein micelles

is responsible for many of the unique properties and processing

characteristics of milk, their structure and properties have been

studied intensively. It has been known for nearly 200 years that

the casein in milk exists as large colloidal particles which are re-

tained by porcelain-Chamberlain filters. During the early part of

the twentieth century, there was interest in explaining the rennet