Food Biochemistry and Food Processing

444 Part IV: Milk

• The turbidity of skim milk increases on addition
  of sodium dodecyl sulphate (SDS) up to 21 mM,
  at which concentration a gel is formed; however,
  at 28 mM (0.8%, w/v) the micelles disperse
  (see Lefebvre-Cases et al. 2001a,b).

There have been few studies on variations in mi-
celle size throughout lactation and these have failed
to show consistent trends. No studies on variability
in the microstructure of the casein micelle have been
published. We are not aware of studies on the effect
of the nutritional or health status of the animal on
the structure of the casein micelles, although their
stability and behavior are strongly dependent on pH,
milk salts, and whey proteins, which are affected by
the health and nutritional status of the cow.

MINORPROTEINS

In addition to the caseins and the two principal whey
proteins, milk contains several proteins at low or
trace levels. Many of these minor proteins are bio-
logically active (see Schrezenmeir et al. 2000);
some are regarded as highly significant and have
attracted considerable attention as neutraceuticals.
When ways of increasing the value of milk proteins
are discussed, the focus is usually on these minor
proteins, but they are, in fact, of little economic val-
ue to the overall dairy industry. They are found
mainly in the whey, but some are also located in the
fat globule membrane. Reviews on the minor pro-
teins include those by Fox and Flynn (1992) and
Haggarty (2002).

Immunoglobulins

Colostrum contains approximately 10% (w/v)
immunoglobulins (Igs), but this level declines rapid-
ly to approximately 0.1% about 5 days postpartum.
IgG1 is the principal Ig in bovine, caprine, or ovine
milk, with lesser amounts of IgG2, IgA, and IgM;
IgA is the principal Ig in human milk. The cow,
sheep, and goat do not transfer Ig to the fetus in
utero, and the neonate is born without Ig in its blood
serum; consequently, it is very susceptible to bacter-
ial infection, with a very high risk of mortality. The
young of these species can absorb Ig from the intes-
tine for several days after birth and thereby acquire
passive immunity until they synthesize their own Ig,
within a few weeks of birth. The human mother

transfers Ig in utero, and the offspring is born with a

broad spectrum of antibodies. Although the human

baby cannot absorb Ig from the intestine, the inges-

tion of colostrum is still very important because the

Igs it contains prevent intestinal infection. Some

species, for example, the horse, transfer Ig both in

utero and via colostrum.

The modern dairy cow produces colostrum far in

excess of the requirements of its calf. Therefore, sur-

plus colostrum is available for the recovery of Ig and

other nutriceuticals (Paakanen and Aalto 1997).

There is also considerable interest in hyperimmuniz-

ing cows against certain human pathogens, for ex-

ample, rota virus, for the production of antibody-

rich milk for human consumption, especially by

infants; the Ig could be isolated from the milk and

presented as a “pharmaceutical” or consumed di-

rectly in the milk.

Bovine Serum Albumin (BSA)

About 1–2% of the protein in bovine milk is BSA,

which enters from the blood by leakage through

intercellular junctions. As befits its physiological

importance, BSA is very well characterized (see

Carter and Ho 1994). BSA has no known biological

function in milk, and considering its very low con-

centration, it probably has no technological signifi-

cance either.

Metal-Binding Proteins

Milk contains several metal-binding proteins: the

caseins (Ca, Mg, PO 4 ) are quantitatively the most

important; others are -la (Ca), xanthine oxidase

(Fe, Mo), alkaline phosphatase (Zn, Mg), lactoper-

oxidase (Fe), catalase (Fe), glutathione peroxidase

(Se), lactoferrin (Fe), and seroferrin (Fe).

Lactoferrin (Lf), a nonheme iron-binding glyco-

protein (see Lonnerdal 2003), is a member of a fam-

ily of iron-binding proteins, which includes serofer-

rin and ovotransferrin (conalbumin). It is present in

several body fluids, including saliva, tears, sweat,

and semen. Lf has several potential biological func-

tions: it improves the bioavailability of Fe and has

bacteriostatic (by sequestering Fe and making it un-

available to intestinal bacteria), antioxidant, antivi-

ral, anti-inflammatory, immunomodulatory, and anti-

carcinogenic activity. Human milk contains a much