Chemical, Physical, and Functional Characteristics of Dairy Ingredients 39They provide offspring with protection
against pathogenic microorganisms and their
toxins, and the mammary gland against infec-
tion. Approximately 0.7 to 1 mg/ml is present
in bovine milk. The basic structural unit of
the immunoglobulins is similar, consisting of
two heavy and two light chains joined
together by disulfi de bonds. The molecule is
approximately 180 kDa in size.
Immunoglobulin G consists of one struc-
tural unit and is the main immunoglobulin in
milk. Immunoglobulin M is a pentamer con-
taining fi ve structural units, which are linked
by the junction component, and it has an
approximate molecular weight of 900 kDa.
Immunoglobulin A is a dimer made up of two
structural units linked by the secretory com-
ponent and junction component.
Bovine serum albumin consists of 582
amino acids and it is the longest protein. It is
approximately 66 kDa and is predominantly
composed of α - helices. It makes up approxi-
mately 1% to 5% of total whey protein. It is
synthesized in the liver and enters the milk
via secretory cells.
Proteose peptones are derived from the
hydrolysis of β - casein. They are considered
whey proteins because they elute in the whey
fraction when isolated from milk. Three vari-
ants have been identifi ed beta - casein f(1 –
105/7), f(1 – 28), and f(29 – 105/7). Proteose
peptones are heat stable and acid - soluble
proteins that are mainly responsible for the
foaming of skim milk. They inhibit rancid-
ity and have an immunological role (Fox,
2003 ).
Lactoferrin is a globular glycoprotein that
is approximately 74 kDa and binds to iron
(Fe) as it contains two metal binding sites.
Bovine milk contains approximately 20 to
200 mg/L and human milk contains 2 g/L.
Several other minor whey proteins exist
in milk including growth factors, vitamin -
binding proteins (folate, vitamin D, ribo-
fl avin, and vitamin B 12 ), angiogenins, and
osteopontin.
Non - protein nitrogen compounds includ-
ing urea, uric acid, creatine, creatinine, andteins have well - developed secondary, ter-
tiary, and quaternary structures and poorer
heat stability because they denature at tem-
peratures greater than 75 ° C.
The two principal whey proteins in milk
are α - lactalbumin and β - lactoglobulin, which
are synthesized in the mammary gland. They
constitute approximately 20% and 40% of
total whey protein in bovine milk, respec-
tively. Other whey proteins are proteose pep-
tones, immunoglobulins, and serum albumin.
α - lactalbumin is a spherical, glycosylated,
compactly folded calcium metalloprotein
that consists of approximately 142 amino
acids and is approximately 14 kDa. It is syn-
thesized and secreted by the mammary gland,
contains four disulfi de bonds and eight cys-
teine residues, and is rich in tryptophan.
Three genetic variants have been identifi ed.
It is the principal protein in human milk.
β - lactoglobulin consists of 178 amino
acids with an approximate molecular weight
of 18 kDa. It exists in both the monomeric
and dimeric form at equilibrium in bovine
milk; however, its association depends on
temperature, pH, protein concentration, and
ionic conditions. The hydrophobic dimeric
form linked by one to three disulfi de bonds
is approximately 36 kDa. The genetic vari-
ants, A and B, are predominantly isolated
from bovine milk; however, overall fi ve
genetic variants have been identifi ed.
Higher concentrations of β - lactoglobulin
are present in bovine milk when compared
with human milk. Furthermore, it has better
heat stability than α - lactalbumin due to the
presence of one free sulphohydryl unit. It
contains an open β - barrel enclosing a hydro-
phobic cleft and a single three - turn α - helix.
It binds to several hydrophobic molecules
including retinol and fatty acids via the
hydrophobic cleft, which in turn stimulates
lipase activity.
Immunoglobulins are antibodies that are
synthesized in response to specifi c antigens.
They are large, heterogeneous molecules
found in the blood. The main immunoglobu-
lins in milk are IgG, IgG2, IgA, and IgM.