438 Part IV: Milk
- Surface activity.Caseinates, whey protein
concentrates, and isolates. - Rheology.All protein-containing dairy
products. - Water sorption:Most dairy products,
comminuted meat products.
Milk proteins have been studied extensively and
are very well characterized at molecular and func-
tional levels (for reviews see Fox and McSweeney
2003).
HETEROGENEITY OFMILKPROTEINS
It has been known since 1830 that milk contains two
types of protein, which can be separated by acidifica-
tion to what we now know as pH 4.6. The proteins
insoluble at pH 4.6 are called caseins and represent
about 78% of the total nitrogen in bovine milk; the
soluble proteins are called whey or serum proteins.
As early as 1885, it was shown that there are two
types of whey protein, globulins and albumins, which
were thought to be transferred directly from the
blood (the proteins of blood and whey have generally
similar physicochemical properties and are classified
as albumins and globulins). Initially, the term casein
was not restricted to the acid-insoluble proteins in
milk but was used to describe all acid-insoluble pro-
teins; however, it was recognized at an early stage
that the caseins are unique milk-specific proteins.
The casein fraction of milk protein was consid-
ered initially to be homogeneous, but from 1918
onwards, evidence began to accumulate that it is het-
erogeneous. Through the application of free bound-
ary electrophoresis (FBE) in the 1930s and zone elec-
trophoresis in the 1960s, in starch or polyacrylamide
gels (SGE, PAGE) containing urea and a reducing
agent, it has been shown that casein is in fact very
heterogeneous. Bovine casein consists of four fami-
lies of caseins: s1, s2, , and , which represent
about 38, 10, 36, and 12%, respectively, of whole
casein. Urea-PAGE showed that each of the casein
families exhibits microheterogeneity due to
- Genetic polymorphism, usually involving
substitution of one or two amino acids; - Variations in the degree of phosphorylation;
- Varations in the degree of glycosylation of -
casein; - Inter-molecular disulphide bond formation in
s2- and -caseins; and- Limited proteolysis, especially of - and as s2-
caseins, by plasmin; the resulting peptides
include the - and -caseins and proteose
peptones.
In the 1930s, FBE showed that both the globulin
and albumin fractions of whey protein are heteroge-
neous, and in the 1950s, the principal constituents
were isolated and characterized. It is now known
that the whey protein fraction of bovine milk com-
prises four main proteins:-lactoglobulin (-lg),-
lactalbumin (-la), immunoglobulins (Igs), and 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 nonpro-
tein nitrogen and trace amounts of several proteins,
including about 60 indigenous enzymes. About 1%
of total milk protein is part of the MFGM, including
many enzymes.-lg and-la are synthesized in the
mammary gland and are milk specific; they exhibit
genetic polymorphism (in fact the genetic polymor-
phism of milk proteins was first demonstrated for-
lg in 1956). BSA, most of the Ig (IgG), and most of
the minor proteins are transferred from the blood.
Methods for the isolation of the individual pro-
teins were developed and gradually improved during
the period 1950–1970, so that by about 1970 all the
principal milk proteins had been purified to homo-
geneity.
The concentration of total protein in milk is af-
fected by most of the factors that affect the concen-
tration 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 days postpartum, mainly due to the
decrease in Ig from approximately 10% in the first
colostrum to 0.1% within about 1 week. The concen-
tration of total protein continues to decline more
slowly thereafter, to a minimum after about 4 weeks,
and then it increases until the end of lactation. Data
on variations in the groups of proteins 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.
- Limited proteolysis, especially of - and as s2-
MOLECULARPROPERTIES OFMILKPROTEINSThe six principal milk-specific proteins have been
isolated and are very well characterized at the mo-