Dairy Chemistry And Biochemistry

162 DAIRY CHEMISTRY AND BIOCHEMISTRY

p. 173, the level of glycosylation varies, resulting in 10 molecular forms of

ic-casein.

Genetic polymorphism. In 1956, Aschaffenburg and Drewry discovered that

the whey protein, P-lactoglobulin (0-lg), exists in two forms, A and B, which

differ from each other by only a few amino acids. The milk of any individual

animal may contain p-lg A or B or both, and the milk is indicated as AA,

BB or AB with respect to P-lg. This phenomenon was referred to as genetic

polymorphism and has since been shown to occur in all milk proteins; a total

of about 30 variants have been demonstrated by PAGE. Since PAGE

differentiates on the basis of charge, only polymorphs which differ in charge,

i.e. in which a charged residue is replaced by an uncharged one or vice versa,

will be detected; therefore, it is very likely that many more than^30

polymorphs exist.

The genetic variant present is indicated by a Latin letter, e.g. a,,-CN

A-8P, c(,,-CN B-8P, a,,-CN B-9P, etc.

The frequency with which certain genetic variants occurs is breed-specific,

and hence genetic polymorphism has been useful in the phylogenetic

classification of cattle and other species. Various technologically important

properties of the milk proteins, e.g. cheesemaking properties and the

concentration of protein in milk, are correlated (linked) with specific

polymorphs and significant research is ongoing on this subject. The genetic

polymorphism of milk proteins has been comprehensively reviewed by

Ng-Kwai-Hang and Grosclaude (1992) and Jakob and Puhan (1992).

4.4.3 Nomenclature of the caseins
During studies on casein fractionation, especially during the 1960s, various
names were assigned to isolated fractions. To rationalize the nomenclature
of milk proteins, the American Dairy Science Association established a
Nomenclature Committee which published its first report in 1956 (Jenness
er al., 1956); the report has been revised regularly (Brunner et al., 1960;
Thompson et al., 1965; Rose et al., 1970; Whitney et al., 1976; Eigel et al.,
1984). An example of the recommended nomenclature is x,,-CN A-SP,
where a,,-CN is the gene product, A is the genetic variant and 8P is the
number of phosphate residues. The Committee recommends that in situ-
ations where confusion may arise through the use of a Greek letter alone,
the relative electrophoretic mobility be given in brackets, thus a,,-CN
A-12P (1.00). The heterogeneity and nomenclature of the caseins in bovine
milk is summarized in Figure 4.8.
In addition to simplifying and standardizing the nomenclature of the milk
proteins, the characteristics of the various caseins and whey proteins are
summarized in the above articles, which are very valuable references.