Dairy Chemistry And Biochemistry

MILK PROTEINS 159

4.4. I Resolution of caseins by electrophoresis

Zonal electrophoresis in starch gels containing 7 M urea was used by Wake

and Baldwin in 1961 to resolve casein into about 20 bands (zones); the two

principal bands were 01,~ - and p-caseins. Incorporation of urea was necessary

to dissociate extensive intermolecular hydrophobic bonding. Electrophoresis

in polyacrylamide gels (PAGE), containing urea or sodium dodecyl sulphate

(SDS), was introduced in 1963; resolution was similar to starch gels (SGE)

but since it is easier to use, PAGE has become the standard technique for

analysis of caseins; a schematic representation of a urea-PAGE elec-

trophoretogram of whole casein is shown in Figure 4.6. Owing to the

presence of intermolecular disulphide bonds, Ic-casein resolves poorly on

-12

Yl

Y3

P

I or

asl -8P

asl -9P

Figure 4.6 Schematic diagram of an electrophoretogram of sodium caseinate in a polyacrylam-
ide gel containing 5 M urea in tris-hydroxymethylamine buffer, pH 8.9. 0 indicates origin.