Methods for separating casein from soluble proteins in a composition. The
method includes: adding a phosphate solution to the composition, mixing
to form a mixture, freezing the mixture to obtain a frozen mixture,
thawing the frozen mixture to obtain casein-containing aggregates in the
mixture, and then centrifuging the mixture to obtain a supernatant
traction and a casein-precipitate fraction. The supernatant fraction
contains the soluble protein, which may be isolated by subjecting the
supernatant fraction to a purification process. The method can be applied
to purify a recombinant protein from the milk of a transgenic mammal.