Consecutive Nouns
When it describes a bovine quadruped, cow is always a noun, but nouns can be used to modify other nouns, and here the noun cow modifies the noun milk. Cow milk has to be seen as a lexical whole, which in turn is modified by the adjective pure. There is thus no more need for a comma after pure than there would be if cow was absent.
If, however, milk was modified by two adjectives, then they would be separated by a comma, as in pure, creamy milk.