Because if there are 2 nouns used with together, second noun must be 3rd possesion form. Kaşar ( a type of the cheese ) peynir + i (his/her/its) On the other hand, second one has "adjective + noun" structure: Beyaz (white) peynir (cheese)

As I said, it is a type of milk which is made from sheep milk. So, it is a noun.