自動詞のturnで補語に名詞を取るときその名詞は無冠詞になるという規則があります。名詞の形容詞用法と考えればわかりやすいかもしれません。たとえば Prime minister Yasuo FukudaのPrime Ministerは名詞ですが、Yusuo Fukudaの形容詞として無冠詞で用いられています。
John started out a music student before he turned linguist.
A series of product mislabeling scandals has probably made the managers of many food makers very much worried about the possibility that some of their employees turn whisle-blower.
Whistle-blower is an employee who reports his/her employer's violation of a law.