A New Yorker editor writing in the Times Literary Supplement debates a grammar textbook writer! Loads of fun. I personally think the “which” in the sentence under scrutiny should be “that.” It clearly refers to the “sourness” and “relentlessness,” and yes, these are appositives, and yes, the point following “which” is essential to the meaning of the sentence. Do you agree?
Aren’t words a hoot?
Just Can't Help Writing 