Straw has that utterly negative connotation of being something that must be discarded. separating the wheat from the straw means to separate what is to be immortal, what is going to survive, from the unwanted portion which is to be burned away.
Von Franz establishes straw as the definitive symbol of psychic and spiritual dross—the futile superficiality that alchemical and biblical tradition alike demands be burned away in the process of transformation.
, Archetypal Patterns in Fairy Tales, 1997thesis