John Dominic Crossan
Crossan discusses this parable as one of several in the section "A Kingdom of Undesirables" [Historical Jesus] (276-82).
The essential point is "that leaven in the ancient world was a symbol of moral corruption," according to Brandon Scott, since it was "made" by taking a piece of bread and storing it in a damp, dark place until mold forms. The bread rots and decays ... modern yeast ... is domesticated." Furthermore, "in Israel there is an equation that leaven is the unholy everyday, and unleaven the holy, the sacred, the feast" (324). Once again, we are confronted with an image of the Kingdom that is immediately shocking and provocative. And it is compounded by the fact that, again from Scott, "woman as a symbolic structure was associated in Judaism, as in other Mediterranean cultures, with the unclean, the religiously impure. The male was the symbol for purity." Furthermore, "the figurative use of hiding to describe the mixing of leaven and flour is otherwise unattested in Greek or Hebrew" (326). With mustard and darnel, then, stands another and triply shocking image for the Kingdom: a woman hiding leaven in her dough. It's there, it's natural, it's normal, it's necessary, but society has a problem with it.
The Seminar considered this item at both the 1986 sessions: Notre Dame and University of Redlands, CA: