The Gospels according to Matthew and Luke mention the parable of the leaven (Mt:16:6 and Lk 13:20) with a positive image . Elsewhere, Jesus uses the euphemism of leaven to expose the hypocrisy of the Pharisees ( Lk 12:1, Mt 16:6 , Mk 8:15).
Mt 13:2 (".. and great crowds gathered about him...") suggests that Jesus told the parable of leaven during the days of Passover Feast.
In those days, a housewife would set apart a small portion of the leavened dough at the end of the day for leavening dough on the coming day. In a sense, the leaven would get older and older as days went by. Now, Israelites had a tradition of cleansing the home, of all leaven during Passover, through various means including sale of the leaven to non-Jews. Food made of leavened dough was in fact taboo during the days of Feast of Passover.
My question is:
Why did Jesus choose to tell the parable of leaven to explain the Kingdom of Heaven, given the negative connotations associated with leaven in the Jewish tradition?
Was it intended to taunt the scrupulosity of the Pharisees?
What do the interpretations of Catholic Church say on the subject ?