I am sure I have heard people discuss the significance of unleavened bread in the scriptures. But the Lord states his meaning plainly enough.
Deuteronomy 16:3 Thou shalt eat no leavened bread with it; seven days shalt thou eat unleavened bread therewith, even the bread of affliction; for thou camest forth out of the land of Egypt in haste: that thou mayest remember the day when thou camest forth out of the land of Egypt all the days of thy life.
Unleavened bread was the only bread that could be eaten on the week of the passoever, which is what the new testament is referring to when it speaks of the "feast of unleavened bread".
While Christ does make other uses of leaven as a symbol, he is clearly not interpreting its use in the law of Moses when he does so. He uses leaven as a symbol as the occasion demands, both using it as a symbol for the Kingdom of Heaven (which a woman hid in a measure of flour but in the end it transforms the whole loaf) as well as for the doctrine of the Scribes and the Pharisees (a little of which transforms the whole loaf).