Pepperdine Bible Lectures 2016 - N.T. Wright (Part 1) 

Leaven is not a symbol of sin. The Passover? Israel was indeed commanded to remove all the yeast from the house. But here we simply do not read the Scriptures very closely. The yeast/leaven is not removed, according to Exodus, because it is unclean. The unleaven bread symbolizes the HASTE in which Israel left. They were in such a hurry there was not time to even make bread. In the Passover it is the bitter herbs that remind Israel of the misery and suffering of slavery in Egypt. The bitter herbs might be considered a negative image . There are sacrifices in which leaven is excluded as well but the rational is never given. On the other side of the coin, leaven/yeast is commanded to be used in thanksgiving /fellowship offerings to Yahweh. The leavened bread is offered to the Lord and eaten (Lev 7.13ff). "With your thanksgiving of well-being you shall bring your offering with cakes of leavened bread." This is one of the most common offerings in the Hebrew Bible. The regulations for the the Feast of First Fruits/Weeks/Pentecost is very different than Passover on the matter of leaven/yeast. No requirement for avoiding yeast is to be found anywhere. Such would have been impractical at best. Grapes and barely harvest are pretty difficult to not have yeast. Grapes naturally have yeast that live on the skin (and if we think leaven is microbes, you would think God knew about those invisible things on the grape) and begin to make wine the instant the skin is broken and barley is used in the beverage known as beer. There was a reason the disciples could have been accused of being slightly intoxicated on Pentecost, because "leaven/yeast" is not excluded from Pentecost. But without any of that, leaven/yeast is simply commanded. "You shall bring your from your home two loaves of bread as an elevation offering, each made with two-tenths of an ephah; they shall be of choice flour, baked with yeast/leaven, as first fruits to the LORD …" (Lev 23.17) This leavened bread is offered along with the leavened drink offering (wine), you invite the priest and the poor and then eat and drink in the presence of the Lord. I add that the shewbread used in the daily offerings in the Temple were made out of leavened bread. Clearly not unclean. The Lord Jesus compares the kingdom of God to leaven, surely he is not casting negativity on the kingdom. "He told them another parable: 'The kingdom of heaven is like yeast/leaven that a woman took and mixed with three measures of flour until it was all leavened" (Mt 13.33) "And again he said, 'To what should I compare the kingdom of God? It is like yeast/leaven that a woman took and mixed in with three measures of flour until all of it was leavened." (Lk 13.20-21). Yeast/leaven is another one of the wonderful examples of what simply assuming can do.