The Bible speaks of leaven as something that "puffs up" things, and there are things in our lives and hearts that can "puff us up" too, including vanity, greed, wicked thoughts, and more. The preparations for this special time calls the Jews to meditate on what may be puffing up their lives, while they physically remove "puffing agents" and puffy baked goods from their cupboards.
Fermentation (leavening) also talks about false doctrine. The Lord said to His disciples "Beware of the leaven (false doctrine) of the Pharisees" (Matthew 16:6; Mark 8:15).
Finally, yeast/leaven represents decay, i.e. the grave.
That was a great reply. I would like to add that many Christian sects and individuals also practice the week of unleavened bread.
It is humbling to think you have your pantry and house cleaned, then a few days later, find a cheese cracker or gold fish. Or, sometime during the week, carelessly be eating a danish that was brought into the office break room.
It really can help you focus on how easy it is to sin anytime throughout the year. It is like teaching a child to count by using counting blocks.
Sourdough is produced by natural yeasts that are in the air, isn’t it? Would sourdough be considered evil?