Yeast is used symbolically in the Bible to represent influence, often negatively. Just as a little bit of yeast, permeates dough and produces a noticeable result, so does influence, be it good or bad.
In the Books of Moses, there are thirty-two references to not using yeast (and more for it’s counterpart, leaven) in various religious practices, which symbolically shows the removal of sin. The theme continues in Judges 6:19, I Samuel 28:24, and Ezekiel 45:21. (Leviticus 7:13 and 23:17 are two exceptions, when yeast should be used.)
Another negative connotation is when Jesus says to beware of the yeast of the Pharisees, which is hypocrisy (Luke 12:1), as well as the yeast of Sadducees (Matthew 16:6-12) and the yeast of Herod (Mark 8:14-15).
Another reference to yeast is in 1 Corinthians 5:6-8, where Paul uses yeast as a metaphor for boasting. Paul says to get rid of bread with old yeast (malice and wickedness) and use new bread without yeast (sincerity and truth).