This is the leaven of the parable in Luke 13:21, when a woman takes and hides leaven in a large batch of flour to produce enough bread to feed a whole village. This is what the Kingdom of God is like.

Leaven, which is a fermented dough containing lactobacillus to help make bread rise, is also mentioned in the Bible. The Parable of the Leaven describes what happens when a woman adds leaven to flour.