In Matthew 13 Jesus talks about leaven.

https://enduringword.com/bible-commentary/matthew-13/

(33) Another illustration of corruption in the kingdom community: the parable of the leaven in the meal.
Another parable He spoke to them: “The kingdom of heaven is like leaven, which a woman took and hid in three measures of meal till it was all leavened.”

a. The kingdom of heaven is like leaven: Jesus used a surprising picture here. Many, if not most, regard this as a beautiful picture of the kingdom of God working its way through the whole world. Yet leaven is consistently used as a picture of sin and corruption (especially in the Passover narrative of Exodus 12:8, 12:15-20). Again, both the content and the context point towards this being a description of corruption in the kingdom community.

i. “There would be a certain shock in hearing the Kingdom of God compared to leaven.” (Barclay)

b. Leaven, which a woman took and hid in three measures of meal till it was all leavened: This was an unusually large amount of meal. It was much more than any normal woman would prepare, and again suggests the idea of massive or unnatural size.

i. “Three measures of meal would be about 40 litres, which would make enough bread for a meal for 100 people, a remarkable baking for an ordinary woman.” (France)

c. Hid in it: The idea of hiding leaven in three measures of meal would have offended any observant Jew. This certainly isn’t a picture of the church gradually influencing the whole world for good. Rather, in the context of increasing opposition to His work, Jesus announced that His kingdom community would also be threatened by corruption and impurity.

i. G. Campbell Morgan wrote that the leaven represents “paganizing influences” brought into the church.