Jesus warns his disciples to: "beware of the yeast of the Pharisees" so, we can immediately begin to understand that "the yeast" represents: "additional teachings" which are added to the word of God.
Yeast is used to inflate bread or make it rise. For centuries many have done exactly that with the Word of God. They have added to the Word as well as take things away from at the same time. They add word to manipulate and control the lives of people who are ignorant of the truth so that they end up with a people who are submissive to them but to God.