Leaven or Yeast
How do ye not understand that it was not concerning bread I said to you, Beware of the leaven of the Pharisees and Sadducees? Then they comprehended that he did not speak of being beware of the leaven of bread, but of the doctrine of the Pharisees and Sadducees. Matthew 16:11-12.
Yeast is a single celled organism that has the ability to produce carbon dioxide and alcohol. For this reason, it is of immeasurable value worldwide in the production of wheat bread, wine, and industry. Most of the yeast used in both baking and in fermentation of grape juice into wine is the ascomycete, Saccharomyces cerivisae. Today, pure yeast may be purchased in almost any store and kept for a long time. But not long ago, it was essential to put aside a portion of the unbaked dough each time to ensure a supply of yeast for the next batch of bread. This lump, or starter, was a mixture of flour and yeast and is known as leaven. Leaven is a more accurate word than yeast to refer to the material used in Bible days to make bread.
There are proscriptions to leaven in most of the offerings. Yet there are offerings where leaven is required as in Leviticus 7:13.
Jesus applies the term leaven to the corrupting influence of the Pharisees and Sadducees.