Leaven is any substance that produces fermentation when added to the dough. Fermentation helps the bread rise. Leaven or yeast can mean two things:
Dough that already has leaven in it. This dough was mixed with flour so the leaven could spread through all of it before baking.
Dough that has grown bigger because of the leaven.
In the past, Hebrew people used a piece of dough with yeast in it to make more leavened dough. Much later, they started using the remains from wine-making as yeast.
Use in Ancient Israel
The ancient Israelites often ate bread with yeast (Hosea 7:4). But when they remembered the Passover, they were not allowed to eat or keep bread with yeast in their homes (Exodus 13:7). This yearly event helped people remember their quick exodus from Egypt when God’s order left no time to make bread with yeast. The people had to carry their mixing bowls and dough. They used these to make flat bread without yeast to eat on their journey (Exodus 12:34–39; Deuteronomy 16:3).
Rules About Leaven in Worship
Maybe because yeast made things break down and go bad, it was not used in offerings given to God on the altar (Exodus 23:18; 34:25). It was also not allowed in grain offerings (Leviticus 2:11; 6:17). The Bible does not tell us if the special bread in the temple, the showbread or bread of the Presence had yeast. The writer Josephus says it did not (Antiquities 3.6.6).
There were two times when yeast was allowed. Bread with yeast could be used in offerings eaten by priests or others. It could be part of the peace offering (Leviticus 7:13). It was given at the Feast of Weeks because it was like the normal food God gave his people (23:17). The Feast of Weeks is also known as Pentecost.
Changes in Using Leaven
The slow action of the yeast was a problem when the Hebrews became farmers, especially during the busy harvest time. So, bread without yeast became more common for everyday baking. Unleavened dough, therefore, became increasingly common for ordinary baking.
This was also because people started to think yeast stood for things going bad, like other foods that ferment. This idea kept yeast out of offerings to God, who was seen as perfect and holy. Plutarch writes about a common belief. “Now leaven is itself the offspring of corruption and corrupts the mass of dough with which it has been mixed.” The apostle Paul quotes a similar saying in 1 Corinthians 5:6 and Galatians 5:9.
Leaven as a Symbol
The important thing about yeast is its power, which means good or bad things. Usually, but not always, yeast was a symbol of bad things in Jewish thought. Jesus used yeast to mean the wrong teachings of some religious groups like the Pharisees and Sadducees (Matthew 16:6, 11–12) and Herod (Mark 8:15). He also said the yeast of the Pharisees was acting or pretending to be what they were not (Luke 12:1; compare Matthew 23:28).
Paul uses the same idea for moral wrongdoing. He warns that “a little yeast affects all the dough.” Paul tells his readers to remove the old yeast, which means the parts of their old lives. They are to live as Christians with the “bread without yeast made of honesty and truth” (1 Corinthians 5:6–8).
It is also true, that Jesus uses the idea of how yeast affects dough in a good way. He gave his followers a short but easy-to-remember story (Matthew 13:33; Luke 13:20–21). In this story, yeast shows how the kingdom of God grows and spreads in the world.