“Watch for the yeast from the Pharisees and the Sadducees, which is hypocrisy.” Then they understood that he didn’t say to watch for the yeast of bread, but for the Pharisees’ and the Sadducees’ teaching.

— The Story of Jesus 16.5

[adapted from Mt 16:11-12; Mk 8:15]

Yeast is a type of single-celled fungus. There are many different species, but they all consume oxygen and carbohydrates, while excreting carbon dioxide and various alcohols. The Bible uses yeast's many qualities to illustrate various spiritual principles.

In fruit juices, yeast ferments natural sugars to produce alcohol, most commonly wine. Once the alcohol gets to a certain concentration, the yeast dies off in the toxic environment it has created. The liquid is then sterile and fit for consumption.

The process of producing wine in ancient times occurred in wineskins. These leather bags were typically made of a whole goat skin, lined with tree sap or resin to prevent its contents from seeping out. The skin of grapes naturally has yeast growing on it. Upon pressing the grapes, the yeast digests the sugars in the juice to produce alcohol and carbon dioxide. New wineskins, full of fermenting wine, expand as gas accumulates. However, wineskins can only be used to ferment wine once – the gas would burst old wineskins if used to hold fresh grape juice again.

No one puts new wine into old wineskins, or else the wine will burst out of the skins and both the wine and the skins will be ruined. They put new wine into fresh wineskins and both are preserved. 

— The Story of Jesus 7.8

[adapted from Mt 16:11-12; Mk 8:15]

In baking, yeast produces carbon dioxide gas, which gets trapped in the sticky dough and causes the loaf to rise. The heat of baking sterilizes the loaf and renders the bread fit for consumption. A small amount of yeast will reproduce exponentially to quickly permeate massive amounts of fruit juice or dough, as long as sufficient water and nutrients are present.

What can I compare God’s kingdom to? It’s like yeast that a woman took and hid in three measures of flour until it had all risen.

— The Story of Jesus 12.2

[adapted from Mt 13:33, Lk 13:20-21]

Don’t you know that a little yeast leavens the whole dough? Clean out the old yeast so you can be new dough, since you’re already unleavened. Because Christ, our Passover, has been sacrificed. So let’s celebrate the Feast, not with old yeast or the yeast of evil and immorality, but the unleavened bread of purity and truth. 

— 1 Corinthians 5:6-8

Any form of yeast or leavening agent was prohibited during the Passover Feast, since, in their haste to leave Egypt centuries prior, Israel had no time to include yeast in their bread and wait for it to rise. The Passover was therefore also called the Feast of the Unleavened.

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