The leaven in the parable is not the same as the yeast used in modern kitchens. In the culture of Scripture, leaven is almost universally understood as something evil or unclean.

The leavening agent of the time was created by setting aside a portion of leftover bread to spoil, in order to create leaven used in future baking. Not spoiled enough, it is worthless and cannot cause the new batter to rise. Allowed to spoil too long, it not only ruins the bread but can result in food poisoning. Leaven can be fatal. Only a small portion—like a mustard seed—is needed to leaven flour. The “three measures of meal” with which the woman mixes the leaven would produce enough bread for a wedding feast. Of additional interest in this parable (as in the parable of the Lost Coin found in Luke’s Gospel) is that Jesus’ role is likened to that of a woman.