Sourdough
When Scripture says “leaven,” it means sourdough — a living culture of wild yeast and bacteria kept as a “mother” lump, not the packet yeast of modern baking. It works by permeating the whole batch and turning it to its own character — which is why leaven pictures an influence rather than a mere additive, and why Jesus’ warning about the leaven of the Pharisees is glossed as “the teaching” (Matthew 16:12).