What is levain?

A natural-leaven pre-ferment built from your sourdough starter, used as the leavening agent in a sourdough recipe. Refreshed 8-12 hours before mix day so it peaks right when you need it. Typically built at a 1:5:5 ratio — one part starter, five parts flour, five parts water — in an amount sized to the recipe (often 100-200g total). The levain is the bread's yeast and bacterial culture multiplied to recipe-strength: starter provides the seed; the long refresh gives the wild yeast time to multiply and the bacteria time to produce the flavor acids that define sourdough. Use the levain when it's domed, bubbly, and either floats in water or has clearly more than doubled. Skipping the levain build (using cold starter straight from the fridge) is the most common reason home sourdough doesn't rise.