Levain

A levain is a small preferment you build the night before or the morning of a bake by combining a spoonful of mature starter with fresh flour and water in the exact amounts a recipe calls for. Unlike your everyday starter, a levain exists for one purpose: to be ready and at peak activity right when you want to mix your dough. Many bakers keep a lean, stable starter in the fridge and always build a levain from it before baking.

Building a levain separately lets you control ripeness precisely, since you can adjust the ratio of starter to flour and water to speed up or slow down how quickly it matures. It also protects your main starter from being weakened by an off bake, since you are not risking the whole culture on one recipe. A ripe levain will have risen well, look domed or slightly bubbly on top, and smell mildly sour rather than sharp or boozy.

When building a levain, use a container with room for at least triple the starting volume so you can watch the rise. A frequent mistake is building the levain too far in advance and letting it over-ferment past its peak before mixing, which weakens the dough's rise. Another is using too little starter to inoculate it, which makes the levain take far longer to mature than the recipe assumes.