A levain is the small offshoot of your starter built specifically for a bake, in the amount the recipe needs. It lets you choose flour, hydration, and ripeness for the loaf without disturbing your storage culture.
A common build is 1:2:2 or 1:5:5, starter to flour to water, made 8 to 12 hours before mixing. Ripe means doubled and domed, with a rounded smell and a visible network; if it smells sharply of acetone or has fallen, it is past peak and will leaven more slowly.
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Build the levain warm, around 24 to 26 degrees, in a clear container so you can see the rise. Use it at peak for the strongest lift, and refrigerate the mother starter separately as usual.