Here’s the next step in mastering the Tartine loaf: mixing a leaven.

You can do this once you’ve trained your bread starter. You’ll use some of the leaven in your bread dough to make it rise, and you’ll save a portion that you’ll keep alive as your starter and will feed regularly.

The night before you’re ready to make bread, you’ll mix your leaven. Use about one tablespoon of your starter (toss the rest) and feed it with 100g of warm water (about 80 degrees) and 100g of the 50/50 flour blend. Cover it with a towel and let it rise on the counter overnight.

In the morning, it should increase in volume by about 20 percent and small air bubbles will be visible. To test if it is ready, drop a spoon-full into a bowl of lukewarm water. If it floats (thanks to CO2 bubbles), it’s ready! If it sinks, let it ferment a little longer in a warm corner and re-check its float-ability in half hour increments.

It should smell like overripe fruit since it is in the beginning stages of fermentation. If it smells like vinegar, it has fermented too long. This is ok, your bread will just taste more acidic. If you want a more mild flavor, like Tartine’s, discard half and feed fresh warm water and 50/50 flour. Let it ferment and aerate until it passes the float test.

Once it passes the float test, you’re ready to mix your dough!

-Learned from Tartine Bread.