Levain is a starter used in various baking applications such as sourdough bread and croissants. A levain is a culture of natural yeasts found in flour, in the air (wild yeast), and on the baker's hands. Bakeries often maintain the same levain day after day, for decades. The daily feeding of the levain ensures it remains active; once the levain is not longer fed, the yeast dies out. Since the yeast found in different areas are different, the levain imparts a distinctive flavor to your baked goods (and yes, you can have a great tasting sourdough without importing yeasts from San Francisco). Starting a levain is simple, add the same amount of flour and water (by weight), mix thoroughly, and cover with a towel or cheesecloth. Let the mixture rest for 24 hours at room temperature. The next day, add the same amount of flour and water (by weight), and mix thoroughly. Cover with a towel or cheesecloth and let the mixture rest for 24 hours at room temperature. Repeat (with a caveat, see below for the correct five days process). After each feeding, the yeasts will feed on the starches of the flour, and you will see micro-bubbles forming, which is a sign that fermentation is taking place. During fermentation, gas is produced. When you smell a levain, it should smell of yeast and fermentation; the smell should be clean.
Making Levain in Five Days. Prepare a total of 200 grams of flour and 200 grams of water. Use all-purpose unbleached flour with a protein content of around 11 percent (10 percent to 12 percent). Pick a clean container to host your levain.
Day One: thoroughly mix 50 grams of flour with 50 grams of room temperature water. Cover with a towel or a cheesecloth, and place in a dark spot (at room temperature) for 24 hours.
Day Two: add 50 grams of flour and 50 grams of room temperature water to your levain container. Cover with a towel or a cheesecloth, and place in a dark spot (at room temperature) for 24 hours.
Day Three: add 100 grams of flour and 100 grams of room temperature water to your levain contained. Cover with a towel or a cheesecloth, and place in a dark spot (at room temperature) for 24 hours.
Day Four: remove 20 percent of the levain. Cover with a towel or a cheesecloth, and place in a dark spot (at room temperature) for 24 hours.
Day Five: the levain should be ready. It should be airy, you should see bubbles, and it should smell clean (fermentation and yeast). If the levain is not ready, add the same amount of flour and water equal to the weight of the levain (before such additions).
Days Six - Ready: repeat Day Five routine until the levain is ready.
Once your levain is ready, you can start using it. For example, if you decide to make croissants, you will use 68 grams of the levain in combination with 560 grams of flour for the detrempage. To keep the remainder of the levain alive, keep feeding it equal weight of flour and water on a regular basis. It is not necessary to feed an active levain every single day, but it is critical to feed it regularly, or it will die. A levain should never smell foul; if yours does, discard it and start from scratch.
Most of the flavors of croissants and breads made with levain comes from the levain. Since levain is made of yeasts, it does contribute to rising a dough as well, but it is not its primary purpose (flavors is). Indeed, often, additional yeast is added (either fresh yeast or dry active yeast) to the dough, to give it more rise. Not two levain will taste exactly the same.