Stained-glass window(13th century) in the Chartres CathedralLevain, a.k.a as wild yeast starter, natural leaven or sourdough, is a culture of flour and water used to leaven bread dough. Unlike baker's yeast which is industrially processed, wild yeasts occur naturally in the kernel of wheat, rye, or spelt or other cereal. Dormant until activated by water and kept reasonably warm, when awakened "they feed on the sugars converted from flour carbohydrate by the action of the enzymes (also naturally occurring)" (Whitley, Bread Matters), producing gas (which raises the bread) and alcohol (which gives it flavor). Levain has been around for centuries: everybody has heard or read the story of the Egyptian baker who forgot a batch of dough somewhere warm and came back to see it considerably inflated. Being of the waste-not/want-not persuasion, he baked it anyway and found out that it had a delicious taste and great shelf-life. This may well be a legend but the Roman writer, Pliny the Elder (who lived in the first century) does mention in one of his books (Book XVIII) that the Gauls - ancestors to the modern French - maintained their levain by feeding it brewer's yeast and that their bread was consequently much lighter than the bread made by other nations (I love that story as it seems to imply that the French are genetically predisposed to making good bread. I wish!). Being a mixture of flour and water, the levain is basically a dough. However, thanks to successive feedings (with various proportions of flour and water), it develops an active microbial flora from the micro-organisms present in the flour. Bacteria - which are also present - are kept in check by the production of lactic and acetic acids. It is when microbial activity is at its most intense and becomes stabilized that this "dough" is used to leaven other dough. Levains can be firm (50% hydration) or liquid, sometimes very liquid (up to 200% hydration). The lower the hydration rate, the slower the fermentation and the more leeway and control the baker has. In the old days, levains were mostly firm. Ruth Allman remembers in her endearing book, Alaska Sourdough, that "while mushing on the trail with the temperature flirting below zero, [her husband] Jack would put some sourdough in an old Prince Albert tobacco can. This he tucked inside the pocket of his woolshirt to make certain it would not freeze". She also remembers the old prospector who "buried his sourdough in the top of his sack of flour - warm and safe. When he arrived at camp, many times he only added flour and water to make the right quantity and consistency, without taking the sourdough from the flour sack. Saved a dish when no dish was available". Ruth Allman goes on to say that when the first attempt was made to climb Mt Mc Kinley, the expedition carried the starter on top of the flour too. "To make sourdoughs (sic), they poured the glacial water - heavy with silt - and made the dough right in the flour sack. Then rolled the sourdough on the end of a stick and baked in front of an open fire". I love all these stories but nowadays levains live a more sedate life. They usually ferment peacefully in the corner of a bakery or kitchen until called to action. There has been a lot of brouhaha around levain in the past few years, so much so that some home bakers are weary of trying their hand at it. In my experience (and heaven knows that I was a complete greenfoot, levain-wise, when I started my first one in the mid-90's ), it is fairly easy (not to mention exciting) to start and keep your own levain. How-to's abound both in books and on the Internet, so I am not going to add my grain of salt, as we say in French. I just want to say that you don't need anything but flour and water. I never used grapes, apples, pineapple juice, milk or yogurt to start a culture although some home bakers have to good results (the idea being that the wild yeasts will feed on the sugar present in these ingredients and so be helped along). I used the Nancy Silverton method (described in her book Breads from La Brea Bakery) but I skipped the grapes and it worked just fine. Go for it!