saw a receipe for rustic rye but can, figure it out. What's a levain? and how is it made?
Levain is another word for a poolish or biga, etc. made with sourdough starter - I believe, though there are far wiser people here than me.
So specifically, a levain refers to a mixture of sourdough starter, plus flour, plus water (and sometimes plus salt). This mixture is left to proof from somewhere between 4 and 24 hours typically, and then mixed into more flour, water and salt to make a whole loaf.
In this case, the sourdough starter frequently replaces the yeast, though additionally may be added when mixing the levain with the remaining flour and water.
As for making the levain, you could do worse than searching this site for a post by gaaarp that describes the process in detail.
Search for "sourdough starter 101" and you'll run right into it. Read the comments too, as there are invaluable battle tested experiences there. Absolutely the best place to start, in my opinion.
You can make levain (or sourdough starter) by mixing any kind of flour with water and culturing the yeasts that are present in the flour over about ten days. Sure there are methods that can make the process go faster, but it's not a complicated process when you get down to it.
I tried to make a flowchart of the process that avoids a lot of the confusing elements.
Here it is:
In the interest of simplicity---there are a number of forms which a pre-ferment can take. The French refer to it as a Poolish, the Italians call it Biga and you will see it referred to as a sponge or levain.
As a general rule a levain usually consists of equal amounts of water and flour with a small amount of leavening (starter or yeast) added to get it cranking and given a 3-18 hour fermentation, depending on which version of the pre-ferment you're producing.
Here are some definitions/information taken directly from The Artisan web site, which is an excellent source for just about any type of bread baking information. This information may help clear up the differences and serve to answer some of your questions about a biga, poolish, etc.
"The biga can be prepared in four ways:
La biga veloce (quick biga): 1 part water, 8-10% yeast, and 1.8 parts flour relative to the weight of the water. Shape the dough into a ring and soak it in lukewarm water for 20-30 minutes. Turn the ring upside-down for approximately 15 minutes. When it floats, it is ready. This is an ideal biga for a rich dough.
La pouliche (poolish): 1 part water, 5% yeast, and 1 part flour relative to the weight of the water. The ambient temperature should be about 77 F ( 25 C). The biga matures in 2 hours. The degree of maturity is judged by the intensity of the bubbles that are formed on the surface. This biga is ideal for products that are characteriazed by a thin, crisp crust, elastic crumb and good cell structure.
La biga corta (short biga): 1 part water, 3-5% yeast, and 1.8-1.9 parts flour relative to the weight of the water The duration of the fermentation is approximately 3 hours at an ambient temperature of about 77 F (25 C).
La biga lunga (long biga): 1 part water, 1-1,2% yeast, and 1.9-2.1 parts flour relative to the weight of the water The duration of the fermentation is from 8 to 14 hours. Depending upon the ambient temperature, the fermentation can last for 18 hours at 42.8 F (6 C)."
Also, check your local library or book store for some of the standard baking books; Rose Levy Beranbaum (Bread Bible), Peter Reinhart (Bread Baker's Apprentice), Jeffrey Hamelman (Bread), Maggier Glezer (Artisan Baking), Michel Suas (Advanced Bread and Pastry), etc. to see how they make pre-ferments (levain, biga, poolish, etc.) and how it fits into the process. There's certainly no shortage of theories and techniques on pre-ferments.
Good luck with your baking adventures,
Levain is a preferment, but not all preferments are levain.
Levain is made from wild yeast. It is sourdough starter. And as such, it's a preferment, but the inverse is not true, not all preferments are levain. Other preferments use commercial yeast.