A liquid levain begins simply as a mixture of flour and water. At The French Culinary Institute, the following guidelines are used to create a liquid levain.
Wild yeast, present in the flour and in the environment, populates the mixture. It is stronger and more stable than commercial yeast, as well as more resistant to acidity. Starch in the mixture is broken down into sugar by the yeast and enzymes, resulting in alcohol and carbon dioxide, the by-products of fermentation.
Starch > fructose/dextrose + alcohol + CO2 (carbon dioxide)