It is a thick, live liquid sold in handy, 220 Imperial gallon-sized tanks (1,000 litres), so don’t even think of trying to find it at present in your supermarket.
It is designed to make Pain au Levain that will respect the French bread decrees on pH, etc. It is used at a ratio of 10 to 15% of the weight of the flour in the recipe. The water in the recipe has to be adjusted down accordingly to allow for the liquid of the Crème de Levain.
Crème de Levain is the registered product name
Can be stored refrigerated for up to 8 weeks.