A blend of 1/3 old lambic that is at least 2 years old and 2/3 young lambic that is at least one year old matured in huge 10,000-litre oak barrels (foudres). Young lambic still contains enough yeast and sugars to continue to ferment in the bottle.
This process produces carbon dioxide, resulting in a slightly carbonated beer with a nice foam.
Due to the second fermentation process in the bottle, its shelf life is the same as that of a good bottle of wine.
Bottles can be stored in a cellar lying on their sides. After transportation, a 3-day rest period will allow the yeast to settle to the bottom of the bottle.
This makes it possible to serve the beer without it becoming cloudy.