Boon geuze Mariage Parfait consists of 95% mild lambic, aged at least three years and specially reserved for this purpose, and 5% young lambic. The latter provides the fermentable sugars and wild yeasts. After mixing in a vessel of 25,000 litres the wort is filtered and chilled before bottling. The bottles are then placed in a climate-controlled space for secondary fermentation. This is followed by maturing at a low temperature for at last 6 months. Want to know the bottling date? Simply subtract 20 years from the best before date!