De Dolle Oerbier Reserva is a specially matured version of De Dolle Oerbier. This version of De Dolle Oerbier is matured for 18 months in Bordeau wine casks. This aging process creates new flavors in three different ways. The flavors of the wood, that are used to improve wine, can come out into the beer, creating oaky, and tannic flavors. The red wine itself that has soaked into the wood can also enter the beer, giving it wine and fruit flavors. Oxygen, which is present in the barrel, will also create smoky, nutty flavors similar to those found in Sherry. Finally, the are micro organisms in both the beer and the barrel that can continue maturing, producing new flavor elements as the beer ages.
This process creates a truly unique and amazing version of Oerbier, that is only available in one small batch per year. Each year is vintaged, and they can be compared to find distinctions in flavor.