Cognac aging takes place over sometimes decades in oak barrels, where a multitude of slow chemical reactions occur. These reactions lead to the formation of hundreds of organic compounds: long-chain even-numbered fatty acids, which act as precursors to odd-numbered methylketones are one such type of reaction. These methylketones are responsible for the desirable 'rancio' aromas that emerge after 15 to 20 years of maturation, lending depth and complexity to the oil: this oil comes from distilling the sediment from barrels at least that old, and the aroma attests to that.