Vanillin, a lignin degradation product, is the most well-known member of the Phenolic Aldehydes group. Noticeably, the more oak barrels are heated (toasted) during their making process, the more they tend to increase the Vanillin and other Phenolic Aldehydes levels. The same thing happens when brand new oak barrels are dried at open-air (Vanillin and other phenolic Aldehydes level increases). Inversely, the more oak barrels are used for alcoholic fermentation, the more it reduces their aromatic vanillin level (that are turned into flavorless vanillic alcohol).