Many spirits like Whiskey and Brandy, including Dutch Genevers, like Bols Corenwyn Genever and Bols Barrel Aged Genever, are matured in oak barrels. In case of the Bols Genevers, french Cognac barrels are used in order to acquire a slightly brandy-tasting Genever. It is common for Whiskeys to use Port barrels or used Bourbon barrels. But what exactly happens when aging a drink in oak? The drinks are matured initially colorless, for they are spirits. The wood maturation gives the drink a golden brown color, but more importantly, it will add flavour of the wood to the spirit.
During the maturation of the distillate in the wooden barrel, a complex set of processes takes place, giving the drink a unique character; extraction and oxidation are the two chemical processes that take place. Extraction is the ongoing process of the spirit drawing flavours from the wood. For oxidation, the supply of air going through the barrel is essential. When oxidation finds place, consequently chemical reactions, through extraction too, ensure the development of substances like vanilla.
A good maturation depends on several factors: alcohol content of the product (water absorbs dyes and bitter substances for example), the type of wood, the size of the vessel (the smaller the vessel, the more contact between the barrel and the spirit), the age of the wooden barrel (the older the barrel, the less extraction is taking place), the ripening time and the temperature and humidity of the environment (the higher the temperature, the faster the chemical reactions take place).