A whisky develops its aroma and flavor mainly through its contact with the wood during maturation.
The vast majority of Scotch whiskies are matured in ex-bourbon American white oak casks (i.e., casks previously used for maturing bourbon whiskey), which imparts honey, vanilla, cereal and sugary note. Whiskies matured in ex-bourbon casks tend to be golden in color.
Another more popular oak casks used for whisky maturation are oak casks that previously held Sherry wine, a Spanish fortified wine made from white grapes. There are two main types of ex-sherry casks - Pedro Ximinez (PX) and Oloroso, as well as other less common ones like Fino, Manzanilla, Amontillado, Palo Cortado and etc. Whiskies matured in PX casks tend to be sweeter, while those matured in Oloroso casks tend to be medium-dry. Also, whiskies matured in ex-sherry casks tend to have a reddish or brown hued color.
European oak grows all over continental Europe, far into Russia and Turkey. Oak casks used for whisky maturation are typically from France and Spain, which tend to be less dense and more impactful on new spirit. European oak casks impart spicy and peppery notes to the whisky, with stronger wood influence.
Sometimes, the denser American white oak casks are also used to store Sherry wines before they are used for whisky maturation. American white oak, which is found in the US and parts of Canada, grows faster than European oak and therefore is less expensive compared to European oak. Also, there are far more ex-bourbon casks than ex-Sherry casks as bourbon is a much larger market compared to Sherry wines. American oak casks imparts a softer taste with vanilla, fruit cake, chocolaty flavour with peel and spice to the whisky.
Other the sherry, whiskies have also been matured in oak casks that previously held other types of wines and liquors, including port, rum, brandy, sauternes, Madeira, burgundy, ruby port, chardonnay, Marsala, and etc.
Sometimes whiskies can also be matured in newly charred virgin oak casks, i.e., oak casks that have not previously held other liquids. Charred virgin oak casks are the hallmark of American whiskies, but they have also been used Scotch and other types of whiskies. Virgin oak matured whiskies then to have flavours of vanilla, cloves and caramel with a dark brown colour.
Other than American and European oak, some whiskies, especially Japanese whiskies, have been matured in Mizunara oak casks. Mizunara oak casks are harder to use due to the wood structure, and also because of the lack of waterproofing oil enzymes, which results in higher “angel’s share” during the maturation process. Mizunara oak casks imparts a special sandalwood, coconut and oriental spice aroma to the whisky.
Whiskies is aged in casks which can be used more than once. First-fill means cask that have been used for the first time to mature whiskies, and second refill means casks that have completed one maturation of whiskies and are now filled with new make spirit. Casks may be used for a third or even fourth time. Each time a cask is filled, the influence of the oak on the maturing whisky is reduced (from as high as 80% for the first fill to 10% in fourth fill).
A relatively modern phenomenon is the concept of “finishing” where a distiller transfers its whisky from one type of casks (e.g., ex-bourbon American white oak casks) into another (e.g., ex-sherry European oak casks) to add complexity to the whisky.
Before maturation, there are also several factors that can affect the flavour of a whisky, including the location of the distillery, water used in the distillation process, natural environment, and also the use of peat in the malting process.