Influence of wood on whiskey - oak casks are used for maturing whiskey

Wooden casks have a major role in defining the flavor profile of whiskey. "Water of life" spends so much time maturing in oak casks, which by law, are required for maturing. Throughout history any type of wood has been used to make whiskey casks but nowadays they have to be made out of oak. Oak is tough but easy to work with, and you can bent oak using heat without splitting it. Oak has tight grain and is porous. So there's no leaking yet oxygen is allowed in and out of the cask.

Wood has lots of natural oils, which are called vanillins. These oils offer much flavors for the spirit in maturation the process. Even though oak is the only wood used for aging whisky, there are lots of different flavor profiles in the world of whiskies, because of different factors. Distillery's character and selections in the making process define what kind of product it's going to be. Selections such as:

  • Ingredients, which type of whiskey is in question
  • Size and shape of the stills
  • Location, what type of barley and peat has been used
  • Length of maturation
  • Type of oak used

Where and how you taste your whiskey has an impact too. Read more about tasting whiskey.

Virgin oak/New oak

Virgin oak is not a species, yet it's a definition for maturing whiskey in oak casks which have not been infused with any other alcohol. For instance, American whiskey a.k.a bourbon is being matured in new oak casks. Some Scottish distilleries have tried virgin/new oak as well. Swedish Mackmyra's whiskies and Japanese whiskies also have new oak influence from casks made in their home country.

Different types of oak casks

European Oak (Quercus robur)

Oak from Europe has traditionally been used for whisky maturing in Scotland and Ireland for almost two centuries. First casks were slow growing Scottish or English oak, which were too twisted to prevent casks from leaking. Imported fast growing Russian oak gave more consistency to whisky production.

1860's saw the importing of Spanish sherry to the UK. Sherry casks were made using Spanish oak, which had similar qualities as the Russian oak. Since Spanish oak cost less than Russian, it gained ground in the whisky industry. Even though sherry casks cost much more than bourbon casks, nearly ten times more. Traditional Spanish oak is grown in Northern Spain (region of Galicia). Since the 1970's, sherry industry has declined but Ex-Sherry casks are still very much used in whisky making.

European oak may also refer to French oak, which means casks from the wine industry. Wine cask maturation is quite commonly used in modern whisky making but mainly for finishing. Finish means that the whisky has been aged in some other casks for a certain period, before transferred to Ex-Wine casks for a different kind of finish.

European oak usually adds flavors like sherry, dried fruits and spices. Raisins, sultanas, nutmeg, cinnamon, wood and caramel are usual suspects of European Ex-Sherry casks as well.

American Oak (Quercus alba)

This oak type has been around since the end of World War II. Since then there's been a law to boost the coopering industry, that all American bourbon whiskey has to be aged in new oak casks. The coopering industry was collapsing during 1920's and 30' Prohibition but after the law was made, there was a great increase in cask production. After the slow recovery of USA whiskey industry from Prohibition, Scottish and Irish whisky producers started to use Ex-Bourbon casks for whisky maturation. Good availability and low price of American oak bourbon casks made them popular when compared to the traditional Ex-Sherry casks, which started to become more expensive due to small availability.

American oak trees are fast growing tall straight trunks with great quality, offering lots of vanillins. So they are perfect for maturing whiskey. American Standard Barrel (ASB) size is considered to age whiskey at the optimal rate. So the ratio between the amount of liquid and the surface area inside the cask is perfect.

Almost 90% of all the whisky in the world today is being matured in American oak Ex-Bourbon casks.

American oak adds flavors such as vanilla, toffee (fudge, butterscotch), honey, spices and nuts (almonds, coconuts, hazelnuts). Even notes of ginger might be a result of American oak.

Japanese oak (Quercus mongolica)

Whisky industry in Japan is famous for using their own oak, the Mizunara oak. This type of wooden casks have been in use since the 1930's, giving away some unique flavors to Japanese whisky. Mizunara oak has also very high levels of vanillins. Unlike the American oka, it's soft and very porous. That's why Mizunara casks are very prone to leaking and can be damaged easily. Due to their softness, Japanese whisky industry has modified the usage to reduce the risks. Nowadays most of the whisky from Japan is first aged in former bourbon or sherry casks, and finished in Mizunara casks for unique flavor.

Usual Mizunara flavors also include vanilla, spices and honey like American oak but Mizunara can add fresh floral or fruity (pears, apples) notes too. Nutmeg, wood and cloves are usually present as well.

Below I've listed few categories, which have different oak types presented. Speyside is famous for great whiskies matured in Ex-Sherry casks. Mackmyra uses their own Swedish Oak casks for maturation, just like Japanese use Mizunaras.

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