Whiskey casks are the wooden vessels used to store, age and transport whisky. Not all woods are suitable for whiskey cask usage, as a number of factors can affect their flavor. These factors include:
Each type of wood imparts different flavor characteristics to the whisky, depending on its properties. For example, American white oak imparts sweet vanilla notes while European oak gives a spicy character. Latest addition being cherry wood by some pioneering distilleries such as Middleton.
|American white oak (Quercus alba)||Vanilla, caramel, soft and mellowing affect|
|European oak (Quercus robur and petraea)||Spicy, bitter, strong affect on the wood|
|Japanese Mizunara Oak (Quercus crispula)||Sandal wood, coconut, (oriental) spices|
|Maple (Acer)||Maple syrup, sweetens the liquid|
|(French) cherry wood||Ginger, coconut and black tea|
Prior to being filled with spirit which will turn into whisky in the cask, wooden casks must be “seasoned” by being filled with predecessor liquids such as:
This process helps to open up the pores of the wood so that spirit can penetrate more easily during maturation. It also helps to draw out some of the tannins in the wood which can give an astringent flavor if not properly prepared beforehand.
|Predecessor||Liquid type||Effect on taste||Effect on color|
|Bourbon||Whiskey||Sweet, creamy, vanilla, caramel||Golden|
|Amontillado||Sherry||Sweet, fresh, dry, acid, nutty||Amber|
|Fino||Sherry||Sweet, dry, light fruits and wood||Bright|
|Manzanilla||Sherry||Dry, fresh, salty, maritime, fruits||Bright|
|Oloroso||Sherry||Deep, nutty, dark ripe fruits||Red amber|
|Palo Cortado||Sherry||Rich, sweet, dry, sweet spices, fruits||Brown|
|Pedro Ximenez (PX)||Sherry||Very sweet, dark fruits, raisins, syrup||Amber|
|Liqueur Muscat||Fortified wine||Very sweet, dark fruits, raisins, syrup||Amber|
|Madeira||Fortified wine||Dry, sweet, spicy, lightly fruity||Dark amber|
|Marsala||Fortified wine||Sweet, complex, spices||Dark red|
|Tawny Port||Fortified wine||Sweet, spicy, dried fruits, nuts||Red|
|Ruby Port||Fortified wine||Very fruity, dark fruits, berries||Red|
|Rosé Port||Fortified wine||Berries, caramel||Rose|
|White Port||Fortified wine||Sweet, citrus||Golden|
|Amarone||Wine||Tannins (bitter), dry, raisins, ripe fruits||Red|
|Barolo||Wine||Tannins (bitter), heavy aromas, fruits, dried fruits||Red|
|Bordeaux||Wine||Strong red fruits, grapes (wine), berries||Red|
|Burgundy||Wine||Lightly sweet and dry, fruity||Dark red|
|Chardonnay||Wine||Lean, crisp, acid, tropical fruits||Bright|
|Muscat||Wine||Sweet, floral, citrus, peach||Bright|
|Sauternes||Wine||Sweet, zest, acid, light fruits||Bright|
|Tokaji||Wine||Very sweet, light, fresh, citrus, mango||Bright|
|Rum (white)||Spirit||Sweet, molasses, vanilla, tropical fruit, almond||Bright|
|Rum (dark)||Spirit||Sweet, syrup, dark fruits, oak, caramel, vanilla||Amber|
|Virgin Oak||–||Fresh, vanilla, cloves, caramel, wood||Dark brown|
The charring process is another important part of preparing a cask for maturation. The inside of the cask will be set on fire in order to create a layer of charcoal which helps to filter out unwanted compounds during maturation such as sulphur compounds which can give off unpleasant odours and tastes in whisky.
Bluegrass Cooperage charring bourbon barrels:
Casks may also be subjected to “toasting” which involves heating them over open flames at high temperatures in order to caramelize some of the sugars present in the wood. Again, giving off desirable flavors such as nutty or honeyed notes that would otherwise not be present if left untreated.
Casks come in various sizes depending on where they are made and their purpose – typically these range from 50 litres all the way up to 500 litres. Though there is no standardized size across different producers.
Many distilleries use smaller casks such as 25-40 litre barrels because they mature faster than larger barrels. Yet they offer less complexity due to having less contact with surface area compared to a larger cask over time; this makes them ideal for finishing whiskies or creating quick-maturing blends where complexity isn’t required.
By contrast, many Scottish distilleries prefer larger 250-500 litre sherry butts due largely because these allow for prolonged maturation times. Which allow for greater complexity in flavour profile over time, compared with smaller ex-bourbon barrels – making them ideal for creating single malts with deeper layers of flavour and aroma profiles.
Below you can find complete table of various cask sizes:
|Type name||Liter size||US gallon size||Imperial gallon size|
Overall, like stated in the beginning of this article: there are numerous factors that can affect how whisky matures when stored in wooden casks. These include type of wood used, seasoning techniques applied prior to filling, level of charring/toasting applied before maturation begins and finally, the size of the cask employed throughout its lifetime.
Each one of these factors play an integral part in helping craft unique flavours found within any given whisky expression today. And the distillers have gotten quite imaginative with all those factors. Which is a good thing.