Types of casks
The cask maturation of whisky is one of the most influential processes that define the taste of whisky. While the length of time the whisky matures is important, the type of the cask however is crutial. The wood of the cask adds the different complex flavours to the distillery character that is already in the new made spirit. Different casks offer different flavours.
You can find a detailed description of maturation process in this article. It describes what kind of flavours casks add to Scotch whisky, Bourbon or Irish whiskey.
There are five major factors that influence the whisky in the cask:
1. Type of the predecessor liquid
|predecessor||Type of liquid||Alternation of taste||Alteration of colour|
|Bourbon||whiskey||vanilla, sweetness, caramel, creamy||golden|
|Burgundy||wine||very fruity, lightly sweet, lightly dry||dark red|
|Madeira||fortified wine||spiciness, light fruitiness, sweetness, dryness||dark, amber|
|Port (sweet)||fortified wine||sweet, dried fruit, spiciness||red|
|Port (semi-dry)||fortified wine||lightly sweet, dried fruit, spiciness||red|
|Port (dry)||fortified wine||dry, dried fruit, spiciness||red|
|Oloroso||sherry||deep, dark, nutty, dark ripe fruits||red, amber|
|Pedro Ximenez (PX)||sherry||very sweet, dark fruits, raisins, syrup||amber|
|Fino||sherry||light fruits, sweetness, dryness, light wood||bright|
|Manzanilla||sherry||salty, dryness, sea flavours, fresh, some fruit||bright|
|Amontillado||sherry||sweetness, nutty, dry, fresh, acid||amber|
|Palo Cortado||sherry||rich, sweet, dry, sweet spices, fruits||brown|
|Sauternes||wine||sweetness, zest, acidity, light fruits||bright to amber|
|Bordeaux (red)||wine||strong red fruits, grapes (wine), berries||red|
|Tokaji||wine||light fresh fruits (citrus, mango), very sweet,||bright to amber|
|Ruby Port||fortified wine||very fruity, dark fruits, berries||red|
|Barolo||wine||fruits, tannins (bitter), dried fruits, heavy aromas||red|
|Chardonnay||wine||lean, crisp, acidic, tropical fruits||bright|
|Muscat||wine||floral , sweet, citrus, peach||bright/red|
|Muscat||fortified wine||very sweet, dark fruits, raisins, syrup||amber|
|Rum (white)||spirit||sweet, molasses, vanilla, tropical fruit, almond||bright|
|Rum (dark)||spirit||sweet, syrup, dark fruits, oak, caramel, vanilla||amber|
|Amarone||wine||tannins (bitter), dry, raisins, ripe fruits||red|
|Marsala||fortified wine||sweet, complex, spices||dark red|
2. Size of the Casks
The sizes of casks are very difficult to define, because there is no ISO standard to define the volume of a standard cask. There is another major problem with the volume of the casks. The cask sizes were also a unit of measurement. Take the Butt for example. The normal butts come in sizes of 500 liters (132 galons). But there is also a measurement unit called a butt, which is 1/2 a tun and is 122 US gallons (477 liters). The following table shows the actual sizes the most barrels come in, not the sizes of the measurement units.
|Name||Liter||US. Gallon||Imp. Gallon|
|American Standard Barrel (ASB) /Bourbon Barrel||200||53||44|
|Port Pipe (tall)||500||132||109|
The sizes of the the casks vary as different coopers produce different sizes of casks. It is important to remember the big differences between the casks.
|Size of casks||Type of casks|
|Big > 400 Liters (>132 US. gallons)||Butt, Port Pipe, Puncheon, Madeira Drum|
|Medium 200 - 400 Liters (53 - 106 US. Gallons)||ASB, Bourbon barrel, any Hogshead, Barrique cask, Cognac cask, Bordeaux cask|
|Small <200 Liters (53 US. Gallons)||Quater cask, bloodtub|
Here is a video of Horst Luening explaining 6 casks at the Old Midelton distillery.
3. Type of wood
|Type of wood||effect on taste|
|American white oak (Quercus alba)||mellow, soft, vanilla, caramel|
|European oak (Quercus robur and petraea)||spicy, bitter, stong on the wood|
There are big differences between the two major types of oak. The American white oak grows in the east of the United States of America and a few parts of Canada. The tree grows rather fast for an oak tree and is therefore a bit less expensive than the European counterpart. Its wood is very dense (770 kg/m³) and contains a lot of monogalloyl glocose. This is later transferred into the typical Bourbon vanilla taste.
The European oak grows all over the European continent far into Russia and Turkey. It grows slower than the American counterpart and is a bit less dense (720kg/m³). It contains Gallic acid that is considered a pseudo tannin. This acid in combination with water gives the whisky a slightly bitter note. The European oak has also a lot of other components that also add to the spiciness of the whisky.
4. Charring and Toasting
The toasting and charring converts the wood sugars into vanilla and caramel flavours. The Cooper distinguishes between charring and toasting. Toasting just darkens the top of the wood and acts in the depth of the staves. It leaves the wood with a black flat layer on top. If you char a barrel then you burn the wood to a point where the surface breaks and leaves the wood with a surface like uneven structure. It looks like an alligator skin.
The level of charring is determined by the time the barrel is burned. This time varies with the cooperage or the specification of the distillery ordering the cask.
Jack Daniel's Cooperage
About 42 seconds of charring
About 15 seconds of charing
5. Reuse of the cask
The last point about a cask is the number of times it has been used. The more often you use a cask the less flavours it will release into the whisky.
Bourbon however has to be matured in fresh casks. So Bourbon always gets the maximum taste from the fresh barrels.
Most often the casks are rejuvenated before the casks are being refilled. For this the casks are milled from the inside and then charred again. This restores more vanilla and caramel flavours for the maturation.
Here is a video of the rejuvenation of casks at the Speyside Cooperage.