Scotch single malt whisky gets its unique taste from a multitude of production and maturation factors- cask maturation being one of them. 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 crucial. 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.
The wood used in a cask is an important factor that determines the whisky. 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.
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 sizes of the casks vary as different coopers produce different sizes of casks. It is important to remember the big differences between the casks.
The toasting and charring converts the wood sugars into vanilla and caramel flavours. 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.
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.