TYPE OF WOOD
The laws governing Scotch whisky specify that casks must be made from oak. The two main types of oak used are American oak (Quercus alba) and European oak (Quercus robur or Quercus petraea).
American oak contains organic compounds such as vanillin and lactone, which give the whisky creamy, vanilla, and coconut notes. European oak has tannins that add spicy and woody notes.
TIMES OF FILLING
Whisky casks* last for decades, and are used more than once.
Casks that are used to mature Scotch for the first time are called first fill casks, while subsequent uses are called refill casks. First fill casks impart the most wood characteristics to the whisky, which decreases over time with refill casks.
Scotch whiskies are usually matured in casks that were previously used to mature other spirits or wine, such as bourbon or sherry. The pre-used casks have additive effects — obtaining distinct notes from the previous content thus enhancing flavours of the whisky.
For instance, sherry casks that have previously aged Oloroso sherry give the whisky deep, nutty, and dark ripe fruit flavours.
LEVEL OF CHARRING
When making a cask, the inside is toasted and charred**. This involves burning the wood over an open flame at varying intensities to break down its structure and release vanillin, which gives the whisky vanilla and caramel flavours.
Sherry and wine casks are usually toasted, while the bourbon casks are charred. The charred layer in bourbon casks also filters out unwanted flavours, such as sulphur compounds, from the whisky.
SIZE OF THE CASK
Whisky casks come in different sizes, such as barrels (200 litres), hogsheads (250 litres), and butts (500 litres).
Smaller casks allow more contact between the whisky and the wood surface, resulting in a faster maturation and a stronger flavour profile. Whereas, larger casks reduce the oak’s effect on the whisky and allow for a more gentle maturation process.