The particular range of flavour compounds the spirit gains from a cask depends on the type of cask used, the usual choice being casks that previously aged Bourbon or sherry.
Bourbon barrels contribute vanilla, honey, various fruit flavours, coconut and spices such as cinnamon, with a light, dry sweetness. Sherry casks add rich sweetness with dried fruit notes including raisins and prunes, fortified wine, spices such as vanilla and ginger, as well as chocolate tones.
These flavour differences are principally due to the species of oak, with Bourbon barrels made from American oak (Quercus Alba) whereas sherry casks are European oak (Quercus Robur).
Another vital factor is the way casks are treated before being used to age Bourbon or sherry. A flame is applied to the interior of Bourbon barrels to briefly ignite the oak, before being extinguished with water. This creates a surface layer of char, around two mms deep, with the heat also toasting an underlying two to three mms layer of oak. A flame is also used to toast (but not ignite) the interior of sherry casks, creating a toasted layer.
Toasting breaks the oak down into various flavour compounds, which the spirit extracts from the toasted layer (the spirit readily penetrates the char in a Bourbon barrel to reach the toasted layer). A certain level of compounds is extracted when the casks age Bourbon and sherry, but significant levels remain to benefit malt whisky.
As soon as a cask is filled with new make spirit (for aging into malt whisky) reactions are initiated and the aging process begins. One of the first flavour compounds evident in the spirit is vanillin, which contributes vanilla notes.
“A small but measurable level of vanillin is extracted even within two to three months. The extraction rate of vanillin is greatest during the first few years of aging, then slows down,” says Brian Kinsman, master blender, William Grant & Sons.
Bourbon and sherry casks can contribute similar levels of vanillin, with the resulting vanilla sweetness also having a significant interaction with other flavours. Just as adding salt can ‘focus’ flavours when cooking, vanilla notes make others in a malt whisky seem richer, smoother and rounder.
Tannins also play a vital role, getting the credit for adding body, structure and balance.
“Tannins add complexity to the mouthfeel by providing dryness, for example, which balances other flavours such as the sweetness of vanillin,” says Dennis Watson, director of technical and scientific affairs, Chivas Brothers.
Tannins are extracted at the greatest rate in the first few years, then the rate decreases significantly. Tannin levels in European oak (ie. sherry casks) can be up to several times higher than American oak (ie. Bourbon barrels). Consequently, malts aged in European oak generally acquire higher tannin levels. However, above a certain level tannins can add astringency and bitterness.
Another change that begins within months is a reduction in the level of cereal notes and sulphur compounds, which comprise vegetal, meaty, rubbery notes (essentially formed during fermentation). This reduction occurs partly due to evaporation from the cask (oak being porous), and through oxidation (ie air entering the cask) which breaks down sulphur compounds into subtler, less flavour-active forms.
However, the greatest reduction is through absorption by the layer of char in Bourbon barrels, essentially within five years.
The toasted layer in sherry casks does a similar job, but can only absorb much smaller amounts, so cereal and sulphur notes diminish more slowly.
Cereal and sulphur notes are quite dominant, therefore reducing the level also enables lighter notes including esters (fruityness) to show through, which significantly alters the flavour profile.
New make spirit is colourless when entering the cask, and develops orangey, amber, dark brown hues in sherry casks, compared to the lighter, golden, straw tints in Bourbon barrels. This difference is typically attributed to the species of oak, in conjunction with the toasted layer. However, exactly which constituents within the toasted layer provide colour is still uncertain.
“You see colour developing after literally a few weeks, with the greatest colour development within four to five years, then it begins reducing and plateaus in the teenage years. Certainly after 20 years the increase in colour is noticeably less year on year. But colour continues developing in later years as much through evaporation, which sees the volume of liquid in the cask decreasing, and so the existing colour becomes more concentrated within this decreasing volume of liquid,” says Brian Kinsman.