Vanilla flavours are extracted from the cask by the maturing spirit. The earliest discernible signs are two to three months after filling a cask. The rate of extraction peaks in the first two to three years, then slows markedly. How much vanilla flavour is acquired depends on the level of vanillin (a flavour compound) within the oak.
The usual choice is Bourbon barrels (American oak) and sherry casks (American or European oak). American oak can have higher concentrations of vanillin than European oak, which also contains more ‘woody’ notes than American oak that mask vanilla.
Additionally, seasoning casks with sherry adds intense flavours such as raisins and Christmas pudding which also mask vanilla, whereas seasoning barrels with Bourbon produces sweeter, complementary flavours including honey and coconut.
The level of vanillin is also determined by the ‘fill’ (i.e. how many times the cask has been used to age malt whisky). A first fill, naturally, contains the highest level. Second and third fills offer progressively less, though how much less depends on the duration of each preceding fill, which can be a few years or many more. But then even small amounts of vanillin have an impact.
“A level of 0.2ppm (parts per million) is enough to shine through. For comparison, a phenolic level of 0.2ppm in a peated malt gives discernible light smoke. When using first-fill Bourbon barrels the vanillin could reach 4ppm, but this could also mean pronounced oak character and the risk of overt tannins,” says Stuart Harvey, master blender at Inver House Distillers.
Different fills also provide varying nuances of vanilla character. Brendan McCarron, head of maturing whisky stocks at Glenmorangie, says, “Glenmorangie Original is a blend of first- and second-fill Bourbon barrels and the vanilla presents differently in each. A first fill tastes like scraped vanilla pod, compared to vanilla ice cream from a second fill. Together, they provide the balance we’re looking for.”
Adding virgin (ie. unseasoned) casks into the mix alters the vanilla quota.
Brian Kinsman, master blender at William Grant & Sons, says, “The recipe for Glenfiddich 15 Years Old Solera includes a marriage of first-fill and refill Bourbon barrels, and European oak Sherry casks, which gives a fruity character with a certain level of sweetness. To this we add 15 Years Old Glenfiddich matured for four to five months in virgin American oak barrels, which give a lot more vanilla sweetness that elevates the fruit and intensifies the spice.”
The result of using only virgin American oak is evident in Benromach’s organic malt. “The vanilla is noticeably rich, accompanied by coconut, together with tropical fruit derived from both the cask and new make spirit,” says Stuart Urquhart, operations director at Gordon & MacPhail, which owns Benromach.
“We bottle this as a 7 Years Old, as aging any longer sees the cask influence begin to dominate.”
Vanilla can also take much longer to become visible, depending on the new make spirit profile.
Stephanie Macleod, Dewar’s master blender, says, “Into its early teenage years Craigellachie has distinct sulphur notes, which reminds me of bonfire night. These aromas mask the vanilla. The Craigellachie 23 Years Old still has a subtle edge of bonfire notes, but the vanilla comes through clearly as luscious crème brûlée.”
A final influence on vanilla is the ‘drinking strength’.
Stephanie Macleod says, “Malt whisky aged in Bourbon barrels delivers greater levels of vanilla at higher alcoholic strengths, with more fruit notes appearing after diluting with water. In malts aged in sherry casks it’s difficult to detect vanilla at higher alcoholic strengths as clove and aniseed dominate, but more vanilla comes through when you reduce it.”
Charring and Toasting
Bourbon barrels are charred on the inside by applying a flame to the interior to briefly ignite the oak, before extinguishing with water.
This creates a surface layer of char, around 2mm deep, with the heat also toasting an underlying 2-3mm layer of oak. A flame is then also used in order to toast (but not ignite or char) the interior of sherry casks, creating a 2-3mm toasted layer.
Applying heat breaks down some of the oak’s structural components which ‘activates’ flavour compounds in the toasted layer of oak.Cooperages offer varying levels of charring and toasting, from light to heavy plus.
“Vanillin offers a complex range of notes, which are determined by the level of toasting. A lighter toasting creates floral and lightly buttery notes, with medium and heavier toasting resulting in vanilla pod and spicy notes. However, there’s no standard for toasting times and temperatures; every cooperage does it slightly differently,” says Alexandre Sakon, founder and owner of ASC Barrels in France.