The World of Single Malt Whisk(e)y
Wood and Whisky:
This month, we begin a two part series looking at the influence of wood on Single Malt Whisky.
The use of wooden vessels for aging alcoholic drinks goes back as far as 350BC, and most likely we have the Celts and Gauls to thank. The Celts were masterful wood workers and ship builders who used their technical prowess not just to build ships, but to build wooden storage containers; a complex process that was passed to the Gauls who used it for transporting primarily wine, as the round shape and sturdy durability made it easier to move and store than the other vessels of the time, such as amphorea. But nowadays, it’s not so much important for transportation as it is for aging the spirit within.
While there are three ingredients in Single Malt Scotch Whisky (barley, water, and yeast), some would say that wood is the fourth ingredient, as about 70% of the flavour comes from the wood’s influence, along with 100% of its natural colouring (Scotland does allow the use of E150a caramel colouring which affects the look but not the taste and is widely utilized - but that’s another conversation for another time). Single Malt Scotch is required to be aged for 3 years in oak casks. Cask is a generic term, whereas barrel is a specific size, but one often sees them used interchangeably.
Casks come in a range of sizes, although the Scotch Whisky Regulations stipulate the cask can’t be larger than 700 litres. Barrels (200L) and Hogsheads (250L) are the typical sizes seen for whisky maturation.
A Butt holds about 500L and is the preferred size for sherry maturation, which used to be the preferred vessel for aging Scottish new-make spirit; but, when sherry cask sourcing became difficult during the Spanish Civil War in the late 1930s, and when Spanish bottling laws changed in the 1980s, many whisky companies turned to America and their readily available bourbon barrels.
As bourbon barrels can be charred to different levels, some Scotch brands don’t want to impart quite so much of that intense bourbon character on their whisky, so they make the barrel into a hogshead (or ‘hoggie’) by adding new oak staves and ends to make the barrel that larger size. Such a seemingly slight change has a large impact, as there’s a greater volume-to-surface ratio (meaning not as much whisky is actually touching the inside of the barrel). As barrels are often broken down for shipping purposes and rebuilt in Scotland, it’s also common practice for 5 bourbon barrels to be rebuilt into 4 hogsheads.
While whisky matures faster in a smaller barrel because of the contact ratio, many argue it matures better in a larger vessel. Larger vessels also have less evaporation, as not as much whisky is in contact with the wood.
Oak is the preferred wood for aging single malt, as it’s cellular structure is water tight, but allows for that vital oxygen interaction with the spirit aging inside. While the wood can be sourced from anywhere, American and European oak are most often used.
American oak (Quercus alba) has a tighter grain and therefore gives less colour, as there is less seeping in and out of the wood. White oak grown from a cooler climate (such as Minnesota) also has a naturally tighter grain structure.
European oak (Quercus robur) has a looser grain, which sees much more whisky/wood integration and results in a deeper colour.
These pictures taken at Glenturret illustrate the differences between American and European oak.
A barrel made from an apple tree doesn’t make the whiskey taste like apple, as one might expect. Oak, on the other hand, be it American or European, whether charred, virgin, or seasoned, imparts those naturally desirable flavours we want in our whisky.
American oak has high levels of vanillins which, when used for aging spirit, impart sweet notes such as coconut, vanilla, and caramel. Secondary flavours include tropical fruits, ginger, honey, butterscotch, and almonds.
European oak gives spicier and richer notes and flavours of dark fruits (such as dates and currants), toffee, christmas cake, and cloves.
Seasoned casks will further impart different flavours and characteristics to the maturing whisky, as well.
A cask being used to mature Scotch for the first time (regardless of if it held bourbon, sherry, or something else previously) is called “First Fill”, and this will impart the most colour and flavour to the whisky. If it is used again, it is called a “refill” or 2nd fill, 3rd fill, and so on. The more times a cask is re-used, the less influence it will give, and the more the spirit dominates.
Many companies are done with the cask after 3-4 fills, but they can be rejuvenated - this is done by removing the ends, scraping down the insides, re-toasting or re-charring, and then putting it back together. Rejuvenated casks can be used for another 3-4 fills (meaning it could last up to 100 years, about the same amount of time it takes for a cask-intended tree to mature) and tend to produce spicier whiskies.