How distillers are playing with oak varieties in their casks — and looking beyond oak altogether

How distillers are playing with oak varieties in their casks — and looking beyond oak altogether

Forget previous cask contents — what if experimenting with oak species could unlock fresh flavours in whisky maturation? With new oak releases increasingly popular, it’s time to embrace the tree's potential

Production | 26 Feb 2024 | Issue 196 | By Kristiane Sherry

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Sturdy. Reliable. Enduring. Oak trees conjure up a feeling of calm. Whether as a childhood memory or the focal point out of the office window, oaks – at least in the UK — usher in a sense of the serene. They change with the seasons, but they remain for centuries, underpinning a grounding stoicism. Therefore it seems almost discordant that the oak finds itself at the centre of a whisky flavour revolution.


Recent years have seen an uptick in new oak releases. (A quick note: while many producers and brands use the term ‘virgin’ oak, in this feature they will be referred to as ‘new’.) Partly, the trend is due to a shifting market that makes traditional used casks more difficult — and expensive — to procure. But it is also attributable to the increased understanding that different species of oak can impart a wealth of flavours in maturation.


It’s commonly estimated that a whisky, or any maturing spirit, will get 60–80 per cent of its flavour, and all its natural colour, from the cask it matured in. This is because while a cask is (hopefully) watertight, it isn’t airtight. Casks ‘breathe’, drawing the liquid contents in and out of the wood over time.


Alasdair Day studied botany at Glasgow University before working in the food industry and ultimately winding up in whisky. The Raasay Distillery co-founder knows a thing or two about how plants impact flavour. “There’s something like 200 species of oak,” he says. It opens up a lot of questions around flavour. “Then it’s ‘will that work with our spirit?’. Because the way the spirit interacts with the oak is as interesting as the oak species.”


Of course, the industry doesn’t look at hundreds of oak varieties – yet. The predominant three are Quercus alba, or American white oak, and Quercus robur and Quercus petraea from Europe. Stuart MacPherson, former master of wood at Macallan who now consults for the likes of Ardgowan, describes them as “the standard”. He also notes that there are sub-species and hybridisations. These subtypes are all distinct.


“The kinds of compounds within plants and trees are like cellulose, hemicellulose, and lignins,” he explains. “Then you have secondary compounds like terpenes, oak lactones, and tannins.” While some compounds such as cellulose never impact flavour, others do, and the volumes present vary by species. “Your Quercus robur has 10 times more tannin than, say, Quercus alba,” he details. “There’s a significant difference in terms of the chemical compounds within the species, and we know the different species will produce a different flavour of mature whisky.”

Casks at Ardgowan Distillery

It’s not just the make-up of the trees – their structure impacts flavour, too. The tightness of the grain is critical. For Max Vaughan, who co-founded Derbyshire’s White Peak Distillery, it’s about tracking what you’re using and where.


“French oak typically has a coarser grain structure, with higher associated oxygen permeability and earlier flavour extract, especially tannins,” Vaughan explains. “We see this with the STRs [shaved, toasted, and recharred casks] that have French oak heads and in particular more spiced and citrus fruit notes, which plays well with our lightly peated spirit.”


Ultimately, the grain tightness influences how porous the cask is — how far into, and how easily, the spirit moves into the wood. “The European oaks tend to be tighter veined and more porous, and the American oak tends to be looser and less porous,” explains Billy Walker, master distiller at GlenAllachie Distillery and a new-oak evangelist.


There’s another factor at play here, too: toasting and charring. By applying heat to the oak before the cask is filled, you’re “decomposing the structure of the hemicelluloses into various sugars,” Walker explains. Essentially, those flavour compounds — whatever they are, depending on the oak species — become more accessible to the spirit. “When you char you are caramelising these sugars so you’re going to get chiefly those molasses notes, toffee notes. We can actually forward-think some of the things that we want to get out of the wood.”


And then there’s a note on how the oak is coopered. David Boyd-Armstrong from Northern Ireland’s Rademon Estate Distillery makes a case for paying attention to how the cask is raised. “For us, we don’t use any oven-dried wood. It’s all a minimum of three years’ seasoning,” he says, describing the process of leaving the wood out before it enters the cooperage. “We do believe that it does strip out the tannins.”


With so many variables at play, it’s easy to understand why oak finds itself at the flavour forefront. Take a step back, and geography becomes paramount, too.


One of the biggest factors on that all-important grain tightness is climate. “Take Quercus petraea that’s grown in a cooler part of France,” Day explains. The oak grows more slowly here. “Its grain’s really tight, and you’re not going to get too much influence from that oak.” Look to a warmer part of France, and the faster-growing Quercus robur is more prevalent. “It’s going to have a much wider grain, so you’re going to get further into the wood and you’re going to get more impact from the oak.” Cold-climate Russian oak, he suggests, will be so quiet with such fine grain that he suspects whisky could mature in it for 20 years.


“What’s interesting is that we’ve got some Quercus humboldtii casks, Colombian oak,” Day continues. “It grows 3,000 feet up in the Andes Mountains in Colombia. It’s a warmer part of the world, but because of the altitude it’s growing more slowly. But I mean, these are incredibly active casks.” Excitingly, he’s got Raasay whisky that’s been fully matured in Quercus humboldtii due to be released in 2024.

GlenAllachie master distiller Billy Walker sampling from a cask with warehouse manager Lindsay Cormie

GlenAllachie’s Walker has also been eyeing up the Colombian oak. He’s used mizunara casks as well, but they are notoriously difficult to source. However, he’s considered playing with geography to find a work-around — a different oak but one that would impart a similar flavour profile.


“We looked at getting casks from the Eastern Seaboard of Russia, somewhere on the same latitude as Hokkaido,” he details. “We thought maybe, just maybe, there might be an interesting relationship.” Spoiler alert: there wasn’t.


“They were very good casks and there were some very interesting flavour profiles, but it didn’t give us anywhere near mizunara,” Walker concedes. But the quest continues with the same theory. He’s fascinated by Mongolian oak, again from a similar latitude. “We think it could be an interesting source of a new style of wood, and these casks will be coming in within the next two months.”


To discover what other experiments are underway, it’s interesting to look further afield. While Scotland’s whisky makers are beholden to oak due to the 2009 Scotch Whisky Regulations, policed by the Scotch Whisky Association (SWA), other makers can push the boat out even further.


Over in Tennessee, Jack Daniel’s vice president, master distiller, and director of quality Chris Fletcher is looking at wood experiments alongside other production trials. “We’ve done baked maple barrels as a finishing barrel,” he states. But they were structured in a way that made them “leak like crazy”, so it was only possible to use them for 6–12 months. “Even in that short amount of time the whisky loss would be the same as four years in a white oak barrel,” he continues. “That really constrains us in using different types.”


In terms of oak, the distillery sources from Tennessee, the Appalachian Mountains, and Missouri. “You can go pretty far north, but you can’t go very far south because it stays too hot,” he says, indicating the desire for a tighter grain. Different oak types is something he looked at during his research and development days. “There were never any real clear conclusions, there may be some very minuscule and small differences. But nonetheless, I think that it is interesting, you know.” It’s an area of innovation he sees as holding “some opportunity”.


Similarly free from the constraints of Scotch is Shortcross, Rademon Estate Distillery’s whisky brand. Boyd-Armstrong is getting excited about some upcoming expressions. “We are actually doing a series of malts that
will have a finish or a secondary maturation in mulberry, acacia, and chestnut,” he reveals. “They’re all different. That’s the beauty.”

Chris Fletcher, master distiller and director of quality at the Jack Daniel Distillery, assesses a sample

Back in Scotland, and oak creativity continues. Walker says he is learning a lot from his trials, but cautions that “we have to follow them religiously”. Casks can “overcook”, meaning the spirit takes on too much flavour from the oak. With these experimental projects breaking new ground, no-one knows yet where that sweet spot might be.


MacPherson’s aim at Ardgowan is to slow things down. Besides the oak species, another variant at the coopering stage is size of vessel — and he’s going big. “I can’t go into too much simply because of the NDAs, but the casks are bigger,” he says, adding that they are SWA compliant (i.e. do not exceed 700 litres in capacity). He’s also explored toasting levels with the supplier to make sure the spirit can stay in the casks for quite some time. “We believe it will be beneficial in the long term for the development of that spirit.”


One tricky part to all this is the sustainability factor. Proper forest management is going to be critical, and some places are better than others. Then there’s the sea miles accrued by shipping casks.


“Clearly, if we could use Scottish oak casks and they were just coming from up the road, that would be fantastic,” says Day. But as well as not being plentiful enough, Scottish oak, like mizunara, has a reputation for being difficult to work with.


Day adds that there’s a “fair point” in critique about sourcing casks from the other side of the world. “It’s a lot easier to bring casks from France,” he continues. “But bringing them from Colombia is probably comparable to bringing the ex-rye casks over.” (He sources these from Sagamore Spirits in Maryland.)


Another question abounds. With increased demand, trees could end up being harvested at a far younger age. This could have environmental consequences for the ecosystem of a forest, but could there be flavour implications, too?


“To be quite honest, I would say so, although I don’t have any scientific evidence to back that up as such,” muses MacPherson. “You would imagine that as a tree grows and develops, as it matures, becoming stronger, the compounds would have an influence.”


There is a collective thought that an answer to the sustainability question would be for the SWA to look at broadening wood types permitted in Scotch maturation. That seems like a big leap right now, but it may become more urgent as the climate crisis progresses.


Really, distillers are near the start of the flavour discovery journey when it comes to new oak. With so many factors at play, so many levers to pull, there’s plenty of space to experiment in. Or as White Peak’s Vaughan puts it, “There are many compounding complexities, which is one of the wonders of whisky making!” What a wonder it is. 

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