“The smell …” says Stuart, a smile spreading across his face. “The smell is amazing. We draw straws to see who gets to fill the casks – but the rest of us stand around inhaling the aromas.”

In his long career at Harviestoun, head brewer Stuart Cail must have filled many thousands of casks, stoically loading the sterile metal and plastic containers with beers like Schiehallion and Bitter & Twisted. However, from time to time this everyday process is interrupted by an occasion that seemingly has Harviestoun’s brewery staff grappling for the filling hose.

The source of the excitement is a barrel – an unwanted cast–off from another industry. For Harviestoun, these deliveries mark the start of something exciting. While the vast majority of the beers produced at their Clackmannanshire facility leave immediately, the arrival of these pungent wooden casks means some of their beer won’t be going anywhere for months.

Back in 2002, Harviestoun’s US distributor suggested an addition to their export line-up – American brewers were experimenting with ageing beer in discarded bourbon casks, causing the beer to develop rich, oaky, vanilla flavours. He wondered if a Scottish brewery could do something similar with Highland whisky barrels. After all, he was importing beer from the most famous whisky-producing nation on earth; surely they would be well placed to follow this new trend?

“We had a discussion at the brewery, and thought it would be possible, so approached the Dalmore distillery,” says Stuart. “We came away with one of their casks, and filled it with Old Engine Oil [Harviestoun’s 6% ABV porter]. We left it for four or five months. The guy came over from the States, had one taste, and took the entire lot. Now, we have a ten-year exclusivity deal with Highland Park for their old whisky casks, which is how we produce Ola Dubh.”

Meaning ‘black oil’, Ola Dubh has become one of Harviestoun’s most successful beers, and is released in numbered format, depending on the age of the whisky barrel used (the beer typically remaining in the cask for up to six months). The three regular versions are Ola Dubh 12, 16 and 18 – but in the past, 30 and 40 have been released. “They are really hard to produce though,” says Stuart. “Highland Park don’t have any 30 or 40s planned – and getting hold of casks is the major limiting factor.”

“The most important factor in choosing the beer is that it will benefit from the flavours already present within the wood. Marrying the right beer with the perfect barrel takes skill and experience”

Once filled, the dark wooden barrels sit in a shady spot and are occasionally hosed if the temperature rises significantly. “Distilleries can lose a lot to evaporation and leakage,” says Stuart, “but we hardly have a problem with that – Old Engine Oil is so viscous that if it leaks out of the cask it doesn’t really go anywhere. The crack basically self-seals, after a while.” The most important factor in choosing the beer is that it will benefit from the flavours already present within the wood. Marrying the right beer with the perfect barrel takes skill and experience. Andy Baker, Managing Director of Yorkshire’s Summer Wine Brewery, says: “People just assume we pick the barrel that’s at the top of the pile, or is free. It doesn’t work that way.

“For starters, what is the original beer’s style and flavour profile? What do you hope to achieve from barrel–ageing? Is the barrel freshly emptied and bunged and therefore wet? This adds ABV pickup and also increases the flavours and phenols that infuse into the beer, and at a faster rate. We spend time before the brew day to source the barrels that we feel would best suit the end product.”

Andy and head brewer James Farran acquired casks from two very different distilleries to begin ageing their 9% imperial stout KopiKat. The beer contains a blend of Madagascan vanilla, muscovado sugar and four different coffee beans – so isn’t short of flavour to start. They aged half in peaty Caol Ila casks, and the rest in softer, fruitier Clynelish, with the original, unaltered KopiKat also released, for comparison. “The intention was to barrel-age for twelve months, but after three we noticed considerable pickup and after six months decided that this was quite sufficient,” he says.

“We were more than happy with the results – two completely different beers to complement the original. The vanilla had subsided and the intense roasted notes from the coffee had rounded out.” Timing is all important. Leave the beer in an Islay cask for too long, and you risk it becoming too peaty and smoky. Decant the beer too soon, however, and the flavours ingrained in the wood won’t have mingled, and the beer will lack balance.

“Understanding when the time is right to bottle is vital,” says Paul Miller, founder of the Eden Brewery at St Andrews. “Every cask is different. The risk in trying to get more from the wood is that the beer quality can quickly turn. This is as much trial and understanding based on experience as science,” he explains. When you speak to brewers who are involved with ageing their beer in whisky barrels, that particular word – experience – crops up time and again. Starting the process is a journey into the unknown, and while they can try to mitigate the difficulties by carefully selecting the beer, once it goes into the wood, anything can happen.

After equal ageing length, for example, Summer Wine’s two KopiKats came out at 9.4% (Caol Ila) and 10% (Clynelish) – yet the original Caol Ila whisky was 57% cask strength, and the Clynelish only 40%. Brewers delight in this unpredictability – after all, the whole point of leaving a beer in a cask is to let the flavours develop. For most, the added dimension that comes with ageing is more than worth the cost, and the pitfalls – including infection.

“It’s a weird feeling,” says Colin Stronge, head brewer at Buxton Brewery. “After the control of the brewing process, the careful sterilisation of the tanks, the time you have put in carefully nurturing the fermentation – to then allow your creation to be placed into a wooden cask, sloshing with the dregs of the cask’s previous tenant – every part of your being tells you it’s wrong. Brewers moved away from wood in the 1800s – using barrels opens the door for all kinds of evils to be unleashed.” Evils including bacteria lurking in the wood which can easily sour the batch.

Brewers have options to combat this. Metal brew kits are kept sterile – unlike wooden barrels, which were previously lined with tar, or rinsed with acid before filling. These methods destroy the flavours so instead some brewers opt to fight the bacteria using hot water.

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One such producer is the Bristol Beer Factory. “We steam sterilise the whisky casks,” says Managing Director Simon Bartlett. “This can strip out some of the flavours, but we think it’s better to have a stable product. We prefer to have a beer with whisky flavours that complement rather than dominate.” Colin Stronge disagrees.

“Hot water techniques don’t really get into the wood deep enough to truly sterilise the parts that the beer will leach into and out of – any bugs hidden deeper in the wood will surely survive, and potentially ruin your product.” For those who decide not to pre-treat casks, infection can be a real problem. Paul Miller says over 20 per cent of the Eden Brewery’s initial barrel-aged programme was ‘wasted’ this way. However, there are some out there who do not see this as a waste at all – quite the opposite. Lauren Salazar, Wood Cellar Manager and ‘Sensory Specialist’ at the New Belgium Brewing Company in Colorado, is one of the growing number who encourage bacteria to flourish, by deliberately introducing them.

“We started the programme in 1998,” she says, “inoculating neutral French oak foudres in our cellar with souring bacteria – Lactobacillus, Pediococcus – and all Brettanomyces wild yeast strains. We filled the barrels with either a dark or a pale lager base beer and let them acidify over time – tasting, blending, topping up, making beer, filling again. We discovered and fell in love with a local distiller, Leopold Brothers. We realised we’d love to have some type of partnership with them. That came in the form of New Belgium having ‘first rights’ to their Blackberry Whiskey barrels.”

“After the control of the brewing process, to then allow your creation to be placed into a wooden cask, sloshing with the dregs of the cask’s previous tenant – every part of your being tells you it’s wrong”

This unusual spirit is one of the specialities of the Denver-based Leopold micro-distillery. They blend their rye whisky with blackberry juice, and then age the liquor in bourbon casks – the barrels eventually used by New Belgium. “We take our dark lager Oscar that has been souring in an oaken barrel for some time and rack it into the blackberry whisky barrel. We’ll let it age there for four months – it picks up the char, the blackberry, the heat,” Salazar says.

Whatever path a brewer takes when they age a beer in whisky casks, the process is ultimately all about balancing the properties of beer and wood to gain flavour. Barrel-ageing is breathing new life into brewing – whilst adding a huge amount of unpredictability into the exact science of making beer. As Andy Baker from Summer Wine concludes: “Some distilleries are even wanting the barrels back after they have been filled with high-end beers, to reuse for whisky again.” The barrel, it seems, is no longer merely a container.

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