A barrel-aged beer is a traditional style or a unique experimental beer, that has been aged for a period of time in a wooden barrel.

“They tend to be premium or super premium beers,” says Dr Gearoid Cahill, director of brewing science at Alltech Lexington Brewing and Distilling Company. “They are usually high alcohol and small batch, and come at a high price, and they deserve that as it is painstakingly prepared. They aren’t easily scaled up and we have to do a lot of actively managing that in order to make sure we get the most consistent product that we can make.”

Bourbon barrels are the gold standard when it comes to used casks. But with the number of breweries in the UK exceeding 2,000 and with distilleries also looking to buy bourbon barrels, producers face more difficulty acquiring them affordably. In the past, used barrels once fetched £50, but now the used barrel commands upwards of £150.

“A lot of people are using the bourbon barrel, but with Irish and Scottish whiskey producers trying to get their hands on these, you’re already competing with them straight away. The competition for these barrels is fierce,” says Dr Cahill.

Alltech Lexington Brewing and Distilling Company has been brewing its Kentucky Bourbon Barrel Ale since 2006, and the company makes 40,000 barrels of barrel-aged beer per year. “That’s a lot of beer going through,” says Cahill. “Barrel-aged beer is hard to do when you’re doing a barrel here and there, but to do it on full scale and get consistency is tricky. While we do a whole range of other beers, the Kentucky Bourbon Barrel Ale makes up about 75 percent of our production and we need to make sure we keep making it as well as we can.”

The right barrel

Knowing the history of the barrel that is going to be used is vital. The nice thing about spirits is that they’re high in alcohol, which means that the barrels they inhabit are more or less devoid of microscopic life forms. But the way the barrel has been looked after still needs to be inspected thoroughly.

“So you have a four year old bourbon that has been emptied and bottled, and you have this barrel. That’s an excellent barrel to use for barrel ageing beer – if it’s fresh,” says Cahill.

He adds: “Let’s say the barrel is two months old and it has been hanging around in a warehouse before you get your hands on it. If that bourbon barrel dries out you’re in a bad place. First of all, if it really dries out it leeks like hell. It’s only a piece of wood at the end of the day made up of a load of sticks. if it shrinks you will not fill it with anything. It may sound like a stupid question, but your first question should be “Is it water tight?” Then you should be asking, “and is it wet?” If it’s watertight but it has dried out then you’ve lost a whole load of bourbon character and flavour.

“Also, if the barrel has been left open and it is dry and there is no more bourbon in there, you can have anything growing in there. You could get fungus and bacteria in there. Therefore, one that is watertight but dried out loses its antiseptic properties. The history and condition of the barrel is very important. But if you source fresh barrels, then you’re on the way to having good success at creating barrel-aged beer.”

Whereas, with wine, bacteria can happily grow in the barrels. Cahill says that while extra care needs to be taken, these barrels can be used. “If you look at a barrel that was used for wine, and it is a sour as hell inside – it has a bit of fungus growing in it – it can certainly still be of use. What you need to do is rejuvenate that barrel.”

Rejuvenation techniques can bring a barrel back to life. The first thing that needs to be done is sterilising the barrel. “We’ve done this. We’ve picked the worst popular barrels that we could and as we would say here in Ireland – we steamed the bejesus out of them. The steam went into the barrel, the bung hole was in the bottom, we steamed them for 10 minutes.

They get really hot, the wood takes a while for it to come through, but a lot of heat builds up in it, we steam them and let them drain out, and then we fill them with beer after they’d cooled down. And we had perfectly good beer, we didn’t have a lot of bourbon character in it, but we had nice oak flavours in there, and the beer was perfectly sound.”

Cahill explains that there are some manufacturers that steam their barrels to get the last dregs of bourbon out, and while the barrel may be wet the bourbon contribution might be quite low, and may need reconditioning. “So if you steam the barrel and recondition it then by adding red wine, or bourbon or sherry, back into it that’s an interesting thing to do and there are some people in Europe that are reconditioning barrels in that way. They’ll do all that work for you and they’ll sell you a reconditioned barrel and it’s pretty good, it is pretty bullet proof at that stage,” says Cahill.

Cahill says that it is important to know when a beer is considered a beer, and when it is considered a spirit. “If you add in bourbon or whiskey into your barrel, rejuvenate it, and you’re leaving liquid in that barrel then technically you’re adding spirit to beer, and it’s not a beer anymore it is a spirit. You will pay through the nose dearly for the duty bill on that. So reconditioning or re-soaking the barrels in the original liquid is fine using a duty paid liquid, but you then need to make sure it is drained off completely, or you’ll be in hot water,” Cahill notes.

Making beer

Filling and emptying is a careful process, and when it comes to using barrels extra care needs to be taken as the environment is not as controlled as stainless steel. “We will change the valve settings and run CO2 through down to the bottom of the barrel – and CO2 is much heavier than air so it basically fills up the barrel with CO2. Then we switch over and fill our beer from the bottom. So we don’t get any air mixing; we fill it up to the brim full, hammer in a bung and off we go,” says Cahill.

The Kentucky Bourbon Barrel Ale is always stored cool. “We can’t afford to have a high level of failures,” says Cahill. “Some people are storing ambient, works really well for sours, it’s a bit risky if you’re not doing that. We collect our own barrels from our distillery, sometimes we have to go further afield as well to collect our barrels. They are fresh, and in the summer we keep them in a giant cold room, even though they are bourbon, they’re empty, they’re stable, but when we use them fresh we keep them cold because we have to minimise the risk. If three percent of our barrels get infected, first of all it is a enormous waste of beer, but secondly if we had any of that beer get out to market our reputation would be spoiled.

“A cool area out of sunlight as a bare minimum and refrigerate it if you can. We mature our barrel-aged beer in the cold at four degrees celsius for six weeks. We find that’s the sweet spot, and it is a minimum of six weeks, we sometimes have to go to eight weeks depending on stock levels and production. Then we go into the cold after six weeks and we take out our beer,” says Cahill.

Some brewers are putting their beer into a barrel for much longer than Alltech Lexington Brewing and Distilling Company. “It’s a brave step, but I’ve tasted some beer that has been aged for a year in barrels and it was fantastic. They had used some souring cultures as well and I think that’s how it was able to survive the journey of one year in a barrel.”

However, Cahill notes that the flavours produced from the barrel aged process are intense, and sometimes hypersensitive to the treatment.

He says: “Bourbon barrels hold about 200 litres. There’s actually two square metres of oak in contact with your beer at that stage, and that is quite a large area for impacting a flavour. But if you take the volume of spirits, so I have a barrel of bourbon, I empty it out, and now I’m going to do something with beer, there’s actually six litres of bourbon at high strength available to you from that barrel, even though technically it’s empty.

“So, that’s fantastic, it’s a wonderful flavour, but you need to be careful that if it’s a well wetted barrel, and if you’re putting in a lighter flavour beer, particularly with whiskey it seems to happen, you can get an over-pronunciation of the whiskey notes so that you get a beer that is pretty honking of whiskey and it is out of balance. It works with some beer styles and not with other ones.”

So what does the barrel give you? The six litres of flavours from the bourbon, and the wood itself, can give you some vanillins, lactones, and a lot of different sugars, which add to the character of the beer you’re maturing. But with sourcing of barrels becoming increasingly difficult and more costly, Cahill notes it can be a “nail biting process and it isn’t for everyone”.