Barrel ageing

This issue, Louise Crane explains why barrel ageing is back, and why it can make good beers great


Leopard print. ‘Mom’ jeans. Foraging. Some trends don’t know how to stay dead, resurfacing to make a glorious comeback decades, or even centuries, after they were first hot. Barrel aged beer might seem like a modern trend, but before the 1900s, most beer was barrel aged simply because nearly all the beer produced was stored in wooden barrels. After the turn of the century, plastic, aluminium, and steel rose to prominence, and their usefulness in creating watertight vessels meant that wood fell out of fashion. But barrel ageing is back in the limelight once again thanks to trendsetters like Goose Island, who were using ex-Jim Beam barrels back in 1992 to finish their flagship stout. The question on our (if not everyone’s) lips is: how exactly does barrel ageing work? What is it about wood that magically transforms an average beer into a star? Well, it’s not magic - it’s science.

Since the 17th century, makers of alcoholic beverages appreciate that wood is not only useful for making storage vessels, but for adding flavour, complexity and colour to wine, beer and spirits. Wood is full of tiny little porous holes and because of changes in atmospheric pressure, air can go in and out of the barrel, or ‘cask’. This sucks liquid into the walls of the casks, where it mingles with the structure of the wood before being pushed out again as the cask ‘breathes’. This process is tempered and controlled by the weather. Megan Kelly works at Harviestoun, where Highland Park barrels are used to age their Ola Dubh beer in warehouses overlooking the scenic Ochil Hills, north of the Forth valley in Scotland. “We believe the local varying temperatures allow the beer to have greater contact with the wood which maximises the flavours extracted from the spirit in the wood and the oak barrel itself. As the temperature gets warmer the wood opens up and allows better contact,” she explains.

As soon as beer is pumped into a barrel, it starts reacting with the wood. Not in the chemistry-set-explosion kind of way, but the slow, measured way of the tortoise that beat the hare. Wood is made up of many chemicals that mingle with the beer. The three main building blocks are cellulose, hemicellulose, and lignins, plus there are tannins and a small amount of lipids (oils, fats and waxes), all with their different flavours and textures. 

“We don’t think of wood aroma as tasty, but if you walk through a cooperage or a barrel hall the aromas are usually pleasant and appealing,” explains Chris Curtin, Assistant Professor of Brewing Microbiology at Oregon State University, who researches barrel aged beer in collaboration with Deschutes Brewery and Block15, both of Oregon, USA. “That said, sometimes what we think of as ‘oak’ flavour should be more accurately referred to as ’toasted oak’ flavour.” When barrels are made, they are fired, or ‘toasted’ on the inside to help tighten the staves against each other. The heat releases compounds from the natural chemicals in the wood to produce a smorgasbord of flavour and aromas just waiting to be absorbed by the beer. By examining just a few of them, we can begin to understand the morphosis.

Lactones are lipids contained within the oak, and in low concentrations they can strike us as simply “oaky”. Ramping it up to medium concentrations releases a rose-like aroma, and it’s only when you get to higher concentrations do they give a powerful impression of coconut. To some, this can be cloying but to others it’s addictively sweet. 

Phenolic aldehydes are derived from lignins, complex polymers that make up part of the oak structure. The most recognisable of these is vanillin, which forms when lignin degrades under the influence of gentle heat or mild acid. Turn up the heat, though, and lignin will break down to even simpler steam-volatile phenols, and when they’re around you’ll feel like you’re in a Victorian hospital, the air suffused with medicinal salves and gases. Other phenolic compounds include guiacaols (sweet spice, cinnamon) and eugenol (clove).

The toasty heat also degrades the hemicelluloses wood structure, polymers made up of several simple sugars. Like a furious child smashing up their Lego, the flames break these compounds down into their constituent sugars (glucose, xylose, mannose, rhamnose, arabinose and galactose to name but a few) which then caramelise into furfurals, maltol, cyclotene, and other compounds that give flavours that the most retro of sweet shops would be proud of: bitter almond, toast, sweet caramel and burnt sugar. Maltol is a key sugar here, because its freshly baked bread aroma can increase your perception of maltiness in beer. 

Oak tannins are the least understood part of oak wood chemistry, complex as they are. They’re described as hydrolysable because they can be broken down into simpler parts in the presence of water and acidity, and they’re formed in the growing tree to store food. Ellagitannins arise when glucose combines with ellagic and sometimes gallic acid, and they’re bitter as a recently uncoupled ex. But introduce them to a lovely warm fire and they break down and become more likable (the tannins, not the ex). They’re also essential to maturation for one major reason: oxygen. 

“The key to ageing beer, or any other alcoholic beverage, is that you need some oxygen to facilitate chemical reactions,” explains Chris Curtin. “Too much oxygen and the beer becomes oxidized and unpleasant, too little oxygen and secondary flavours won’t develop.” Wood tannin reacts with oxygen in the presence of a ‘transition metal’ such as iron, copper or manganese, which releases activated oxygen in the form of hydrogen peroxide, a compound that goes forth to create new ones in a cascade of reactions with the other chemicals in the beer and wood. When properly controlled, oxidation can soften hop bitterness, producing malt flavours to step up a place, and add complexity. 

Brewers would never use virgin barrels, as the flavour compounds would be totally overwhelming. Damian Doherty of the recently founded Emperor’s Brewery describes sourcing his barrels: “We only make *very* small scale imperial stouts and porters (20l per batch). Finding oak casks this small was a little bit challenging as I didn’t want new oak casks, which would add too much oak character to the beer too quickly. In the end I sourced used pin casks from Theakston’s Brewery. This meant I could mature the 12+% stouts over many months (6-12 months or more) in oak whilst giving it more subtle oak notes.”

“When brewers obtain used barrels,” says Chris, “they may have been used once (e.g. bourbon barrels) or many times (e.g. rum barrels). These already used barrels are chock full of other delicious flavours from the liquid previously stored in them, and sometimes the barrels still contain some of that liquid!” Even if dry, up to an inch of the wood will be soaked with the previous occupant, which may also be extracted into beer, bringing higher alcohol flavours and grappa-like notes.

A bit of leftover whisky might not be the only thing a brewer finds in her used barrel. Micro-organisms absolutely love wood, especially damp and in the dark. “Microbes are able to penetrate the porous oak staves as far as the liquid seeps in, and they are adept at forming biofilms. All of that is to say that beer going into wood will be exposed to yeast and bacteria,” says Chris. “Mainly we’re talking about Brettanomyces yeasts, lactic acid bacteria, and acetic acid bacteria. Brewers can manage this risk by being vigilant, and sometimes they may find that the microbial impacts are interesting.” For example, when furfural meet active yeasts, they can transform from a bitter almond character to smoky, meaty, and leathery flavours that taste just great in aged beers. But unless a brewer is looking to take advantage of this impact, by making a lambic or a guezue for example, sterility is key. Damian Doherty says, “At large breweries, this is done with an industrial sized steam generator… at Emperor’s, we use a modified wallpaper stripper!”

The results of successful maturation are rich, funky and sophisticated aged beers that have all their flavours in harmony. “Oak has been tried and tested over thousands of years to be a great material for containing beer and allowing it to mature and become more tasty,” says Chris Curtin. Barrel ageing depends on wood, weather, bacteria and the former life of the barrel, a mix of factors that mean that outcomes are almost infinitely variable. As Chris wisely sums up, “Brewers have the opportunity to take a base beer in many different directions flavour-wise when you take all of this into consideration, which is what makes tasting them so much fun.”

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