Understanding Oak Barrel Maturation

Understanding Oak Barrel Maturation

Published : 15-01-2020 - Categories : Spirit Enhancement

If there is one thing that we all go a bit crazy for, it’s oak matured spirits. Even if you are not a big spirits lover, odds are good you’ve indulged frequently on other such oaked products like Balsamic Vinegar, Worcestershire or Tabasco sauces. For those who have discovered the world of a well-aged whisky, tequila, brandy, rum or fortified wine, a lot of appreciation is given to a bottle carrying buzz terms like extra añejo, solera, single barrel, cask strength, sherry finish or paradise.

But what do we really understand about the influence of the oak barrel?

The Language of Oak Barrels

Anyone who has ever attended a Whisky Tasting or Appreciation evening (more specifically a Masterclass) would have been exposed to terms like vanillin, tannin, lignin, lactone, alligator char, conditioning– and something about ‘greedy angels’ – all thrown at them by an overtly enthusiastic brand ambassador.

These terms belong to a language few understand, and while the science may be more than a little confusing, the results are anything but.

As age statements are being removed more and more from bottles, understanding oak selection and cask finish is becoming more and more important.

The History of Oak Barrels

Despite archaeological evidence placing the art of the ‘cooper’ as far back as 100 B.C., the wooden cask has evolved little over the last two millennium.

 When the Romans eventually acquired the skill of coopering off the Gauls around third century AD, it was with the promise of cheap manufacture and improved strength which saw the wooden barrel replace the traditional clay amphora’s of old. And it would have ended there if early cognac, navy rum and American whiskies weren’t forced to spend long periods of time sitting in oak casks, in hot climates.

Time proved to be the secret ingredient. Previously sharp and volatile components dissipated, flavors of vanilla and toffee developed beyond the potential of the raw materials used, and a golden hue was imbued throughout the liquid as if in recognition of this magical transformation.

Let us attempt to understand more behind the mysteries of maturation, explore the importance of oak, the different casks they’re made into, and what really takes place inside them.

Common Barrel Sizes

Note: All Barrel volumes are rounded to the nearest whole number. Exact volumes can vary on Barrel use and between traditional and modern cooperage’s.

Firkin: 41 litres

Taking its name from old Middle Dutch for “fourth” implying a quarter of the size of a British Barrel. Traditionally used for dispensing cask ale

Quarter Cask: 50 litres

A quarter of the size and proportion of an American Standard Barrel generating a higher wood to liquid ratio. Used for rich oak finishes in Scotch and American whiskey

Rundlet: 70 litres

Roughly half the size of a British Barrel. Traditionally used to transport wine

Tierce: 160 litres

Closest cask in volume to that of a modern oil drum and one third of a Pipe. Traditionally used to transport wine, mature rum or store salted goods

British Barrel: 160 litres

Roughly half a Hogshead. Traditionally used to store ale or lager

ASB Barrel: 200 litres

The ASB – American Standard Barrel is used throughout the US Whiskey industry after which most are exported for reuse in maturing other spirit types including rum, tequila, Scotch and Irish Whiskies

Hogshead: 250 – 300 litres

The most popular cask used in maturing Scotch and Irish Whiskies commonly consisting of rebuilt ASB’s from the US which have already held American Whiskey. With a slightly smaller oak to liquid ratio, it’s believed that Hogsheads react better to the cooler Scottish climate. Twice the size of a Barrel, half the size of a Butt, quarter of a Tun. Also used in wine and beer.

Barrique: (cognac) 300 litres & (wine) 225 litres

Standard barrel used for old French wine and cognac although at two different volumes. Traditionally coopered with wooden hoops instead of metal.

Puncheon or Tertian: 450 litres

Also known as a Tertian from the Latin for “third” implying a volume roughly one third of a Tun (330 litres) although modern day puncheon’s are closer to 500 litres. The modern rum industry favours a short, fat puncheon with thick staves known as a Machine Puncheon while the sherry industry prefers a more traditional tall, slim puncheon with thin staves called a Sherry Shape Puncheon.

Butt: 500 litres

Twice the size of a Hogshead, tall and narrow with thick staves and a nice set of hips true to its name. Commonly used for sherry.

Pipe: 650 litres

Tall cask yet stockier and rounder than a Butt with thick staves. Commonly used for port.

Drum: 650 litres

True to its name, short fat and dumpy with wide staves. Common cask for Madeira wine.

Gorda: 700 litres

Commonly used in North America for the marrying or vatting of different whiskies

Tun: 982 litres

Roughly twice the size of a Butt and equal to four Hogsheads. Traditionally used for the fermentation of beer or marrying of spirits. Designed to represent one perfect imperial ton of liquid.

What number of Barrels are used in the World?

It’s believed that at any one time, 20 million oak casks lie maturing in bonded warehouses throughout Scotland. Of these, 90% are said to have come from America. And with global demands for Scotch outstretching its current supply, there is added pressure on American whiskey distilleries to supply them with high volumes of ex-whiskey casks.

According to digital magazine Drink Spirits, a single qualified worker at the Brown-Forman Cooperage in Louisville, Kentucky can assemble up to 500 barrels a day, creating a total cooperage output of near 2000 new American oak barrels each working day.

But they’re not even close to the biggest.

There is only one distillery in the world that makes its own barrels. Classified as a Tennessee whiskey, Jack Daniels is limited by law to using only new American oak barrels to mature their annual yield of 24 million US gallons. In return the company shares more ex American whiskey barrels to the global spirit industry than any other brand.

How do we Innovate with Barrels?

All the while there is an increase in different spirit types being matured in barrels. Different types of wood being utilized to construct the barrels used. Different enhancement techniques (charring, smoking, flavoring).

With this increase in cask finishes and barrel types filled by so many different spirits and beverages around the world, the spirit consumers have never been offered such a range of products to choose from. But are some of these finishes just window dressing, hiding a younger mass-produced spirit underneath?

What is the difference between Wooding and Maturation?

According to the earliest use of the word maturation, it’s in 14th century medieval Latin where we find the birth term maturatio(n-) or maturare to describe a doctors act of “encouraging suppuration”  (the discharging of pus from a boil or wound). Charming.

From there, this early definition evolved through the years to define anything that is “ripe” or “ready of age” until hitting a Wikipedia page some 700 years later as “the emergence of personal and behavioral characteristics through growth processes”. 

For our purposes however, we’ll focus on the modern use of the term and the processes that take place between a new fill spirit and a cask made of oak.

Looking at ALL barrel matured products internationally is not feasible. There are just too many producers, too many different types of products, and honestly – not enough information. The Whisk(e)y Industry is however quite forthcoming with facts and figures.

The current location of Whisky Distilleries outside South Africa (which currently has 5) are:

109 Scotland
5 Ireland
1 Wales
3 England (many new start-ups in the works)
6 Australia
8 Canada
6 India
6 Japan
9 Sweden


Additional nations like New Zealand, Thailand and Korea are also getting into the Whisky production game.

None of these nations however compare to the United States. With over 700 registered distilleries to date, and more opening every week, the USA is by far the leader in the Whisky Distilling Industry. Interestingly enough that merely a decade ago, the US only had 70 registered distilleries. The massive increase over the last 10 years being attributed to the immense growth in the Craft Spirit Industry.

This obviously led to a relative increase in the demand for oak as well.

While each of the above distilleries will be using their own unique techniques to produce their own unique styles of matured spirit, for the sake of simplicity, we will be focusing primarily on the two most iconic; Single Malt Scotch and Bourbon Whiskey.

 According to the Scotch Whisky Association, 60%-80% of a Single Malt whisky’s flavor is attributed directly to maturation. For American whiskey it is around the same ratio however it’s important to point out that the Scotch industry use casks which have already held American whiskey in them while Bourbon is restricted to only using new American oak casks for maturation.

Like using one tea bag for two cups of tea, the first cup will get the greatest infusion of tea in the shortest period while the second receives a slower, longer infusion at a more delicate release.

So while oak influences a similar percentage of flavor in both American and Scotch whiskies, the oak release is quite different. Add to that further flavor variables such as the origin of the oak, a casks previous use and number of uses, cask sizes, stave widths, stave conditioning, maturation environments, toasting levels, etc. and it becomes clear that to truly understand what’s really behind a bottle of ‘fully matured whisky’, one must pay attention to the detail.


Angels’ Share

Whether known as le ángeles cuota or la part des anges, ‘angel’s share’ is an inescapable side effect of the maturation of any spirit despite large quantities of money invested by companies every year into researching ways of preventing it.

Scottish Excise Laws allow Whisky producers to write off 2% of their production volume every year as a natural effect of angels share. Due to climatic influences, temperature, humidity, air pressure, etc. the actual percentage lost in the Angels’ Share varies from location to location. Scotch distilleries in the Orkney Islands (Highland Park or Scapa), have an average loss through angel’s share of only 0.5%-1% annually due to their unique micro climate.

Cognac Factoid: For the Cognac industry, angel’s share equates to a loss of around 22 million bottles per year, making the angels the second biggest consumers of Cognac globally after America. Cognac also has an entire ecosystem built around the angel’s share with a black fungus growing in the rafters and ceilings of maturation warehouses feeding off the alcoholic vapour. Officially called Baudoinia compniacensis, the fungus is enjoyed by local insects which feed off it, in turn feeding the spiders which in turn keep other wood-boring bugs off the barrels and effecting the Cognac.

For the Bourbon, Rye and Tennessee Whiskeys, the angels share is closer to 4% annually. With Kentucky recording average summer highs of around 32°C compared to 19°C in Scotland, both the level of evaporation and rate of maturation are greatly accelerated.

It is commonly believed that if two exact casks of liquid were placed in Scotland and Kentucky at the same time, the cask in Scotland would have to remain there for around three years to equal just one year in Kentucky. A good example of not judging a whisky merely by the age on its label. As the new industry mantra states: “Maturity, not age”.



A process rarely mentioned by brand ambassadors yet regarded by many coopers as holding a greater influence on the final flavor characteristic of a mature spirit than the species of oak itself.

Seasoning refers to the vital practice of weathering freshly cut ‘green’ oak staves by naturally drying them to the point of tightening. As this happens large amounts of unwanted sap, moisture and heavy tannic compounds leach out of the wood yet retain core flavor elements such as sugars and vanillins.

Often referred to as ‘air-drying’, the process is more accurately ‘weathering’ as wind, rain, heat, UV, air and even bugs play an important role in well-seasoned oak. As with all things, there are however shortcuts. Kilns can be used to speed up the process although it is well believed that only natural seasoning delivers the best aromatic profile with the softest level of tannins.

In the case of American mills, most freshly cut oak staves spend at least 18 months seasoning. More specialist coopers only accept staves which have seen 36 months minimum.

In Europe, oak is grown in a more temperate climate than the Americas and as such requires longer seasoning periods. But like maturation, old isn’t always best. Too much seasoning and oak loses too much ‘life’. This can result in the wood becoming brittle and the chance of the stave breaking during lamination (bending) increasing.

In a classic Cognac Barrel Mill, once an oak tree has been selected and cut down, the main trunk section is transported whole to a mill for splitting. An average of 4 barrels can be made from one good tree.

The trunks are quartered to obtain lengths for the barrel staves by a means of splitting. Cutting is not used, to prevent destroying wood veins important in later maturation. Splitting is done while the wood is still green as it is less susceptible to snapping.

The staves are then planed into rough shapes (called merrain) and stacked outside for between three-five years after which they develop a silvery-grey finish darkened as a reaction to the seepage of heavy tannins. Only after this period, and once each stave has been inspected, are they replaned to the specifications of the cooper and shipped out for cask assembly.

Staves can even be stacked for seasoning in different ways, depending on how the millers want the air to move between the wood piles relative to the species and age of oak (wider grain oak generally have more tannin and as such require longer seasoning). End piling, edge piling, crib piling, package piling, lap piling or combinations of them all, are stack variations specific to how the miller wishes to control the amount of airflow, impact of rain water, UV exposure or damp).

Back in the realm of aged spirits, the vast majority of color and flavor imparted on the distillate is done so in its first twelve months of maturation. After this period the characteristics of badly or well-seasoned staves becomes more and more influential on flavor and the charring or toasting influences drop off.

Of all the wood which arrives into a barrel mill, only about 20% will be used for making casks as the vast majority is discarded due to damage, knots or an abundance of sap. A common rule of thumb for millers is to season wood in or near its natural environment as cold frosty or hot and humid environments can negatively affect the seasoning process.

According to experienced coopers and barrel brokers, not even the stave width or thickness contributes as much to a spirits’ final flavor profile as well-seasoned oak does.

Many coopers in the wine industry prefer to season their own staves so they have a more direct control over this important process. For the spirits industry, producers are normally more concerned with buying casks from coopers with a reputation for producing tight barrels, rather than those with a reputation for well-seasoned oak.

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