Maturation is undoubtedly one of the most important stages in whiskey production. In fact, we wouldn't be wrong to say that it is the most important thing. Because it is said by many sources that 70-75% of a whiskey's character is formed during the maturation stage.

Whiskey consultant who has been working in the whiskey industry for over 40 years and has done important work in many distilleries Dr Jim SwanThere is a very good saying that emphasizes the importance of maturation:

“The maturation of alcohol in a barrel and its transformation into whiskey can be compared to the transformation of a caterpillar into a butterfly. The barrel is the cocoon of whiskey.” (Dr. Jim Swan)

In this article, I will try to share this transformation of whiskey with you in many details. In my article, I will talk about the importance of barrel and maturation for whiskey, oak and barrel types and the effect of barrel on whiskey character.

To my notes from the Speyside Cooperage, the barrel-making workshop I visited in Scotland from this link .

A Brief History of Whiskey & Cask

Whiskey was not always an aged/ripened spirit as it is now. In ancient times, whiskey was more distilled, i.e. “New make spirit” was drunk.

As you can imagine, the "new make" whiskeys produced in the past were very different from the whiskeys we know because they were not matured. Compared to now, they had much harder, angular, in short, very difficult to drink characters.

So what happened that barrels started to be used?

In the past, barrels were used for purely functional purposes, for the purpose of storing or transporting whiskey from one place to another. Therefore, it did not matter in which barrel the whiskey was stored, and any barrel could be preferred. There was a "cheaper, better" mentality in the selection of kegs.

Over time, it was realized that the whiskeys that were forgotten in the back of the warehouses or that came out of the barrels from long journeys had more balanced flavors that were more suitable for drinking, and thus the period of maturation in the barrel began.

Why “Oak” Barrel?

Today, only oak barrels are used to mature whiskey. Of course, it hasn't been this way since day one. In the years when the importance of maturation in whiskey was discovered, we see that many different types of barrels are used.

Just like in maturation, the importance of oak and that it is the most compatible wood species with whiskey were recognized over time.

Oak wood has significant advantages over other types in maturing whiskey.

Because oak is a hard wood type, it prevents the liquid in it from leaking out. In addition, oak can be easily formed into the shape required for barrel production. For this, it is sufficient to heat the oak to a certain temperature.

One of the most important reasons for choosing oak is that oak has a porous structure. This porous structure allows the liquid in the oak barrel to contact with air and oxidize. In this way, the coarse alcohol in the barrel softens and gains different characteristics. In other words, oak allows whiskey to breathe.

Oak can be easily shaped, preserves whiskey in a way that it does not leak out, and provides oxidation thanks to its porous structure.

Oak Types and Effects on Whiskey

In the whiskey industry, we see that 2 types of oak are generally used for barrel making:

  • American oak (Quercus Alba)
  • European oak (Quercus Robur)

American Oak (Quercus Alba)

Amerikan meşesinden (Beyaz meşe olarak da biliniyor) yapılmış fıçılar, günümüzde olgunlaştırma için kullanılan fıçıların %90-95’ini oluşturuyor.

Growing faster than other oak species, Quercus Alba is also a very productive oak species for barrel producers with its long trunked structure.

Barrels made of American oak impart aromas to whiskey, such as vanilla, caramel, coconut and honey.

European oak (Quercus Robur)

Traditionally used in Shariah aging, Quercus Robur can be found geographically in much of Europe.

This type of oak has a more dense and tannic structure compared to American white oak. Therefore, it gives whiskey a slightly dry bitter character in the early stages of maturation.

Casks made of European oak lend the whiskey a rich spice, aromas such as apple, pear, hazelnut and toffee.

Mizunara (Quercus Mongolica): In addition to American and European oak, there are also barrels made of Japanese Oak, whose name we have started to hear more often with the increasing popularity of Japanese whiskeys in recent years.

Mizunara barrels are known for imparting aromas to whiskey with intense spice, honey, flowers, cloves, nutmeg and sandalwood.

Although Mizunara barrels provide complex flavors to whiskey, the difficulty in growing this oak and its inaccessibility is the biggest obstacle to further use of these barrels.

Another obstacle is that this oak has a very soft and porous structure. Due to this structure, the evaporation rate of whiskey during maturation is much higher than in normal barrels (Angels' share).

Different Cask Preferences from Country to Country

As in all production stages of whiskey, we see differences from country to country (even brand to brand) in the maturation phase.

Scotch whiskey is mostly aged in used oak barrels. Although this is not required by law, we see that the use of new barrels is very limited in Scotland.

In America, the situation is the opposite. American whiskeys are matured in newly made oak barrels and these barrels can only be used once. This rule is clearly stated in the law. (The rules differ for different types of whiskey)

These different laws in Scotland and America pushed the two countries to cooperate on a large scale. In the current order, barrels used in America are sent to Scotland for reuse.

In this way, the Americans sell the barrels they cannot use again to Scotland, and the Scots can meet their keg needs cheaply in this way. In other words, we can say that this cooperation is quite profitable for both parties.

Why are used barrels used in Scotland?

Many sources mention that previously used barrels are used to mature whiskey in Scotland, but not many details are shared as to why. Actually, the reason for this is quite simple.

As you can imagine, barrels lose their effectiveness with each use. In other words, after each use, the effects of the whiskey in the barrels are reduced.

To unpack a little; There is a clear difference between the effect you will get if you put a whiskey in a barrel and the effect you will get when you put it in a barrel that will be used for the 2nd or 3rd time.

If the Scots had matured their whiskeys in unused casks, that is, in the most active state of the casks, the flavors of vanilla and oak would be very dominant in most Scotch whiskeys we will taste. So we would get a Bourbon-like feel rather than Scotch.

That's why Scots use second-hand barrels to keep the flavors coming from the barrel at the optimum level and to obtain a more balanced whiskey.

Why are only new barrels used in America?

Americans, unlike Scots, want the maximum effect from the cask and use only new casks for this. This is the reason for the intense vanilla and oak flavors we get in almost all American whiskeys.

In different sources, it is stated that the big players in the timber and barrel industry of the period were behind the "new barrel" rule. In this way, during the prohibition era in America in the 1920-30s (Prohibition), it is said that it is aimed to bring the sector back to its feet again.

Transitioning from Sherry Casks to Bourbon Casks

Until the 1940s, most of the barrels used in whiskey production were European oak, in which sherry wine was kept. This is because there was a large demand for sherry wine in the UK at that time.

In those years, sherry came to the country in bulk in barrels and was bottled in the UK by the carrier. Therefore, there was a significant amount of sherry barrels in the middle.

With the decline in demand for Sherry wine over time and the European Economic Union's ban on trading in barrels in the 1980s, it became more difficult and costly for whiskey producers to access sherry barrels.

Therefore, many producers had to make special agreements with Bodegas (Winemakers) in Spain for the supply of these barrels.

Today, sherry casks make up about %5 of casks used for maturation. Among the distilleries that stand out with the use of sherry barrels, Macallan, Glendronach, Highland Park, Glenfarclas such as distillery.

How Is a Keg Made?

The barrels used in the maturation of whiskey are made by expert teams in the workshops (Cooperage) specially established for this work.

First of all, dry oaks are cut into strips in certain sizes according to the size of the barrels to be made. The strip boards are put together and fixed without using materials such as nails, glue or glue. The only thing holding the keg together are the metal rings around it.

These rings are placed around the barrel with special hammers after the boards are put together. Of course, in order to place the rings, it is necessary to give the oak boards a slight camber. For this, the boards are heated and stretched as desired, and thus the rings are placed.

After the barrels are made, they are once again heated according to the desired character. (Toasted) or burned. (Charred).

Heating / Combustion has two important purposes. The first is to add extra flavor to the alcohol that will go into the barrel. Another purpose is to increase the sealing and natural filtration properties of the keg (for undesirable molecules in alcohol).

The amount of heating/burning of the barrel is also very important in gaining the color of the whiskey.

Heating (Toasting) and Incineration (charring) We can briefly explain the difference between them as follows:

Toasting : The process of burning the inside of the barrel at 100-200° degrees for 15 to 45 seconds.

This process enriches the Vanillin and Eugenol components in the barrel. Thus, we get more flavors such as vanilla, grape and cinnamon from the whiskey matured in the barrel.

Charring: The process of burning the inside of the barrel at 250° for 15 to 45 seconds until it chars. With this process, the inside of the barrel takes on the appearance of crocodile skin. After carbonization, the filtration feature of the barrel increases even more.

We expect to experience intense and rich flavors such as caramel and honey in whiskeys matured in barrels that have undergone this process. We see that whiskeys matured in these barrels are generally dark in color.

The temperature and how much the barrel is burned directly affects the character of the barrel and the whiskey waiting in it.

From left to right, you can see the color of the barrel when it is burned at different levels.

Barrel production is generally done by traditional methods as I mentioned. But there are also workshops full of technological equipment using much more modern methods (For example, Speyside Cooperage)

We can compare barrel making, which requires serious experience and effort, to a craft. To the barrel masters who do this craft, "Cooper" it is called.

How Does Whiskey Ripe?

Olgunlaşma, damıtım sonrası elde edilen yaklaşık %60-70 dereceye sahip alkollü sıvının meşe fıçıya girmesiyle başlar. Fıçıya giren yüksek alkollü sıvı, fıçıyı adeta içeriden kemirmeye ve yavaş yavaş farklı aromalar/tatlar kazanmaya başlar.

Depending on many variables such as the time spent in the barrel, the type of barrel used, the size of the barrel, how many times it has been used before, what has been kept in the same barrel before, these aromas/flavors gained by the whiskey from the barrel vary.

The preferences of these details about the barrel vary according to the country, the distillery or the whiskey to be produced.

According to Scottish law, whiskey must mature for a minimum of 3 years in oak barrels with a maximum volume of 700 litres.

Although there are different rules in each country regarding maturation, we see that the 3-year rule of Scots is applied by many countries. We can point to America as an exception to this.

according to american law Bourbon The ripening time set for only 3 months! Bourbon matured for 2 years or more Straight Bourbon it is called. If a Bourbon is younger than 4 years, it must be stated on the bottle.

One of the most important things we need to know about maturation is the fact that whiskey matures only in barrels. As soon as the whiskey is taken out of the barrel, maturation/aging stops. This is also called "extinguishing" or "waking up" in different sources.

In short, if you take a 10-year-old Talisker today and hold it for 20 years, you still have a Talisker, not a 30. Talisker 10 you are holding.

Talking about the maturation of whiskey, a whiskey term that I like very much. "Angels' Share"It is impossible not to talk about it.

Whiskey, which has been waiting in barrels for years, evaporates at a certain rate every year and mixes with the air. While the evaporation rate in Scotland is %2, this rate may vary depending on climate and environmental conditions.

The Scots believe that this whiskey, worth millions that flies out of their barrels every year, is taken by the angels as a price for making good whiskey.

What is the Finish in the Maturation Process?

On some bottles “Finished in Oloroso Casks” or “PX Sherry Cask Finish” We see phrases like I can briefly explain the Finish process as follows:

Finish: A process applied in addition to the standard maturation process. In this process, the purpose of which is to add richness to the whiskey, the whiskey is kept in a different barrel for a certain period of time.

Just like in maturation, how long the whiskey sits in a barrel is very important.

Many different barrels with different types of beverages can be used for this process: Mizunara, Sherry (Oloroso, Fino, Amoroso…), Stout, IPA, Rom, Cabernet Sauvignon, Sauternes…

To give a few examples of whiskeys that have been finished in different barrels: Bowmore Darkest 15, Chivas Regal 15 XV, Jameson Caskmates Stout Edition, Langatun Old Deer, Benriach 12 Herodotus Fumosus, Johnnie Walker Blenders' Batch Wine Cask Blend.

What are the Effects of the Cask on Whiskey?

We can examine the effect of the barrel used in maturation on whiskey under 3 headings.

1. Subtractive (Natural Filter)

The fired oak barrels act as a carbon, allowing the removal of unwanted molecules (such as sulfur) in the high alcohol liquid from the distillery.

2. Additive (Character and Color)

The barrels contain components such as Cellulose, Hemicellulose, Lignin, and Tannin. When the barrels are burned, these components enter into chemical reactions and release different aromas.

E.g; Hemicellulose releases caramel flavors when heated. Thus, it adds sweet flavors and color to the whiskey. Lignin brings out flavors like vanilla and coconut. Tannins give whiskey a pleasant smell and color.

3. Interactive

Oak is a porous type of wood. This ensures the contact of the whiskey waiting in it with the air (oxidation). In short, the oak barrel allows the whiskey to breathe, and the breathing whiskey softens over time, moving away from the appearance of coarse alcohol.

The alcohol in the barrel is also affected by the environment it is in as it breathes. For this reason, the environment (temperature and humidity) where the barrel is located is also very important in the character of the whiskey.

Keg Types

Until this part of the article, we have seen that many different variables such as the type of oak used in the barrel, the way the barrel was burned, the number of times it was used, and what was kept in it before could affect the character of the whiskey.

In the last part of the article, I will share the main types of barrels used in the whiskey industry.

Bourbon / Barrel: Also known as the American Standard Barrel, this type of barrel has a volume of approximately 200 liters. Made from American white oak, these barrels cover most of the barrels available on the market.

Hogshead: These casks, which have a volume of about 225-250 liters, are generally used for aging the Sherry.

Butt: Butt barrels, with a volume of about 475-500 liters, are usually made of Spanish oak and are used to ripen sherry.

Quarter Cax: As the name suggests, the “quarter barrel” has a small volume of 45-50 liters. Being small in volume, it directly affects maturation and character by allowing the liquid in it to contact more of the barrel. (Laphroaig Quarter Cask)

Barrique: These barrels, which have a volume of 250-300 liters, are generally produced from French oak and are preferred for aging Wine and Cognac. (Kavalan Vinho Barrique)

Puncheon: Puncheon casks with a volume of 450-500 liters are generally preferred for sherry and Greek aging.

Port Pipe: Port Pipe barrels made of European oak have a volume of 600-650 liters. It is generally used for aging port wine. (Glenmorangie Quinta Ruban)

Madeira Rum: These barrels, which have a volume of 600-650 liters, are generally produced from French oak and are used for aging wine.

While bourbon (Barrel) barrels are among the most used barrels in the industry, we see that other barrels are generally used for finishing.


Broom, D. (2014). Whiskey The Manual. Great Britain: Octopus Publishing Group Limited.

Fred Minnick (2015). Bourbon Curious: A Simple Tasting Guide for the Savvy Drinker. USA, Zenith Press.

Jackson, M. (2015). Michael Jackson's Malt Whiskey Companion. Great Britain: Dorling Kindersley Limited.

Maclean, C., Maclean's Miscellany of Whiskey. Great Britain: Little Books Limited.

Maclean, C., Broom, D., Gardyne, TB, Buxton, I., Mulryan, HO, Smith, GD (2016). World Whiskey. Great Britain: Dorling Kindersley Limited.

If this summer caught your interest;

You can check out the "Whiskey Information" section where I share general information about whiskey!

3 Responses

  1. triumph

    Thank you very much for your articles that I learn something new every day and read without getting bored. This article is a very good article in which the basic information that everyone interested in whiskey should know is definitely useful to know, in a simple way that everyone can understand. I hope your enthusiasm will continue and we will continue to learn new things from you, from every possible platform 🙏🏻


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