Oak barrels & wine - seeing the wood for the trees

Explore / The Road Less Travelled

Winemaking: Why is Oak Used for Ageing Wines?


Caroline Gilby Caroline Gilby

Caroline Gibly MW takes a scientific look at what makes oak ageing so special.

Why the mighty oak?

Cherry, chestnut, acacia, redwood and even mulberry - many species of tree have been turned into casks and barrels for winemaking, but none have become so indispensable as the mighty oak.

Today's winemaking fashion may be to switch back to concrete or even clay, but it's still hard to imagine the world's great wines without a little seasoning and polishing from time spent in oak.

Questions about oak in wine have become pretty much routine: 'New or old? French or American? Size of barrel and how long?' However, in practice our scientific understanding of how oak influences wine rather lags behind our knowledge of the grapevine itself.

In part, I suspect this is due to the long timescales involved in growing oak trees, which are often well over 150 years old by the time they are felled to make into barrels.

What is it about the mighty oak that makes it have an affinity with wine?
What is it about the mighty oak that makes it have an affinity with wine?

Here's what we do know about oak

The principle constituents of oak wood are:

  • Cellulose 45 to 50%
  • Hemicellulose 20-25%
  • Lignin 25 to 35%
  • Tannins 5-10%

Cellulose is the structural part and doesn't change much during barrel making.

Hemicellulose acts as a binding agent in the wood itself, and is mostly made up of small sugars such as xylose and arabinose. Not only can these sugars be hydrolysed when in contact with wine, but they decompose when heated - forming compounds like furfural, maltol and ethoxylactone which can give roasted, malty and caramel notes.

The lignin also breaks down during both seasoning of the raw planks and as a result of heat during the cooperage process.

The heat comes into play because oak planks (or staves) are straight and need to be softened to form the curved shape of a barrel. Most often for wine, this is done by heating the oak staves over an open fire and at the same time the inside of the barrel becomes toasted or even charred.

Barrels can also be shaped with hot water or steam but these are more typically used for bourbon. Nowadays, the heating process and toasting is much more closely controlled and coopers will offer barrels with a range of different styles and levels of toasting according to the flavour profile and oak tannins a winemaker might want.

Some of the key breakdown products of lignin and the flavours they give are:


  • Vanillin
  • Guaiacol
  • 4-ethylguaiacol
  • eugenol


  • vanilla
  • smoky, roast coffee
  • floral, smoky, spicy medicinal
  • cloves, spice

A vast number of compounds

And of course there are lots more. Another group of compounds found in oak in small amounts are the lactones. These have important sensory characters - giving pleasant oaky notes at low levels but giving coconut characters and even resin notes at higher levels.

Toasting the oak increases levels of these compounds too. Tannins from oak are chemically different to those from grapes themselves, particularly two compounds called ellagic acid and gallic acid. Their levels vary with toasting of the barrels too.

Both the origin of the oak tree and its species have an influence on the flavour profile.

There are three species of oak used in winemaking:

Seasoning oak staves in Hungary
Seasoning oak staves in Hungary
  • Quercus alba, the American oak
  • Quercus sessilis (aka Quercus petraea), the French oak of eastern France and central Europe
  • Quercus robur, the oak of western France, also known as the English oak

This last is also the species that predominates in the forests of inland Croatia (the Slavonia region) and also is reported to be the species used for barrels in Macedonia

Quercus sessilis is very much regarded as the finest species for top-quality barrels, usually sourced from the forests of western France, but also increasingly from Hungary and even Russia.

This wood has the tightest grain and it has higher levels of extractable aromatic compoundssuch as eugenol, lactones and vanillin and a lower concentration of oak tannins than Quercus robur.

In comparison, Quercus alba has even higher levels of lactones and is often associated with a noticeable coconut character in wine.

It seems barrel makers don't always distinguish the species of oak though, often marketing their oak based on tightness of grain and sometimes geographical origin.

Using local oak to enhance sense of place

There is a logical rationale for attempting to use local oak to enhance a sense of place in winemaking. However, it's sometimes a concern that oak flavours can dominate a wine, overwhelming fruit character, though of course in certain styles like Rioja, oak is an important flavour component.

Barrel Tightening
Barrel Turning
Barrel Toasting
Oak is key to the style of wine in Rioja. Bodegas Muga have their own in-house cooperage making barrels from a variety of different oaks that they source themselves, the better to improve their wines.

Also, use of the most famous oaks from France and USA, have been accused of giving an international 'sameness' to the wine. Increasingly most good winemakers would prefer to see the place expressed by the fruit rather than being encumbered by dominant oak flavours so oak use is becoming more subtle.

French oak is widely regarded as the 'bees' knees' but increasingly oak from Hungary, Austria and even Russia is proving to be good. Some local oaks are less good though and it's not always clear whether this is due to the oak species, the forestry management, the climate or the process of ageing and coopering the wood.

In Macedonia for instance, research is going on into factors such as levels of cis and trans lactones and how 'sappy' the wood is and thus how long a period of seasoning is required to make good barrels.

In contrast, Slavonian oak (from inland Croatia) serves a different purpose, usually coopered using thick staves into big wooden casks or vats which may be used for many years. Here there's a need for fairly neutral oak flavours but some gentle micro-scale oxidation to help round out mouthfeel, soften the tannins and fix colour.

The importance of the maturation process

Other factors to consider in barrel making are maturation of staves in the open air to reduce greenness and astringency - a minimum of 24 months is required for decent results and a full 36 months seems to give less oaky, toasty character - i.e. more subtlety, though at a cost of course.

Seasoning Oak Staves in Serbia
Seasoning Oak Staves in Serbia

…and the species

And in cost terms, the species of wood matters too. American oak has more tyloses in it - little plugs that stop the oak being porous as the heartwood forms. When it comes to making barrels, coopers can saw American oak up and it will still be liquid proof, whereas Quercus sessilis must be split carefully along the grain to avoid leaks.

American wood gives as much as 40% more useable material, making these barrels much cheaper. And of course nowadays the big coopers offer a range of barrels in different sizes, with different (and carefully controlled) levels of toast to be matched to the wine style.

An expensive investment

Using oak barrels is actually an expensive investment so only the best wines tend to get this treatment. Top-quality French oak may be well over €800 per barrel (and typically a barrel holds 225 litres, making just 300 bottles of wine). Assuming a barrel gets used for three years, this is around 1 euro per bottle just for the oak, never mind the cost of a special barrel cellar and all the extra handling.

Style of wine does come into it, but for so many great wines, oak is simply an essential ingredient. I can't see oak losing its mighty crown any time soon.

Caroline Gilby is a Master of Wine and a scientist by training. She is a wine writer with a passion for the wines of Central and Eastern Europe and contributes to several winebooks, magazines and websites.

Members' Comments (3)

"The article on oak and ageing I found particularly interesting because I have a great interest in wood, do a fair bit of woodwork, and pay a lot of attention to the different ones available and the qualities of specific timber I acquire. And oak is the timber for which I have particular fondness. So, thank you very much for the article."

Professor David Margolies (31-Aug-2019)

""Thank you for the article on the use of oak for wine casks. The article helped me to further understand the significance of the wine cask, amongst other items, in accounting for the different flavours on the palate when drinking wine. The contrasts with other woods and materials in the storage of wine in casks is most interesting. My knowledge advances little by little."

Dr Robert Culbard (9 May, 2020)"

Mr Robert Culbard (09-May-2020)

""Great article. Tied up lots of loose ends. Thanks very much.""

Mr Timothy Nattrass (14-May-2020)

Want more inspiration?

Sign up for a carefully-curated selection of recipes, guides, in-depth expertise and much more.

Our website uses cookies with the aim of providing you with a better service. By using this website you consent to The Wine Society using cookies in accordance with our policy.


4.4. Cookie Policy

By using The Wine Society website, you agree to cookies being used in accordance with the policy outlined below. If you do not agree to this, you must alter your browser settings to turn off cookies or block those types which are unacceptable to you or cease using the website.

The Wine Society uses cookies to enable easy navigation and shopping on the website. We take the privacy of all who use our website very seriously and ensure that our use of cookies complies with current EU legislation. The following guide outlines what cookies are, the types of cookies used on The Society's website and how they work.

You may alter your browser settings to turn off cookies or block those types which are unacceptable to you, but this will cause difficulties when accessing and using some areas of the site. Instructions on how to do this can also be found below.

4.4.1. What are 'Cookies'?

  • Most major websites use cookies.
  • A cookie is a very small data file placed on your hard drive by a web page server. It is essentially your access card, and cannot be executed as code or deliver viruses. It is uniquely yours and can only be read by the server that gave it to you.
  • Cookies cannot be used by themselves to identify you.
  • The purpose of a basic cookie is to tell the server that you returned to that web page or have items in your basket. Without cookies, websites and their servers have no memory. A cookie, like a key, enables swift passage from one place to the next.
  • Without a cookie every time you open a new web page the server where that page is stored will treat you like a completely new visitor.
  • More recently, cookies have also been used to collect information about the user which allows a profile of their preferences and interests to be created so that they can be served with interest-based rather than generic information about available goods and services.

4.4.2. How do Cookies help The Wine Society?

Cookies allow our website to function effectively. Cookies also help us to arrange content to match your preferred interests more quickly. We can learn what information is important to our visitors, and what isn't.

4.4.3. How does The Wine Society use cookies?

The Wine Society does not accept advertising from third parties and therefore, as a rule, does not serve third-party cookies. Exceptions to this include performance/analytical cookies (see below), used anonymously to improve the way our website works, the provision of personalised recommendations, and occasions when we may team up with suppliers to offer special discounts on goods or services.

The Society uses technology to track the patterns of behaviour of visitors to our site.

4.4.4. What type of cookies does The Wine Society use?

We use the following three types of cookies: Strictly Necessary Cookies
These cookies are required for the operation of our website, enabling you to move around the website and use its features, such as accessing secure areas of the website. Without these cookies, services like shopping baskets or e-billing cannot be provided. Under this heading, we currently use the following cookies:

  • Authentication Cookie and Anonymous Cookie
    These cookies remember that you are logged in to your account – without them, the website would repeatedly request your login details with each new page you visit during your time on our website. They are removed once your session has ended.
  • Session Cookie
    These cookies are used to remember who you are as you use our site: without them, the website would be unable to tell the difference between you and another Wine Society member and facilities such as your basket and the checkout process would therefore not be able to function. They too are removed once your session has ended. Functionality & Targeting/Tracking Cookies
These cookies are used to recognise you when you return to our website and to provide enhanced features. This allows us to personalise our content for you. Under this heading, we currently use the following cookies:

  • Unique User Cookie
    This cookie is used to:
    • store your share number in order to identify that you have visited the website before. Without this cookie, we would be unable to tell whether you are a member or not.
    • record your visit to the website, the pages you have visited and the links you have followed. We use this information to make our website, the content displayed on it and direct marketing communications we may send to you or contact you about more relevant to your interests.
    • This cookie expires after 13 months.
  • Peerius Cookies
    These third-party cookies are used to provide you with personalised recommendations based on your purchase and browsing history. They expire within 4 hours of your visit. Performance/analytical cookies
These cookies collect information about how visitors use a website, for instance which pages visitors go to most often, and if they get error messages from web pages. These cookies don't collect information which identifies a visitor. All information these cookies collect is aggregated and therefore anonymous. It is only used to improve how a website works. Under this heading, we currently use the following cookies:

  • Google Analytics Cookies
    These are third-party cookies to enable Google Analytics to monitor website traffic. All information is recorded anonymously. Using Google Analytics allows The Society to better understand how members use our site and monitor website traffic. Authentication Cookie
In order for us to ensure that your data remains secure it is necessary for us to verify that your session is authentic (i.e. it has not been compromised by a malicious user). We do this by storing an otherwise meaningless unique ID in a cookie for the duration of your visit. No personal information can be gained from this cookie.

4.4.5. How do you turn cookies off?

All modern browsers allow you to modify your cookie settings so that all cookies, or those types which are not acceptable to you, are blocked. However, please note that this may affect the successful functioning of the site, particularly if you block all cookies, including essential cookies. For example, In Internet Explorer, go to the Tools Menu, then go to Internet Options, then go to Privacy. Here you can change the rules your browser uses to accept cookies. You can find out more in the public sources mentioned below.

4.4.6. Learn more about cookies

4.4.7. Changes to our cookie policy

Any changes we may make to our cookie policy in the future will be posted on the website and, where appropriate, notified to you by email. Please check back frequently to see any updates and changes to our cookie policy.