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?
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:
- 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
- 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.
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
…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.