Welcome to the seventh part of Terroir & the chemistry of wine. We have followed the wine from soil and acids through phenols and aroma compounds to fermentation and the malolactic conversion. Now we let the wine rest. Maturation and ageing are not a pause, but a long series of chemical reactions that keep shaping flavour, colour and bouquet after the first racking is done.
At expert level it is worth remembering a basic distinction: maturation denotes the biochemical reactions that, after the first racking, lead to quality improvement rather than spoilage. The wine's ageing happens in two main phases, bulk ageing on a vessel and ageing in the bottle. In this part we concentrate on the barrel phase, where oak, oxygen and time work together.
What you will learn
- What the compounds of oak add to the wine, and how they vary with the barrel's type and age
- What role slow oxidation and micro-oxidation play during ageing
- Which chemical reactions drive the maturation in the barrel
- How barrel type, age and toasting leave their mark on the final result
The compounds of oak
The heartwood of oak contains the most widespread constituents that can potentially be extracted into the wine during ageing. When the wine lies in the barrel, three types of reactions occur simultaneously: extraction of the wood's constituents, oxidation of the wine's components, and reactions between organic substances that form new congeners. The barrel is thus both a source of new compounds and an environment where the wine's own compounds are transformed.
Among the compounds that oak contributes to red wine are polyphenols in the form of ellagitannins. Their profile clearly depends on whether the wine is matured in new barrels, in second-fill French oak or in American oak. This is one of the reasons that the choice of barrel is not only about flavour nuances, but also about the wine's structure and its ability to mature further.
Oak can also reinforce certain aroma attributes that are released from the wine's own bound precursors. Hydrolysis of glycosides increases, among other things, the intensity of wood, honey and smoke notes at elevated temperatures. It is a good example of how the barrel and the wine's chemistry do not function separately, but intervene in each other.
Slow oxidation and micro-oxidation
A barrel is not a hermetic vessel. Through the wood and during handling, a small amount of oxygen continually reaches the wine, and it is precisely this slow oxidation that causes the principal changes in flavour and bouquet during ageing. The point is balance: controlled oxidation matures the wine, while excessive oxidation is harmful to quality.
For reference, the oxygen saturation level in wine lies around 6-7 mL per litre. In practice the winemaker works far below this level during maturation. Note here a well-known paradox: topping up the barrels to avoid oxidation can in fact introduce more oxygen, because the added wine has itself been aerated. Oxygen is thus a quantity that must be actively managed, not merely kept out.
In red wine production one can deliberately control the oxidation with micro-oxidation, where oxygen is dosed to the wine in controlled quantities over a period from four days to a month. This gives the winemaker the possibility to mimic the barrel's slow oxygen contribution in a more reproducible way.
Oxygen's counterpart: sulphur dioxide
Sulphur dioxide (SO2) plays a significant role in maturation and stabilisation. SO2 delays ageing by consuming oxygen through the oxidation of sulphurous acid, and thereby functions as a buffer against too rapid oxidation. We go in depth with SO2 and oxidation in the next part of the series, but it is worth having on board already here, because oxygen management and sulphur are two sides of the same equation during ageing.
Reactions during maturation
The maturation itself is a quiet chemical industry. Over several months to years, several things happen at once: particles settle, CO2 equilibrates, the colour stabilisation begins, and bound aroma compounds are gradually released. Ageing enriches the wine with aroma-active compounds, stabilises the colour and improves the complexity of the mouthfeel.
At the molecular level several mechanisms are in play. Aldehydes and acetals are responsible for the development of new flavour in immature table wines. Higher alcohols can be converted into acids that form esters with ethanol and other alcohols, which contributes to the aged wine's flavour and bouquet. Ester formation is thus linked to the fixed acids in the wine, which include tartaric acid, malic acid, succinic acid and lactic acid.
Temperature is a decisive variable. The temperature coefficient Q10 doubles for every 10 °C the temperature rises, so the reactions accelerate markedly with the heat. This explains why the storage temperature is such a central parameter. The optimal maturation temperature for varietal white wines lies between 7 and 21 °C with an optimum around 13 °C, while varietal red wines on average mature best around 15 °C within a range from 7 to 23 °C.
The release of the aromas
A large part of the wine's aroma reserve lies bound as glycosides, and this pool typically exceeds the free volatile compounds in quantity. During maturation the odorous aglycones are gradually released, either enzymatically or with the aid of acid and heat. Hydrolysis of terpene glycosides increases the varietal aroma in aromatic wines such as Muscat. Elevated temperatures accelerate the hydrolysis of glycoside precursors and increase compounds such as vitispirane, 1,1,6-trimethyl-1,2-dihydronaphthalene and damascenone.
It is also here that the wine's age makes a difference in robustness. Older wines aged in oak are more resistant to heat-induced changes than younger wines, which have higher levels of yeast-synthesised esters. The chemistry of maturation is thus dynamic: the wine's starting point determines how it reacts to time and temperature.
Barrel type, age and toasting
The character that ageing in small oak barrels gives depends on a whole series of factors: the oak species, growing conditions, type of drying of the staves (stave seasoning), the degree of toasting, how many times the barrel has been reused, and the duration of the ageing. Each of these parameters nudges the balance between extraction, oxidation and transformation.
The frequency of reuse is especially important. As mentioned, new barrels, second-fill French oak and American oak give different ellagitannin profiles in red wine. A new barrel releases the most compounds and the most distinct oak character, while a barrel that has been used before contributes more subtly and functions to a greater degree as an oxidation vessel than as a source of flavour. The choice between French and American oak is thus both about compound profile and about the stylistic direction the winemaker wants.
When one does not want the barrel's full effect, but still wants oak character or oxygen protection, there are alternatives. Oak chips, infusion spirals and staves in steel tanks give advantages such as shorter ageing, better protection against oxygen and less evaporation loss. They are not the same as maturation in barrel, but they illustrate that the compounds of oak and the barrel's oxidation can be separated and controlled independently.
For sparkling wine a different logic applies. During bottle ageing of Champagne the wine remains in contact with the yeast (lees) for full maturation, and even a contact period of a minimum of four months shows a significant effect on the final quality. Here it is the yeast's autolysis on the lees, rather than the oak, that shapes the complexity.
In brief
- Maturation is quality-improving biochemistry after the first racking, divided into ageing on a vessel and in the bottle.
- The heartwood of oak releases compounds, among them ellagitannins, whose profile depends on barrel type and reuse.
- Three reactions run simultaneously: extraction from the wood, oxidation and the formation of new congeners.
- Slow, controlled oxidation matures the wine, while excessive oxidation harms it. SO2 functions as an oxygen buffer.
- Temperature governs the speed (Q10 doubles per 10 °C), and glycoside hydrolysis gradually releases bound aroma compounds.
Frequently asked questions
Why does an old barrel give less flavour than a new one?
The heartwood of oak contains the compounds that can be extracted into the wine, and the pool is gradually emptied with each use. A new barrel therefore releases the most ellagitannins and the most oak character, while a second-fill barrel contributes more subtly and works to a greater degree as a vessel for slow oxidation.
What is the difference between micro-oxidation and ordinary barrel ageing?
Both build on the slow oxidation that matures the wine. The barrel delivers oxygen naturally through the wood and the handling, while micro-oxidation doses oxygen deliberately and in a controlled way over a defined period. Micro-oxidation can thereby mimic parts of the barrel's oxygen contribution in a more reproducible way.
Ready for the next step?
Maturation is constantly about managing the oxygen, and that leads naturally on to the wine's two most important tools against oxidation. In the next part, Stabilisation: Sulphur and oxidation, we take a closer look at how SO2 consumes oxygen and keeps the wine stable, and where the line between beneficial and harmful oxidation runs.
If you want it all from the start, you can always return to The aroma compounds of wine or Phenols: Tannin, colour and structure, both of which interplay with what happens in the barrel. And when you want to feel the theory in the glass, remember that the best pairing is the wine you like with the food you like. Come by our selection, and let curiosity decide the direction.