During barrel ageing, the wine is exposed to oxygen in various ways:
The barrel naturally contains oxygen trapped in the wood. When the wine impregnates the barrel, this oxygen is transferred to the wine. Desorption starts very quickly at the beginning of the ageing process, even before the transfer of ellagitannins, which appear after about ten days.
The desorption of oxygen follows specific kinetics (Alexandre Pons 2014). According to tests on staves immersed in vats, three phases are obtained:
Kinetics of oxygen desorption
Phase 1: the first 5 days when the concentration of dissolved oxygen in the environment increases rapidly. This is partly due to the fact that the initial concentration gradient between the wood and the solution is very high.
Phase 2: stabilization of dissolved oxygen (sixth to eighth).
Phase 3: decrease in dissolved oxygen due to the extraction of polyphenols which react with O2. The difference between an environment with and without staves is quite clear.
It should be noted that if a barrel is already wet, the oxygen desorption time is longer. If wood is moistened for 48 hours, the transfer rate is three times lower.
Oxygen desorption in new barrels is the first point of entry during ageing. Conversely, used barrels have a layer of several millimetres that is impregnated with wine, so the oxygen has already been desorbed.
The proportion of oxygen transfer is not fixed over the entire duration of the ageing process. Its behaviour varies depending on several parameters, including:
The barrel's oxygen transfer points are distributed over three zones: the bunghole, the joints between the staves and the pores in the wood. Depending on the stage of ageing, the proportion of each varies.
As an example, here are the proportions for ageing with the bunghole on top during the first six months. There is no opening for topping up, so the conditions are stable.
| Circulation area | Percentage of total oxygen |
|---|---|
| Bung hole | 0.21 |
| Spaces between the staves | 0.63 |
| Wood pores | 0.16 |
Note that the results would be different in a situation where, for example, the bunghole was opened every 15 days for topping up.
To significantly modify oxygenation, it is possible to choose different grains of wood or to change the location of the barrel and particularly the type of bung. The type of bung and its position has a major impact on the oxygen supply:
| Position and bung type | Oxygen supply mg/l/an |
|---|---|
| Wooden bung on the top | 28 |
| Wooden bung on the side | 36 |
| Silicone bung on the top | 45 |
It is clear from these results that the choice of bung and position is a fairly simple tool to use if a change in the oxygen supply is desired.
Ageing can be carried out in barrels of various sizes, which makes it possible to modulate the contribution of wood as well as the exposure to oxygen.
The reactions described above are the same. The difference lies in a lower oxygen input for higher volumes. The quantity of dissolved oxygen will be less in a demi-muid (600 litres) than in a standard 225-litre barrel. This difference will be even more pronounced in very large casks (some hold more than 3,500 litres).