Oxygen can be added in a controlled way at various stages in the wine-making process, each time with different benefits and rules of use:
- During alcoholic fermentation (phase 1)
- Between alcoholic fermentation and malolactic fermentation (pre-FML aging) (phase 2)
- After malolactic fermentation or post-MLF aging (phase 3)
Oxygen can also be added in punctual (or violent) form in phase 1 (to cover yeast needs) and phase 4 (to prepare wines for bottling).
A distinction is made between pre-MLF oxygenation (during AF and between AF and MLF) and post-MLF oxygenation, because the results achieved are not the same, and oxygenation is not carried out under the same conditions.
Before MLF, oxygenation builds the wine's structure, whereas after MLF, oxygenation fleshes out the wine. So there's an opposition between construction on the one hand and finishing on the other.
Before MLF, micro-oxygenation builds the wine's backbone
In the construction phase, the addition of oxygen creates complex polymers of anthocyanins and tannins linked by ethyl bridges (derived from ethanal), as shown in the figure.
Ethanal production is one of the keys to successful micro-oxygenation. It is important to produce enough but not too much, as this could be detrimental to the quality of the final wine.
Controlling micro-oxygenation therefore depends on the user's ability to perceive ethanal, the description of which varies according to the concentration in the wine, from cocoa powder to chewed apple.
Moreover, depending on the relative concentration of tannins and anthocyanins, it is possible to obtain totally different polymers and results.
Tannin-anthocyanin balance must be taken in consideration
In the case of imbalance (much more tannins than anthocyanins), oxygenation leads to long polymerization, with a negative impact on wines (dryness, yellowish color).
On the other hand, when the ratio is favorable to anthocyanins, polymerization is limited and the impact on wines is positive (more color, more fat and better resistance to oxidation).
Unfortunately, the tannin/anthocyanin ratio is not stable over time and tends to increase over the life of the wine, due to the extreme fragility of anthocyanins.
It is therefore advisable to try to rebalance this ratio as soon as possible through early blending.
Don't forget wine temperature and turbidity
At the same time, 2 limiting factors need to be taken into account when considering adding oxygen to a wine:
- Temperature
- Turbidity
Temperature has a direct influence on the dissolution of gases in liquids, and therefore of oxygen in wines. Adding oxygen to wines that are too cold can lead to the accumulation of large quantities of oxygen, resulting in significant oxidation in the spring when temperatures rise.
It is therefore necessary to maintain a temperature of 16-18°C, or reduce the flow rate according to temperature if the latter cannot be managed.
The figure shows the flow rate limits not to be exceeded as a function of temperature.
The other limiting factor concerns wine turbidity during addition. Oxygen consumption by the lees during the aging phase is far from negligible. This can interfere with the objective of creating tannin/anthocyanin polymers.
It is therefore advisable to clarify sufficiently (between 100 and 200 NTU) at the end of AF (or at run-off for wines from maceration) to attenuate the influence of the lees.