Building micro-oxygenation before MLF: promoting tannin/anthocyanin polymerization
The construction phase requires significant oxygen inputs to enable the production of ethanal, which is involved in the tannin/anthocyanin polymerization process.
The ageing period before MLF is ideal for several reasons:
- There is no free SO2, so ethanal can be produced without combining with SO2.
- The level of free anthocyanins is higher.
- Anthocyanins are predominantly represented in their colored form at pre-MLF pH, which is no longer the case after MLF.
- Wine temperature is also higher, which is favorable to the chemical mechanisms at work.
- MLF (and especially the lactic acid bacteria that consume ethanal) and post-LMF SO2 addition have not yet taken place, leaving two safeguards to manage any excess ethanal.
The effectiveness of micro-oxygenation then depends on several parameters:
- tannin/anthocyanin ratio ;
- tannin concentration ;
- dominant tannin type;
- purpose of oxygenation.
There are 3 types of tannin: reactive tannins, firm tannins and dry tannins. They are differentiated by their reactivity to oxygen and their capacity to evolve in contact with oxygen.
Unfortunately, there is no analytical method for quantifying them. Tasting is the only way to assess their presence and, above all, their evolution.
This is why twice-weekly tastings are necessary to assess the wine's reactivity to oxygen and anticipate its reactions.
The importance of tasting to control micro-oxygenation
Wines with a high concentration of reactive tannins tend to generate ethanal quickly, but also consume it very quickly if the flow rate is reduced.
Conversely, non-reactive wines take longer to react, but as soon as ethanal appears, it is more difficult to manage.
In the first case, it will be possible to control with low flow rate evolutions (+/- 10-15 ml/l/month), whereas in the second case, flow rates will have to be significantly reduced (by 50%) before ethanal appears.
During tastings, we evaluate not only the presence of ethanal, but also the evolution of the wine's openness, maturity and volume in the mouth. All these elements are used to manage oxygen intake.
We also keep a close eye on limiting factors (temperature and turbidity) and the evolution of dissolved oxygen concentration, whose excessive progression means that the wine is receiving too much oxygen.
Finishing micro-oxygenation after MLF: roundness and smooth tannins
After MLF and the addition of SO2, in the finishing phase, the addition of oxygen increases roundness on the palate and softens tannins.
Oxygen must be added at low flow rates, very close to those used during barrel ageing, i.e. below 1 ml/l/month. Beyond these flow rates, the construction phase continues and the end of aging is postponed.
During the finishing phase, the rules are completely different:
- the production of ethanal is no longer desired. As the two safeguards of SO2 and MLF have already been implemented, we no longer have any means of reducing ethanal in the event of excess;
- the aim of this phase is to gain roundness and soften tannins; it should be prolonged until the desired result is achieved. In general, the more important the construction phase, the more necessary it is to prolong the post-MLF contribution;
- if an increase in dryness is observed, even though the wine did not initially have this character, oxygenation should be stopped. This may be a case of over-oxygenation, the effects of which can be retrograded by not prolonging oxygenation.
Tasting remains the preferred means of control, but given the flow rates used, it is no longer necessary to carry it out twice a week, but rather every 10-15 days.
On the other hand, it is very important to monitor wine temperature closely, and to adjust flow rates downwards if this parameter is not being managed.
Dissolved oxygen should also be measured regularly. Values of less than 0.2 mg/l should be respected to avoid wine evolution.