When to add oxygen
Red wine is often made by aerating must during pumpovers, which then stimulates yeast growth as well as the formation of tannin-anthocyanin bonds. Oxygen can be added to the fermenting juice during a closed pumpover, thereby simulating this practice. This technique is successfully used in France (ICV, 2000) where in some cellars every tank is fitted with its own pump and sparger, coupled to a computer. What makes this method so ideal is the fact that it is much easier to clean up afterwards and that the amount of oxygen can be controlled.
The amount of oxygen added during fermentation is normally 2 ml/ L/day for the two days during peak fermentation (Ducournau-Laplace, 1998). Some winemakers have found however that this technique inhibits fermentation and are now using air instead of medical grade oxygen. Near the end of fermentation no oxygen should be added.
If reductive flavours form during fermentation, the oxygen can help get rid of these flavours without a messy open pumpover. The CO2 that evolves during fermentation would in any way have sparged these flavours out, therefore the cause of these flavours should rather be addressed.
After fermentation before malolactic fermentation (MLF)
The main time MO is used is before MLF on wine and specifically on press wine. At this time the tannins are more susceptible to oxidation due to the lack of SO2 . To delay MLF, the use of Lysozyme has been suggested. The addition of oxygen induces polymerisation and softening of tannins. The green character of the skins and reductive flavours are also reduced. The promoters of this technique often use the example of a barrel being micro-oxygenated and a sample is drawn immediately afterwards. This is a bad experiment because the wine takes between eight and ten days to absorb the oxygen, depending on the temperature and the phenolic composition of the wine. (Vivas & Glories, 1995). The wine may also taste different because of the loss of CO2 and green flavours when it is sparged with oxygen.
The temperature of the wine has a big influence on the amount of oxygen the wine can absorb. If too much oxygen is added then the risk exists that the wine can have a residual dissolved oxygen level higher than 0,03 mg/L, which can lead to a diminishing in wine flavour and acetic acid (Ducournau-Laplace, 1998). Figure 1 shows the maximum amount of oxygen that can be added at a given temperature. This level should not be exceeded, especially if the wine has a light tannin structure. It can be seen that nearly half the oxygen can be absorbed at 11C rather than at 13C.
Micro-oxygenation can offer the following benefits (Vinovation, 2000)
- Tannin / mouth feel change. More body. Softer richer tannins.
- Colour stability.
- Early polymerisation, more intense and more oxydation resistant colour.
More fruit and more integration of wood and vegetal aspects.
- Lowering of reductive flavours.
- Ageing potential.
- MO will not make a wine market ready sooner. It does not prematurely age wine.
MO should not be used during MLF. The reason for this is unknown.
Red wine ageing
During the ageing of wine the wine comes into contact with oxygen during topping and racking.
The racking of wine with air can be simulated by using the MO technique and by adding the oxygen to barrels instead of racking them. A lot of labour can be saved in this way. The amount of oxygen taken up during these rackings range between 2,2 mg/L for a protected pumpover to 7,4 mg/L for a pumpover with deliberate splashing (Vivas & Glories, 1995).
Red wine aged in the presence of wood chips or staves can also benefit from the addition of this oxidation component. It was found that the amount of oxygen taken up during normal ageing in a barrel is on average 2,5 mg/L/month. Rowe & Kingsbury (1999) suggest the rate of micro-oxygenation post fermentation to be between 0,75 and 3 mg/L month.
This technique may be used to mimic the full wood ageing process when white wine is aged in the presence of staves or chips.
Monitoring the process
The wine should be tasted regularly during the process and one should be rather conservative with the dosages at first. A too high dose of oxygen is characterised by the presence of acetaldehyde and at a too low dosage by the presence or even increase in reductive flavours. If possible the dissolved oxygen levels should be monitored (Vinovation, 2000).
This technique has opened a new field of which even the pioneers have not discovered the boundaries. Every wine’s tannin structure is different and therefore every wine will react differently to treatment. Even at this stage we are not able to understand the complex matrix that constitutes wine. The outcome of MO is difficult to predict as there are so many compounds that can react at many different speeds.
We are also not sure what the effect of MO will be on the bacteria population. Vivas & Glories (1995) found that the bacteria count and volatile acidity of a wine rose after exposure to air, but this will of course differ from wine to wine. The effect the oxygen has on the SO2 levels is also not known.
The tempo at which oxygen should be added to simulate barrel ageing is not fully known yet. The barrel takes up the oxygen a little bit at a time over a long period, but it is difficult to add a little oxygen over a long time. In France with permanent, automatic systems installed in cellars, the oxygen is released very slowly over a long period of time.
Until we are very sure what we are doing we should monitor the SO2 and volatile acidity levels in treated wine very carefully.
The MO technique shows great possibilities and we should investigate and learn more about it. If anything, it will make us concentrate a lot more on the development of our wines.
Ducournau-Laplace,1998, Micro-oxygenation apparatus, Operating Manual, March.
ICV, 2000 . Short maceration: a new Mediterranean vision. http://www.icv.fr/kiosqueuk/ flash/flash3.htm.
Rowe, D., Kingsbury, D., 1999. The micro-oxygenation technique, Australian and New Zealand Wine Industry Journal, Volume 14 no.4, July-August.
Vinovation, Inc., 2000. Goals of Micro-oxygenation, http://www.vinovation.com/MOxgoals.htm.
Vivas, N. & Glories, Y., 1995, Racking of Red wine Matured in Barrels – A tentative Classification of Racking Techniques, Australian and New Zealand Wine Industry Journal, Volume 10 no.3, pp241-243.