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WHAT'S UP IN THE VINEYARD AND CELLAR
The role of oxygen in vinification
Charl Theron
In the case of humans oxygen is indispensable, but in vinification this wonderful guest can be either friend or foe. Oxygen is found all around us, however, and winemakers should therefore be aware that the role of oxygen in the handling of juice differs entirely from that of vinified wines.
Most of the facts regarding the role of oxygen in vinification are not new, but often these are either forgotten or otherwise winemakers do not adapt their practices to take them into account. The following facts regarding the role of oxygen in the wine cellar are supposed to be well-known:
- Lees may be used as protection against oxidation, but only to the extent that the available oxygen may be neutralised by the lees.
- The uptake of oxygen benefits from lower temperatures. A decrease of 5°C increases the oxygen solubility by 10%. Do take care when oxidising cold wines.
- The rate of alcoholic fermentation is increased by the exposure of the juice to additional oxygen.
- The formation of volatile acid during fermentation is mostly caused by a lack of oxygen for the yeasts, thereby putting stress on the latter.
- By properly managing the application of oxygen during fermentation, it is possible to obtain fruitier wines that are also able to remain fruity for a longer period.
- The racking of juice during the fermentation of red wine does not add any additional oxygen to the crushed grapes, seeing that it is saturated with carbon dioxide.
- If oxygen is applied for 1 second at a pressure of 3 bars to 100 litres of wine, it will be equivalent to 1 mg/l dissolved oxygen.
- If wines with a light colour, low tannins and a high pH are treated with micro-oxygenation, the process should be managed cautiously.
- Micro-oxygenation, like other vinification practices, is not a magic formula with which wines can be improved.
During the maturation of wines and the eventual bottling thereof, many practices and factors may contribute to the oxygen status of the wine. The following facts should always be borne in mind in this regard:
- Oxygen may react with phenols, resulting in the formation of hydrogen peroxide. The latter may react with alcohol, which may be converted to acetaldehyde. (Acetaldehyde is defined as an oxidation product and is reminiscent of an apple character.)
- Sulphur dioxide as an anti-oxidant does not react directly with oxygen, but rather with the oxidation products such as acetaldehyde. (Sulphur dioxide therefore eliminates the perceptibility of oxidation products.)
- The racking of wine from one tank to the next may result in an increase of 0.1 to 0.2 mg/l dissolved oxygen.
- The aeration of a still wine may result in an increase of 7 mg/l dissolved oxygen.
- Ordinary handling and maturation in barrels may result in an increase of 26 mg/l dissolved oxygen.
- The aeration of wines that have a rotten egg smell is not necessarily the best solution, since sulphites may be converted to mercaptans, which may in turn lead to disulphites. It is practically impossible to get rid of the latter.
- The measurement of dissolved oxygen depends on the position of sampling. The closer the sample to the surface of the wine, the higher the dissolved oxygen quality.
- In order to limit the oxygen uptake during maturation, maintain the cellar temperature between 14 and 18°C and the humidity between 60 and 70%.
- If the relative humidity during maturation is below 60%, water will evaporate from the barrels and the alcohol quality of the wine will increase. If relative humidity is above 60%, the alcohol will evaporate. In both instances an air bubble and vacuum will be created inside the barrels.
Micro-oxygenation (MOX) has become standard practice at many cellars, but the question is often asked why it is applied.
- It is possible to produce wine with a better structure and possibly use less sulphur dioxide.
- With micro-oxygenation the texture of the wine will be characterised by different phases. This cycle may be described as green, hard, firm, soft, full-bodied, complex and once again green. It is best to bottle the wine during the firm-soft phase.
- A so-called MOX recipe for a wine with maturation potential may be summarised as follows: Apply 60 ml oxygen / litre wine / month after wine has fermented dry and gradually reduce it to no oxygen just before the completion of MLF. Once the sulphur dioxide adjustment of the wine has been done, the dose may be increased for two weeks to 40 ml / litres / month and then again gradually reduced to 5 ml / litres / months before being pumped over to the maturation barrels.
MOX is a technique for managing the style of the wine so that when it comes to bottling, it will be unique to the particular cellar or winemaker.
Reference:
Crowe, A. 2007. The Role of Oxygen in Winemaking. Wine Business Monthly. February 2007: 85 - 86.
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