Oxygen: the double edged sword

Published by Sally on September 21, 2009

A shorter version of this article first appeared in Wine and Spirit in 2007, since merged into Harpers Wine and Spirit.

Oxygen, friend and foe: can’t live without it, eventually die with it. Managing highly reactive oxygen gas during winemaking and bottling is like walking the tightrope. At one end is reduction, at the other, oxidation, both to be avoided. Oxidation leads to loss of primary aromas and fruit flavours, with browning of colour and deadening of flavour.

This means a constant balancing act depending on the style of wine being made: at some point towards the reduction end for a pure varietal expression of unoaked sauvignon blanc; near the oxidation end for tawny port and Oloroso sherry. It’s all about using the right amount of oxygen, at the right times to make the desired style, and there are no linear equations that make the process easy.

From the moment grapes are picked, oxidation is a threat

From the moment grapes are picked, oxidation is a threat, but some processes need oxygen. It is essential for the efficient start and smooth completion of alcoholic fermentation. Other benefits of appropriately timed oxygen introductions are more commonly seen in reds: colour stabilisation, flavour intensification, the building up and softening of tannic structure, limiting reduction aromas such as those of onion, rubber, garlic and cabbage, and encouraging natural clarification as less stable phenols are precipitated. It is also thought to help with the amelioration of green characters.

Red wines are thus less susceptible than whites and rosés to oxidation. Red juice and must contains higher levels of phenols (colour, tannin). Oxygenation, the deliberate and wanted introduction of oxygen, usually via air, is important to the resulting quality of red wines. In the cellar, maturing wine in barrel (old if no new flavour is wanted), racking, with smaller or larger amounts of air, and topping up, are ‘oxygenation’ techniques. Leaving the barrel bung at 12 o’clock is a more oxygenating technique than at 2 o’clock, where the bung remains below the wine surface.  Micro-oxygenation is the modern apparition.

microbial instability is an additional risk of too much oxygen 

Apart from direct oxidation, microbial instability is an additional risk of too much oxygen in contact with wine. Wine-spoiling acetic acid bacteria grow when there is enough oxygen. This is why volatile acidity can be a precursor of oxidation, though there are notable reds such as Château Musar and Grange, where otherwise high levels of volatile acidity are integral to the style. Also brettanomyces yeast multiply in must and wine in the presence of oxygen, producing medicinal, horsey, elastoplast aromas, a little of which add complexity, a lot of which is a fault.

At the opposite end of the spectrum, reductive winemaking aims to avoid oxygen all the way, and is mainly used on white wines, especially aromatic and semi-aromatic grape varieties. Oxygen reacts with phenolics, of which whites have fewer than reds, and whites need their few phenolics to contribute to aroma and mouthfeel. Dry ice, inert gases, closed winemaking kit, and regular sulphur dioxide monitoring are the hallmark of reductive winemaking. Low temperature is another tool of reductive winemaking, because reactions are slowed, but oxygen is absorbed more easily at low temperatures.

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