Oxygen: sometimes a good thing for wine…

Oxygen can also increase the quality of wine in certain instances. What, you might say? This is just the opposite of what I have been preaching in the past few posts. However, under certain circumstances limited amounts of oxygen can actually change the composition of wine in a  favourable manner. Read further to find out…

Winemakers sometimes employ oxygen to change the composition of wine. During the first phases of alcoholic fermentation oxygen is often added to the fermenting red must in different ways. This is done so that the yeast can form more fatty acids in their cell walls, which makes them more alcohol tolerant. This is a good practice as the yeast have a better change of completing the alcoholic fermentation, especially in musts with high sugar contents that will result in high alcohol wines. Oxygen is added in red wine ferments (a few mg/oxygen/L wine/day) by pumping the wine out of the tank into another container where it picks up oxygen, or air or oxygen can be directly pumped into the fermenting tank. We measured this oxygen pick up and many factors such as the pump speed and temperature of the wine affects how much oxygen is added to the wine. We also measured how quickly the oxygen disappears during this process and within about 15 min most of it has been consumed by the yeast.

This pumping of oxygen into wine is obviously risky if too much is oxygen added and should be controlled by the winemaker. Oxygen is sometimes also added to red wines (although at a lower dosage) after alcoholic fermentation. This process is called micro-oxygenation. During micro-oxygenation oxygen is added at roughly the same amount as would diffuse through an oak barrel (around 1-4 mg/ oxygen/ L wine/ month). Special equipment is required to control the flow of oxygen, but it basically works like your son’s aquarium, where air (containing about 21% oxygen) or pure oxygen is pumped into the water in a fine bubble form. (One of my other loves in life is aquarium fish, but my wife told me to choose between all the tanks and wine, so the choice was pretty obvious).

During micro-oxygenation in a tank or ageing of red wine in a barrel the small amounts of oxygen coming into contact with the wine induces the binding of anthocyanin molecules (giving the colour to red wine) with tannin molecules. This leads to the wine’s colour becoming more stable in the wine, as well as an increase in the colour of the wines. It can also influence the taste of the wine to a certain extend.

red wine in tank

In white wine small amounts of oxygen can also be beneficial. When yeast are stressing they can produces some unwanted sulphur flavours in wine (especially whites wines actually). These are called reductive aromas and are reminiscent of rotten eggs, rotten vegetables, sulphur etc. These are obvious not very sought after aromas in wine (except if you are a sulphur addict, which I think does not exist) and by adding small amounts of oxygen to the wine these compounds can be oxidised and thereby removed.

Oxygen are even sometimes added to the fresh must to remove some of the tannins and other so-called phenolic compounds when white wines are made. This is a process called must hyper-oxidation and are sometimes employed with certain cultivars such as Chenin blanc and Chardonnay, depending on the style of wine required. Recent scientific publications suggest that a compound called the Grape Reaction Product which are formed due to this oxidation process in must/juice can increase the mouth feel of white wines.

Oxygen can thus be an ally of the winemaker, but as the old saying go, too much of a good thing…

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