Oxygen in white wines: not only inside but on top!

So last time we looked at oxygen in juice and I mentioned that oxygen can be the enemy or friend of the wine maker.  Oxygen reacts with a number of different compounds in wine and can thus change the aroma and taste of the wine. Oxygen levels in wine can range from around 0 mg/L to 7 mg/L.

So let’s discuss a bit the detrimental effects of oxygen in white wine. Too much oxygen exposure in white wine can lead to the wine initially losing its fruitiness and becoming oxidized after too much oxygen exposure. This leads to the wine having a brown colour with a sherry, honey like aroma in extreme cases.

Wine makers will often add sulphur dioxide (SO2) to the wine to prevent it from oxidizing, as this a powerful anti-oxidant. I’ll write a bit later again on the role of SO2 in wine, but let’s remember at this stage that it’s a powerful anti-oxidant. Interesting enough when oxygen enters wine it reacts with phenolic compounds, such as caftaric acid and tannins in white wines. One of the by-products of this reaction is hydrogen peroxide or H2O2. Now peroxide might ring a bell with you. Yes, the same stuff you find in anti-acne salves and what we used to bleach our hair with (a standard practice with the coming of age of some Afrikaans singers. I  love Afrikaans music, just not all associated hairstyles). H2O2 is a very powerful oxidant and can damage the wine quickly beyond repair. Now the SO2 in wine does not react with the oxygen, but rather with the H2O2, thereby preventing it from further damaging the wine. (If you don’t believe me buy some H2O2 from the pharmacy and add a few drops to a bottle of wine, the next day it will be completely oxidized).  SO2 thus indirectly protect the wine from the detrimental effects of too much oxygen by removing the H2O2.

The other way of protecting white wines against oxidation is to prevent oxygen from coming into contact with the wine. One of the most important things I teach my students is to always keep a tank or barrel full with wine. A little bit of oxygen in wine is not always the end of the world, but oxygen on top of wine for an extended period of time is really looking for trouble (remember the old Cremora coffee creamer add, not inside it’s on top!). This is due to the fact that in a tank that is not completely full a lot of oxygen might still be in the headspace on top of the wine and might cause damage. If the tank cannot be filled completely winemakers will use an inert gas, such as nitrogen or Carbon dioxide to displace the oxygen from the headspace in such a tank.Headspace O2

In cellars and bottling plants the oxygen levels dissolved in the wine just before and after bottling should be measured as part of the QC systems of wine producers. However, the older oxygen measuring technology has only allowed for the dissolved oxygen inside the wine to be measured. A good dissolved oxygen level in bottled white wine is less than 1.5 mg/L. However, what wine producers often not measure is how much oxygen is in the gaseous phase in the headspace of the bottle (the part between the wine and closure). Recent research has found that this section can sometimes even contribute more oxygen to the final product than the wine itself if not controlled. Fortunately new technology now exist where one can measure the dissolved oxygen levels inside a bottle’s headspace as well as in the wine itself, without opening the bottle. Bottling equipment that work correctly should thus remove the air in the bottle, thereby forming a vacuum before filling it with wine as well as replace the headspace with an inert gas. We thus nowadays classify oxygen levels at bottling as total packaged oxygen (that in the headspace and dissolved in the wine) and this level should preferably be lower than 3 mg/L just after bottling. New technology and sound winemaking techniques can help the winemaker to protect his/her wine better against too much oxidation

Keep in mind that low levels of oxygen can also have some positive effects in certain wines, which we will discuss next time!

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