Oxidation in Red Wine

By Dr. Aude Watrelot –

Oxidation is one of the most widespread wine faults, that can be managed during winemaking. The goal of this research focus is to provide fundamental and applied information about oxidation that can occur in red wines and tools to manage it.

For specific wines such as fortified or Port wines, oxidation can be positive and brought by micro-oxygenation but for other wines, the oxygen in tank and bottle can lead to oxidized wines if not controlled. Usually red wines are more protected toward oxidation thanks to their antioxidant phenolic compound contents. It is noteworthy in red wines made from hybrid grapes such as Marquette, Frontenac, Noiret, Corot noir, the concentration and the type of anthocyanins and tannins are different and lower than in Vitis vinifera grapes. Therefore, red hybrid wines are more susceptible to oxidation than V. vinifera red wines.

Figure 1. Marquette (left) and Frontenac (right) wines opened for 3 days (upper glass) and for 3 weeks (lower glass)

What is an “oxidized red wine”?

• The first indication is the change of wine color (Figure 1). The wine loses the deep purple-red or ruby color and becomes orange or brown. This is mostly because of the oxidation of anthocyanins responsible for the wine color and tannins that tend to form brown compounds after oxidation.
• The second indication is the aroma. The most common compound associated to oxidized wine aroma is the acetaldehyde that has green apple, nuts aroma. Phenylacetaldehyde and methional have the aroma of honey-like and boiled potatos.
• Then the mouthfeel might be slightly less astringent because oxidized tannins tend to precipitate during wine aging and have shown a lower reactivity with proteins from saliva, responsible for the astringency perception (Watrelot et al., 2019).

In a recent study on the perception of oxidized wines, experts were able to distinguish oxidized wines, but the separation between faulty and not-faulty wines was more difficult to establish in red wines probably due to the matrix complexity (Franco-Luesma et al., 2019).

What is oxidation?

Oxidation is the loss of electron from an atom or a molecule during a chemical reaction. In wine, two types of oxidation exists that involve either metal ions such as copper or iron present in wine, or enzymes (Oliveira et al., 2011 and Waterhouse et al., 2006).

• The chemical or non-enzymatic oxidation process is the oxidation of polyphenols containing a galloyl group or a catechol group such as catechins, gallic acid and its esters. These groups are oxidized to semiquinones and then to quinones through the redox cycles of iron and copper while oxygen is reduced to hydrogen peroxide (H2O2). Those quinones are highly reactive and can react with other phenolic compounds to form stable compounds that are brown. H2O2 can react with ethanol to form acetaldehyde that is responsible for the “oxidized” smell.

Figure 2. General mechanism of oxidation of (+)-catechin. From Janeiro and Brett 2004, https://doi.org/10.1016/j.aca.2004.05.038

• Enzymatic oxidation occurs mainly in the musts when the grape cells are disrupted. It releases polyphenols and polyphenol oxidases (PPO) that will start oxidation. In Botrytis cinerea infected grapes, laccases are enzymes also responsible for enzymatic oxidation of polyphenols. The PPO catalyzes catechol group of polyphenols such as catechin and form quinones that are highly reactive and able to undergo condensation reactions to form insoluble brown compounds. In enzymatic oxidation, the concentration of polyphenol oxidase, polyphenols, pH, temperature and the available oxygen are key factors. This reaction also happens when you cut an apple in pieces and you observe a browning color on the cut pieces.

When does oxidation occur?

The level of oxygen present at each step of the winemaking is the main source of oxidation that depends on the temperature and alcohol level. At room temperature, wine can absorb about 8 mg/L of oxygen before becoming saturated. The solubility of oxygen increases when working at low temperature leading to a higher dissolution of oxygen to wine. Therefore, a greater care must be handled during cold stabilization.

1. At crushing, oxygen from the air is in contact with enzymes and phenolic compounds can start enzymatic oxidation. That is the reason why sulfur dioxide is commonly used at this step.
2. Pumping wine from tank to tank results in an average oxygen addition of 0.1 to 0.2 mg/L.
3. While pumping-over, a lot of air is in contact with the must which lead to oxygen addition of 7.4 mg/L compared to protected pump-overs that added 2.2 mg/L of oxygen.
4. At aging, the oxygen dissolved in your wine will keep oxidizing the wine if you do not have enough antioxidants. If your wine is poor in polyphenols such as tannins and you do not use enough sulfur dioxide, oxidation will happen.
5. During filtration, depending on the type of filtration, diatomeous earth filtration tend to add0.7 mg/L of oxygen to the wine and membrane and tangential filtration add about 1.7 mg/L of oxygen.
6. At bottling, if you do not sparge your bottles with an inert gas either nitrogen or argon, oxygen in the bottle will be in contact with your wine and will oxidized during the time it will be in bottle until the consumer drinks it. That is also the reason why it is recommended to add sulfur dioxide in the appropriate dose at bottling.

How and when to reduce oxidation?

1. To prevent oxidation, minimize the exposure of wine to air.
2. Try to transfer wine as quickly as you can without too much mixing.
3. Prefer punch-down over pump-over.
4. When transferring in a tank, barrel or carboy try to remove the oxygen from it prior, to get the wine in with an inert gas.
5. If bulk wine is required to remain unchanged for many months, oxygen must be totally excluded.
6. Remember to top up the wine to avoid too much headspace.
   1. When it’s in a carboy leave not more than ¾ inches (1.9 cm) of headspace between the wine and the bung.
   2. In barrel, because the oxygen goes through the staves and there is more evaporation than in carboy, the wine should be topped up to the bung twice a month or more often depending on the room humidity
7. Use antioxidants such as sulfur dioxide at the appropriate levels (below 350 ppm or mg/L in the finished wine) at crushing, aging and bottling.
8. The addition of some enological tannins can also help to reduce the oxidation during aging and bottling.
9. The best barriers to limit oxygen transfer into wine through the closure are bottle caps and screw caps, and with technical cork stoppers ranking second.

How to identify oxidation in red wine using chemistry?

• Measurement of the dissolved oxygen level in wines can be carried out at all stages of winemaking with a dissolved oxygen meter. Some false results can be provided with this meter due to calibration that needs to be checked very often also because we are surrounded by atmosphere that contains 20 % of oxygen.
• Determination of the hue or tint:

As explained above, oxidation in red wine leads to a change of the color. This orange-brown color of oxidized red wine can be observed and can be determined by the measurement of “red wine hue”. The hue or tint is determined using a spectrophotometer as the ratio of the absorbance of red wine at 420 nm to the absorbance at 520 nm. At 520 nm, anthocyanins that are the molecules responsible for the red color absorbs a lot. When oxidation occur in red wine, those molecules tend to interact with other compounds which lead to a decrease of this absorbance at 520nm and the compounds formed absorbs more at 420 nm. Therefore as the red wine get oxidized, the absorbance at 420 nm increase and the hue value increase.

Figure 3. Absorbance of red wine during aging. From Somers 1998.

References to learn more:

Franco-Luesma, E.; Honoré-Chedozeau, C.; Ballester, J.; Valentin, D. Oxidation in Wine: Does Expertise Influence the Perception? LWT 2019, 116, 108511. https://doi.org/10.1016/j.lwt.2019.108511.

Karbowiak, T.; Gougeon, R.; Alinc, J.-B.; Brachais, L.; Debeaufort, F.; Voilley, A.; Chassagne, D. Wine Oxidation and the Role of Cork. Critical reviews in food science and nutrition 2010, 50, 20–52. https://doi.org/10.1080/10408390802248585.

Oliveira, C.; Ferreira, A.; Freitas, V.; Silva, A. Oxidation Mechanisms Occuring in Wines. Food Research International 2011, 44, 1115–1126. https://doi.org/10.1016/j.foodres.2011.03.050.

Waterhouse, A. L.; Laurie, V. F. Oxidation of Wine Phenolics: A Critical Evaluation and Hypotheses. 2006, 8.

Watrelot, A. A.; Day, M. P.; Schulkin, A.; Falconer, R. J.; Smith, P.; Waterhouse, A. L.; Bindon, K. A. Oxygen Exposure during Red Wine Fermentation Modifies Tannin Reactivity with Poly-l-Proline. Food Chemistry 2019, 297, 124923. https://doi.org/10.1016/j.foodchem.2019.05.197.