Oxygen and Wine
# **Oxygen and wine** In this post, we’ll take a look at the impact of oxygen on wine across fermentation, barrel or bottle maturation and finally during decanting prior to serving. Depending on when must or wine is exposed to oxygen, and to what extent, oxygen can have both a beneficial and a detrimental impact on wine. **Oxygen Saturation** Oxygen has limited solubility in wine. This is between 6 – 8 mg of oxygen per liter of wine at 20°C room temperature. The solubility of oxygen increases by 10% if the temperature is dropped by 5°C. Unless more is added from external sources, oxygen dissolved in wine depletes over time due to chemical reactions. **Winemaking Operations** Oxygen is a requirement for yeast to be able to complete **[alcoholic fermentation](https://modernwineclub.com/blogs/news/alcoholic-fermentation)** – the key metabolic process during which the wine yeast converts grape sugars to ethanol and carbon dioxide while heat is released. Ethanol is toxic to yeast at higher concentrations and the presence of oxygen acts as a positive survival factor that increases the ethanol tolerance of yeast and makes it less likely that the fermentation will get stuck. This makes the presence of oxygen critical to the early stages of most winemaking efforts apart from the exception case of carbonic maceration used to make Beaujolais Nouveau wines. Beyond oxygen exposure during alcoholic fermentation, received wisdom has been to try to limit subsequent oxygen exposure, especially for white wines given that oxidation has a color impact turning white wines yellow or pink and later even brown. Oxidase enzymes hold the key to the browning reactions that are seen after fruit and vegetables have been cut with a knife as well as when wine changes color towards brown. After grapes have been crushed, three reactants play a role to cause the browning reaction: oxygen present in the air, oxidase enzymes and a substrate – the latter typically a phenol when it come to grapes. The oxidation reaction happens to both white and red wines but is most pronounced for white wines given lower levels of phenols (tannin) as well as the pale color that is more sensitive to color changes. The higher phenolic content in red wine leads to slower oxidation and the darker color makes consumers less sensitive to color changes for red wine. As a result, winemakers are generally less careful with limiting oxygen exposure for red wines early on during winemaking. The most common oxidase is polyphenol oxidase (PPO) and it is the enzyme responsible for most browning reactions in fruit and vegetables. So when you cut an apple and leave the pieces to turn brown, it is the work of PPO that you witness in front of you. PPO reacts very fast, is inhibited by the presence of sulfur dioxide and/or ascorbic acid and becomes inactivated after alcoholic fermentation has been completed. So post alcoholic fermentation, PPO is no longer a headache to winemakers. To prevent oxidation during the early stages of winemaking, winemakers act to limit one or more of the three reactants above (oxygen, enzyme or phenolic substrate). To protect the wine from oxygen, the most effective method is often to remove or limit the empty “headspace” in tanks and in barrels so that wine has less oxygen to come into contact with. The headspace can also be flushed with argon, an inert gas that is heavier than air, or with the functionally inert gases carbon dioxide and nitrogen. Argon is the most expensive of these gases and the higher cost is sometimes a consideration when making inexpensive wine. Carbon dioxide is more soluable in wine than the other gases and for that reason it is more often used in the beginning of winemaking operations to leave time for it to be released so that still wine does not end up mildly carbonated. Hence, argon and nitrogen are the gases of choice for later stages of winemaking operations. It is worth pointing out that gases are not solids. As such, they do not provide a complete blanket cover for the wines, given that gas molecules move around, but they significantly reduce the amount of oxygen that the wine comes into contact with. Wine can also be protected from oxidation through the application of sulfur dioxide and ascorbic acid. Sulfur dioxide has the added benefit that it is also anti-microbial and inhibits the growth of unwanted spoilage organisms such as Acetobacter and Brettanomyces. Sulfur dioxide does not react directly with oxygen present in the juice or must but it effectively blocks the reaction of the oxidase enzyme. It is usually enough to apply 50-75 ppm of sulfur dioxide to inhibit enzymatic browning. On the other hand, ascorbic acid does react directly with molecular oxygen and therefore plays the role of “oxygen scavenger” in wine. However, ascorabate does not have any anti-microbial function and does not inhibit the oxidase enzyme so if oxygen later returns, and the ascorbate has been depleted, the oxidation reaction simply starts again. As such, sulfur dioxide and ascorbate are often combined to inhibit the oxidase enzyme, prevent the growth of microorganisms and to scavenge for oxygen already dissolved in the wine. On a chemical level, oxygen is usually the limiting factor for oxidation reactions rather than the oxidative enzyme or the phenolic substrate. This means that once oxygen is used up, the chemical oxidation reaction stops unless more oxygen is introduced. **Hyperoxidation and Mixro-oxygenation** Historically, most winemakers have been on a quest to limit oxygen exposure. But there are always exceptions to the rules and modern winemakers have been experimenting with hyperodixation and micro-oxygenation to deliberately expose the must or wine to oxygen. Hyperoxidation is exposing white wine must to significant amounts of oxygen prior to alcoholic fermentation to deliberately trigger an early oxidation reaction. This causes the must to go brown so that most of the oxidation that can happen to the must has already happened. In addition, hyperoxidation reduces the amount of phenolic compounds capable of causing bitterness and astringency in white wines. During alcoholic fermentation, most of the particles causing the brown color precipitate out of the wine and can be filtered away to return the white wine to its desired pale color. And remaining particles can be fined away using clarifying fining agents such as isinglass and PVPP (Polyvinylpolypyrrolindone). This makes hyperoxidation a kind of shock treatment to get oxidation over and done with to promote color stability further down the line for white wines in order to extend the shelf-life. Micro-oxygenation (also called MOX) was invented in France in the early 1990s by a winemaker working with the highly tannic varietal Tannat. The purpose of MOX is to promote color stability, reduce tannins causing astringency and bitterness in red wines and to reduce vegetative characteristics. It works by administering a controlled flow-rate of oxygen through either a membrane or a diffuser into the wine. MOX can be used both before and after **[malolactic fermentation](https://modernwineclub.com/blogs/news/malolactic-fermentation)** (MLF). If used prior to MLF, more oxygen is typically applied and many winemakers believe the results are more favorable when oxygen is infused early during the winemaking process. For less expensive wines, destined for early consumption, MOX is often used in tandem with oak adjuncts (i.e. oak barrel substitutes including oak staves and oak chips) to simulate longer term maturation in more expensive oak barrels. As such MOX plus oak adjuncts can be used to “hack” wine to more quickly produce wine that appears to be aged and to have oak barrel treatment. The dangers include premature aging of the wine, reduced aging potential and that too much oxygen exposure can generate overproduction of acetaldehyde as a bi-product causing the wine to acquire a sherry-like aroma. The table below, compiled by **[Schmidtke, Clark & Scollary](https://www.tandfonline.com/doi/abs/10.1080/10408390903434548)** summarizes the oxygen application rate in mL per month, and the time duration, for different red wine varietals. When used prior to MLF, there is quite a lot of oxygen applied for a shorter period of time while the opposite applies after MLF with much less oxygen applied over a longer period of time. When wine is matured in oak barrels, the normal rate of oxygen infusion is usually around 3mL/L per month which is in the same ballpark as the post-MLF microoxygenation flow-rates in the table above. As such, micro-oxygenation post MLF essentially doubles the amount of oxygen infusion. **Oxygen and Wine Aging** Quality wine is often destined for maturation in oak barrels and later in bottles. While aging in barrels, the same general principle applies of limiting headspace and adding sulfur dioxide and sometimes also ascorbic acid. White wines, and sparkling wines, are sometimes matured on the lees (sur lies) to acquire a richer, creamier and more full-bodied character with aroma notes of toast, brioche, caramel and nuts. The lees are the dead yeast cells left at the end of fermentation. These yeast cells break down in a process known as autolysis and this releases amino acids, lipids and polysaccharides into the wine adding mouthfeel and aromas. When aging on the lees, wine is left on the lees for a period ranging from a few months to three years for vintage champagnes. In relation to oxygen, the yeast lees act as an oxygen scavenger by absorbing dissolved oxygen in the wine. This has an anti-oxidant property and reduces the need to apply sulfur dioxide for anti-oxidation purposes for white and sparkling wines. Wine that is rich in tannins has a greater ability to absorb oxygen and there is a more favorable impact as oxygen reacts with the tannins. This is the key reason why natural corks and extended bottle maturation often has a positive impact on quality red wines and especially on red wines high in tannins. It is also why white wines in general benefit less from aging and why advances have been made with new type of closures (e.g. screwcaps) for white wines to limit oxygen exposure once the wine has been bottled. **Special Case of Pinot Noir** The color in red wine is due to anthocyanins which chemically have a ring structure. When an acyl group is attached to the ring structure this makes the wine less sensitive to color bleeching as a result of oxidative reactions. Pinot Noir is a special case among red grape varietals in that the anthocyanins are not acylated making wine made from Pinot Noir much more sensitive to oxidation than most other red wines. It is exactly this effect you may have noted if you have preserved bottles of Pinot Noir from one day to another. We don’t recommend this given the high risk of oxidation as a result of the Pinot Noir anthocyanins not being acylated. Hence, it is always safest to consume a bottle of Pinot Noir on the same day as it is opened. **Bottling** Bottling equipment used in modern wineries has been designed to limit oxygen exposure to the wine. Prior to filling, the glass bottles are often flushed with carbon dioxide or with nitrogen to push out oxygen. Many fillers also fill from the bottom of the bottles to have the wine push out oxygen and then add an extra dose of nitrogen at the neck of the bottle just prior to corking. In other words, great care is taken to bottle the wine with as little oxygen as possible. A term used when bottling wine is TPO for Total Package Oxygen. TPO is a measurement for the oxygen exposure of the wine during bottling procedures and afterwards. It is the sum of oxygen that becomes dissolved into the wine, oxygen left in the neck of the bottle and oxygen entering the bottle later on through the closure. Oxygen entering the bottle through the closure is referred to as OTR for Oxygen Transmission Rate and closure suppliers provide technical data sheets with the OTR specified. Screwcaps generally have very low OTRs in comparison to the higher rate for natural corks. These days, screwcaps as well as many technical corks can be ordered with different OTR levels. Interestingly, emerging research from Australia has shown that red wines can age gracefully under screwcaps with zero or very low OTRs. This is going some way towards dispelling the myth that red wines need natural cork with relatively high OTR to age well. In addition, screwcaps and technical corks avoid issues associated with TCA cork taint in wine. **Premature Oxidation (Premox)** Premature oxidation (premox) of white wines emerged as a term to characterize the too early oxidation of white Burgundy wines especially from 1996 to 2002 vintages. The result was white wine that turned yellow to brown at a too young age with a loss of fresh and fruity aromas. The leading hypothesis for why this happened was sub-standard corks, leading to too high oxygen uptake, together with a trend to limit the application of sulfur dioxide. Much good wine was wasted this way and winemakers have since become more careful with closures as well as with their sulfur dioxide additions. **Decanting** Decanting serves three key purposes for wine. First, as red wine ages, oxygen triggers finely dispersed tannin monomers to polymerize and eventually to precipitate out of the wine as sediment as the bottom of the bottles. Decanting allows for the gentle removal of these polymerized tannins. This is much less of an issue for white wines given the presence of less tannins. Second, decanting wines aerates it and this releases accumulated gases that may have been trapped in the bottle. This is especially beneficial in the case of hydrogen sulfide, a volatile compound associated with burnt matches or rotten eggs. It is also the case for mercaptans that can give wine aromas of garlic, onion and canned vegetables. A decant of half an hour to an hour allows for such unwanted gases to evaporate out of the wine. This is also why most white wine is not decanted as aromatic, fruity aromas are also easily lost to evaporation. Third, aeration of wine can help to open up and soften tannins to make the wine smoother and more approachable. This is typically what is meant with the term “letting wine breathe”. It is worth noting here that academic wine chemists have not been able to establish any structural change to red wine tannins as a result of a short decant from 30 minutes to a couple of hours. Oxidative polymerization reactions of red wine tannins simply take longer time than that. Given this, it is well worth trying double or even triple decanting of young, tannic red wine. That is, decant the wine for an hour to saturate it with oxygen. Then funnel the wine back into the bottle and put in the fridge to leave the oxygen work through the tannins in the wine. Then decant again before serving. This can be repeated multiple times over several days to saturate the wine with oxygen and then let the bottle sit in the fridge while the oxygen gets to work. A side effect of decanting is the evaporation of alcohol in the wine. Academic studies have shown that wine lose between 0.9-1.9% alcohol over the course of a two hour decant and up to 3.2% of alcohol when the decant is extended to six hours. This is generally not mentioned in relation to decanting and worth taking note of. And this is a key reason for why it may well be beneficial to decant the wine for a shorter period of time to saturate it with oxygen and then funnel the wine back into the bottle and close it. You may also wish to cover the decanter to reduce the rate of evaporation. We hope you found this overview of the impact of oxygen on wine interesting.