The benefits and drawbacks of dissolved oxygen in wine can be discussed at great length. This article will provide an initial overview of some of the benefits and negative aspects regarding the impact of oxygen in wine. However, to extend aging potential and prevent undesirable changes in wine due to excess oxygen, a winemaker must recognize that in most cases oxygen is considered to be detrimental to the production of premium-quality wines. Key areas where oxygen can be introduced into the winemaking process will be identified to help winemakers recognize practices that prevent excess oxygen absorption.

Types of oxidation and potential negative effects
There are three basic types of oxidation that can have negative effects on wine: enzymatic oxidation, chemical oxidation and microbial oxidation.

Enzymatic oxidation: Juice and must oxidation is catalyzed by an enzyme called polyphenol oxidase (PPO). This enzyme oxidizes certain phenolic molecules to produce quinones. These compounds form polymers that influence wine color and flavor along with the loss of varietal character. Typically, these wines develop a brownish tint and may produce an oxidative odor of acetaldehyde (sherry-like aromas). At correct sulfur dioxide (SO₂) levels, these enzymes are easily inhibited in musts. It should be noted that their oxidative activity does not occur in wines.

Another enzyme to produce oxidative reactions is laccase, which occurs in unsound or rotten fruit. This rot is caused by Botrytis cinerea and is often called bunch rot. Winemakers consider laccase a more serious problem than PPO. This enzyme has more resistance to SO₂ and has a wide range of oxidative substrates. When oxygen is present in wines, laccase activity can continue to cause browning with a decrease in varietal aroma.

Chemical oxidation: This is the main oxidative process in wines, and it involves the oxidation of polyphenols such as catechin, epicatechin, anthocyanins and other phenols present in grapes. Through a series of reactions with oxygen, certain phenols form quinones and another by-product, hydrogen peroxidase (H₂O₂). This oxygen-containing compound is a stronger oxidizing agent than PPO. This oxidizing agent converts ethanol to acetaldehyde, resulting in a Sherry-like aroma in the wine. If wines are not protected with SO₂ and from oxygen exposure, chemical oxidation yields several negative sensory notes. These include brown-yellow coloration and off-odor formation with aroma degradation.

Microbial oxidation: Spoilage microorganisms such as acetic acid bacteria (AAB), film yeasts (Candida) and Brettanomyces (“Brett”) are dependent upon oxygen. AAB produces acetic acid (vinegar odor) from ethanol and may also yield acetaldehyde and ethyl acetate (fingernail polish) under certain conditions. In addition, certain “wild” yeasts belonging to the group Kloeckera and Hanseniaspora in the absence of sulfur dioxide can be abundant in must and juice at the beginning stages of fermentation (Zoecklein, 1995).

These native yeasts are often associated with cold soak procedures of red and white wine varieties that can produce high levels of ethyl acetate and acetic acid as an off by-product. This ester has a distinctive spoilage odor reminiscent of nail polish remover. Growth of “Brett” in wines can express off-odor descriptors such as horsey, barnyard and medicinal. This yeast is usually associated with wood cooperage, and its taint production is linked to volatile phenols. Film yeasts such as Candida spp. and Pichia spp. form a chalky layer on top of stored wine when containers are not filled to capacity. They are associated with oxidative defects such as acetic acid, aldehyde and volatile esters of acetate.

Benefits from dissolved oxygen in must and wine
There are several critical times during the winemaking process that dissolved oxygen in must and wine can be beneficial. These include hyperoxygenation of juice or musts, the role of oxygen during initial fermentation and the use of the micro-oxygenation technique to soften and increase color stability in red wines.

Hyperoxidation: In some cases, oxygen exposure in the must/juice (known as hyperoxidation) has been associated with stabilizing white wines from further browning oxidation during the vinification process, and it is believed to help extend the shelf-life potential of those wines. This enzymatic oxidation occurs in white wine must and juice devoid of sulfur dioxide (SO₂), where certain phenol groups react with oxygen to produce yellow quinones. These compounds in turn react with more oxygen to yield brown-colored products that fall out as a precipitate and are racked off the juice prior to fermentation. This process stabilizes further browning reactions in wine from this source (Ough, 1992).

The difficulty of this procedure lies in knowing the actual oxygen capacity of the must/juice related to the total amount of phenolic substrates present, the variation in enzyme activity and the proportion of phenolic components serving as substrates for enzymes. This can vary based on cultivar and vineyard (Boulton, et al., 1999).

There have been mixed reviews from studies indicating the final effect on sensory evaluation of wines treated by the hyperoxidation procedure. Based on my experience, I consider hyperoxidation of grape juice to be slightly less delicate in expressing varietal character in sensory evaluation, and I believe more studies need to be conducted to examine the effect this procedure has on the shelf life stability of different white wine varieties. Also, this oxidative process is not implicated in oxidative reactions occurring in wine.

Fermentation: Oxygen is also essential during the initial stages of alcoholic fermentation for healthy yeast propagation and fermentation profiles. Most of the dissolved oxygen occurs at grape crushing, pressing and racking, which can approach the saturation point at 6-9 mg/L depending on temperature, equipment and handling procedures. When musts lack oxygen, the fermentation process becomes slow and often stops to yield an unacceptable wine. After a few generations of yeast growth, sterols are initially used up and not renewed. Oxygen is critical to sterol synthesis and thus the successful completion of primary fermentation (Peynaud, 1984).

The lack of oxygen can cause stuck or sluggish fermentations, resulting in typically dry wines being sweet on the palate in addition to potential yeast stress producing off by-products such as hydrogen sulfide. Essential concentrations of oxygen to fermenting must/juice exhibit levels near 4-6 mg/L at the beginning and end of yeast cell growth to ensure a successful finish to primary fermentation (Specht, 2010). A majority of the oxygen will be scavenged by the yeast cells or blown off by the production of carbon dioxide (CO₂) during the fermentation process.

Micro-oxygenation: Some controlled oxygen exposure also may be beneficial in select red wines during barrel aging by a cellar procedure known as micro-oxygenation. This controlled process increases phenol polymerization, improves color stability and has the effect of softening the palate (reducing harshness) in red wines (Zoecklein, 1995). It is important to understand that micro-oxygenation is intended to avoid excessive accumulation of dissolved molecular oxygen, which causes oxidation in the must or wine (Smith, 2002). However, the advantages of micro-oxygenation need further research and should be performed by trained personnel only in recommending this technique.

Oxygen elimination post primary and pre bottling
Oxygen is generally understood to be detrimental to wine quality, especially from the end of fermentation through wine storage and bottling. The presence of oxygen after primary fermentation and during the latter stages of wine production can increase browning reactions, chemical and microbiological instability and result in the production of off-aromas such as acetaldehyde.

Attention must be given during the cellaring process to avoid those potential sources for oxygen pickup and prevent excess oxygen from dissolving into the wine. Key sources for oxygen pickup include: racking, excess headspace, pumping, cold stabilization, filtration and bottling. Depending on temperature, dissolved oxygen levels can range from 6 to 9 mg/L in wine (Peynaud, 1984). Higher levels are expected at lower temperatures. Since the rate of oxidation increases with temperature, it is critical to add the appropriate amount of SO₂ based on wine pH. Furthermore, when kept at low temperatures such as during cold stabilization, protecting the wine from air and keeping tanks full is essential to minimizing oxygen absorption (Gallander, 1991).

When wine is moved in the cellar from tank to tank or barrel to barrel, it is vulnerable to increased amounts of oxygen dissolving into the wine. Consequently, it is vital to limit movement of wine as much as possible. Critical aspects regarding maintenance and the use of pumps and filtration equipment according to manufacturer’s directions are essential in keeping excess oxygen from entering the wine during this time. Inspect for leaky pump seals, secure loose hose connections and make sure filter plates are tight to help minimize oxygen pickup.

When racking, it is vital to purge all transfer lines/hoses with an inert gas such as nitrogen, carbon dioxide or argon to help displace oxygen. Prior to racking, it is recommended that the receiving tank or vessel be purged with an inert gas as well. It is good practice to rack from the bottom of one tank to the bottom of the receiving tank or vessel. Be sure to plan accordingly so there is no headspace in any tanks or barrels during wine storage. If any headspace exists, an inert gas blanket is vital to drive off the oxygen in the headspace.

Another area of concern for excess oxygen dissolving into wine occurs during cold stabilization. Since oxygen dissolves into wine more readily at cold temperatures, it is essential to recognize this and make sure SO₂ levels are up based on wine pH prior to cold stabilization procedures. Avoiding headspace during this process is also critical in protecting wine from oxygen absorption.

Prior to bottling, excess oxygen in wines can be removed by using an inline sparger. This introduces an inert gas like nitrogen (N₂) or CO₂ through a porous stainless steel cylinder suspended in the wine. As the wine passes around the sparger, gas bubbles enter the product and displace the dissolved oxygen. The bubbles will rise to the top of the tank, releasing the inert gas and oxygen. For this procedure, the use of CO₂ as an inert gas is less effective and may excessively carbonate (saturate) the wine prior to bottling; therefore, N₂ is preferred (Ough, 1992).

Oxygen elimination at bottling
Bottling is the last winemaking process during which dissolved oxygen can be added and have a significant negative impact on the aging potential and quality of wine being released to the consumer. Thus, extreme care must be employed in minimizing the amount of oxygen entry at bottling.

Oxygen has the potential to dissolve into the wine at every stage of the bottling process. A recommended level for total dissolved oxygen should be below 1.25 mg/L
in bottled red wines and 0.6 mg/L for white, blush and rosé wines (Fugelsang, 2009). Major sources of oxygen diffusion into wine at bottling occur during wine transfer, filtration, filling and headspace levels of the bottling tank, filler and bottle. Each process will be described in further detail below.

When transferring wine to the bottling tank, it is advisable to purge the tank and transfer lines as mentioned above with N₂ or CO₂ prior to filling. If any headspace is present after filling the bottling tank, it is important to use an inert gas on the surface to prevent oxygen from dissolving into the wine. Often, a mixture of N₂ and CO₂ can be beneficial, especially for white wines. Maintaining a slight but constant pressure over the headspace is recommended. Although CO₂ levels around 300-600 mg/L can enhance a young white or light red wine (Peynaud, 1984), caution must be used that excessive pressure may cause too much CO₂ absorption, providing a noticeable tactile sensory perception and possible bubble formation.

In addition, excessive CO₂ levels can cause an increase in pressure, possibly pushing the cork out after bottling. Therefore, the use and monitoring of CO₂ in the wine prior to bottling by Carbodoser is beneficial in adjusting concentrations up or down accordingly for these purposes. The Carbodoser is a relatively simple technique involving a glass tube measuring the amount of CO₂ out-gassed from a fixed volume of wine. Comparing actual results with a calibration curve provides the concentration of CO₂ in mg/L of wine.

Wine filtration prior to bottling is another source for oxygen pickup. During filtration, it is important to operate the filtration unit according to the manufacturer’s directions, making sure all connections and pads are tight to prevent oxygen entry. Purging of air from the filter pads and transfer lines is also a recommended practice.

Wine entering the filler bowl is typically one of the most problematic sources for oxygen pickup. The filler bowl should also be covered with an inert gas. Depending on the type of filler used, the process of filling wine bottles can increase the levels of dissolved oxygen by 0.5 to 2.0 mg/L (Peynaud, 1984). The length of the fill spouts as well as the type and force of the jet may influence the amount of dissolved oxygen. Therefore, it is advisable that filling tubes be as long as possible depending on the bottle. Providing vacuum prior to filling and flushing with 2 to 3 volumes of N₂ has been reported to lower oxygen absorption at bottling (Boulton, et al., 1999).

After filling, bottle headspace is another source of oxygen absorption. This is due, in part, to the variability of the bottle headspace, which is influenced by such factors as wine temperature, solubility of gases in the wine, bottle size and shape. To help reduce oxygen ingress at this stage, the injection of an inert gas such as N₂ or CO₂ can reduce the amount of oxygen in the headspace. According to Peynaud (1984), a small amount of CO₂ supplied to the bottle headspace will help replace the oxygen and diffuse into the wine, causing a depression that also helps prevent the problem of wine leakage due to expansion. In addition, a bottling line supplied with a vacuum filler is also effective in reducing the amount of oxygen in the headspace. A controlled dosage of liquid N₂ into the wine after filling is another good option to flush oxygen from the bottle headspace for screwcap operations (Crochiere, 2007).

The corking machine also may vary as to whether it supplies a vacuum prior to cork insertion. According to Crochiere (2007), if set up properly, supplying a vacuum at corking can help reduce the amount of oxygen absorption into the wine.

Whether using inert gas sparging, pulling a vacuum, liquid N₂ dosing or a combination of these procedures, it is advisable to keep the time and distance from the filler to the corking machine as short as possible. In addition, if there is an interruption in the bottling line process, down time may cause the inert gas to escape, allowing oxygen to concentrate back into the headspace of the bottle. Therefore, if a bottling line stoppage has occurred, it is advisable to remove all bottles in question and dose them again or discard them from the bottling line.

The last important item that influences oxygen absorption during the bottling process and ultimately affects wine aging potential is the closure. Today there are many wine closures available, each with different properties. Two major functions affecting oxygen pickup in bottled wine include closure recovery time from compression and the rate of oxygen permeation. Lopes et al. (2007) indicated that the level of oxygen permeation is lowest for screwcaps and “technical” corks, intermediate for conventional natural cork stoppers and highest for synthetic closures. Further, they showed that differences in oxygen pickup varied among grades of each closure. This variability could then provide an explanation for bottle to bottle variation. This finding was in agreement with the results reported by Crochiere (2007). Both studies reported the need to be more consistent in production standards of each type of closure as it relates to compression recovery and oxygen ingress rates.

Author’s note
With this important information in mind, the Ohio State University/Ohio Agricultural Research and Development Center Enology Program is taking a closer look at dissolved oxygen-management strategies in Ohio’s commercial wineries. Through the Ohio Grape Industries Committee, the Enology Program recently purchased the NomaSense Trace Unit oxygen analyzer from Nomacorc. This equipment provides a non-invasive way to measure dissolved oxygen concentrations at the critical times of the winemaking process described above, with the ultimate goal of further increasing wine quality. A subsequent article will report the results obtained from trials with this equipment.

Todd Steiner has been with the Ohio State University/Ohio Agricultural Research and Development Center for 23 years and has led the enology program since 2001. He serves as state enologist to the Ohio commercial wine industry with both research and extension responsibilities.