By: Becky Garrison
During the Oregon Wine Symposium, held virtually from February 15-17, 2022, two sessions on the role of oxygen in winemaking. Following is a summary of some of these key findings.
In explaining the role of oxygen, Dr. Gavin Sacks, professor and associate chair of food science at Cornell University, broke down how wineries utilize oxygen pre-fermentation, during fermentation and post-fermentation. When handling must and juices pre-fermentation, winemakers use the terms hyperreductive, reductive, oxidative and hyperoxidative. As these terms do not have rigorous regulatory definitions, winemakers use these terms in different ways. Generally, those winemakers, who talk about reductive versus oxidative, add sulfur dioxide in reductive winemaking, but they won’t add it in oxidative winemaking. Hyperreductive means that not only will sulfur dioxide be added, but there will also be an effort to minimize air contact pre-fermentation. Conversely, hyperoxidative means that while sulfur dioxide is not added, air is intentionally added.
Under these conditions where one is using fresh must with no sulfur dioxide present, Sacks notes that the main route by which oxygen is consumed or reacts is going to be enzymatic enzymes either from the grape or enzymes from spoilage organisms like detritus. The reactions are classified under the generic term polyphenol oxidases. In the presence of oxygen, they will get converted into oxidized forms called quinones. As quinones are pretty short-lived, they will only form following mechanical damage, such as crushing and pressing fruit.
According to Dr. Sacks, the most common way to slow down this enzymatic browning in a winery involves using antioxidants such as sulfur dioxide. These antioxidants will react with the quinones, but even more importantly, they will deactivate enzymes but is less effective on laccase found in molds. Other effective options are ascorbic acid and glutathione, which are in grapes and yeast (lees), as well as slowing it down to cooling. In addition, charcoal and bentonite can be used to bind to and remove some of the browning products and inactivate enzymes. Also, hyperoxidation followed by the brown product via flotation or filtration tends to decrease the browning potential of that eventual wine.
Pre-fermentation oxygen exposure might not have a major effect, especially with aroma compounds, as most aroma compounds found in finished wine are not present in the juice or the must. Instead, they exist in precursor form or are produced de novo by the spore bylactic bacteria.
In Dr. Sack’s estimation, oxidation matters much less than just letting the fruit sit around before fermenting. “This allows time for the glutathione 3-MH precursors to form. The resulting wine will have more intense aromas.”
During fermentation, oxygen consumption continues to be relatively rapid due to the formation of carbon dioxide and the yeast utilizing oxygen enzymatically. Yeast cells have cell membranes composed of phospholipids, which have fatty acids. The yeast will try to modify these fatty acids in response to their environment. For example, under colder temperatures, yeast will increase the concentration of unsaturated fatty acids, thus increasing the need for oxygen.
Post-fermentation, Sacks recommends looking at the oxygen consumption rate. Fresh must in actively fermenting wine is consuming oxygen at a rate of a few milligrams per liter per minute. In comparison, in post-fermentation, it’s down to one milligram per liter as non-enzymatic oxidation goes much more slowly. The main effects of oxygen on finished wine are attributed to microbial growth due to the presence of oxygen. This can result in an off flavor and haze formation, along with possible regulatory issues.
Chemical Changes in Wine Due to Oxidation
Sacks refers to the main pathway for wine with little or no oxidation as the iron phenolic pathway because it involves oxygen, iron and diophenol. “The difference here is instead of having an enzymatic catalyst (TPO), now we’ve gotten iron as a catalyst,” he states.
As the reaction proceeds, it will form an oxidized diophenol, just like when must is oxidized pre-fermentation. However, the big difference is that this also makes hydrogen peroxide. These two compounds are highly reactive and can result in the loss of sulfidryls (tannin reactions).
Hydrogen peroxide will react with iron to generate hydroxyl free radicals. And then those hydroxyl radicals can direct indiscriminately with wind components to generate compounds like aldehydes, including acid aldehyde by oxidation ethanol. These compounds result in the oxidized smell of wine, such as acid aldehyde, which smells like bruised apples, cherry, walnut, baked potatoes or soy sauce. Also, hydrogen peroxide produces browning particles.
One way some winemakers intentionally oxidize their wines is through Micro-oxygenation (Micro-ox), which is the treatment of wine with well-controlled small doses of oxygen over a short period of time. This will result in compounds that are referred to as wine pigments. They’re less bleachable by sulfur dioxide and not as prone to hydrolysis, so they’re more stable in a wine environment. Also, they’re the major contributors to the color of aged wines. Dr. Sacks referenced several experiments showing that if Micro-ox is done at roughly the same concentrations as an air saturation offering of six to nine milliliters per liter (milligrams per liter per month), this could have modest effects by increasing in the color intensity and wine pigment and slightly decrease astringency.
Also, when sulfur dioxide is added to a wine, a portion will stay free, but a portion will also form strong chemical bonds with other components in wine, referred to as binders. They act as a reducing agent to prevent oxidized changes or chemical oxidation from happening to the wine.
In a research study exploring assessing the impact of free and total sulfur dioxide in bagged wine, Sacks observed that when they measured dissolved oxygen in these wines, it was always almost always near zero and undetectable. “So, oxygen is getting in, but it’s being consumed by the wine, but it’s also happening relatively fast with all the SO₂ being consumed in a year.”
How to Control Redox Potential Using Air During Fermentation
Roger Boulton, a consultant for RB Boulton Inc. and emeritus professor of enology and chemical engineering at UC Davis, offered an in-depth analysis of the redox potential (reduction-oxidation potential) by first noting that dissolved oxygen in wine cannot and does not oxidize anything until it gets activated by components in solution (iron and copper tartrate complexes), temperature or light. Once activated, hydrogen peroxide is produced, which in turn causes a rapid rise in the redox potential of the juice or wine. Secondly, there is no relationship between dissolved oxygen level and redox potential. As might be expected, this is a major cause of confusion when winemakers and others talk about winemaking practices, oxygen exposure or oxidation of the wine.
Once the fermentation begins and even before the yeast begins to grow, one of the components they secrete to control the redox potential around them is glutathione. As they do this, the redox potential declines. The decline in the potential will continue until yeast growth has ceased, typically at the point of the maximum fermentation rate. The higher the fermentation temperature, the faster the onset of fermentation and the quicker the decline in redox potential occurs.
Introducing a small amount of air (resulting in less than one mg/L of dissolved oxygen) enables this amount of oxygen to be activated. This generates a burst of hydrogen peroxide that causes the redox potential to increase, usually by about 100 mV, over a period of approximately 30 minutes. Due to the reaction between peroxide and glutathione, the redox potential declines again, usually over the next few hours. The pattern is repeated if the air is added again, but this cannot begin until the redox potential has returned to a stationary value. The addition of dissolved oxygen at higher concentrations has no further effect. This is why controlling redox potential during fermentation is very different from simply controlling air addition or establishing a certain level of dissolved oxygen. Once yeast growth has ceased, there is no need to keep adding periodic amounts of air. And the redox potential will slowly return to its final level at the end of the fermentation.
The motivation for controlling the redox potential during wine fermentation is to prevent the formation of hydrogen sulfide and other alkyl thiols and ethyl thioesters. If elemental sulfur is present as a residual from vineyard applications, it will produce small amounts of hydrogen sulfide when the redox potential is at low levels. Many juices can reach these levels during fermentation. The aim of controlling the redox potential during fermentation is to prevent this from happening. While the yeast creates these changes in the redox environment, it is the initial level of the potential and the sensitivity to change that is determined by the juice composition. This is why the formation of hydrogen sulfide varies so much across juices and yeast strains and why there is some confusion as to this being a property of the strain alone.
For those looking to integrate a redox system into their own winery for fermentation control, Boulton recommends a Hamilton electrode probe ($2,000), which is the only probe he knows of currently that is food grade.
Once fermentation has begun and significant levels of ethanol form, the addition of air and the activation of dissolved oxygen lead to the formation of a radical called the hydroxyethyl radical. The dihydroxy phenols (including tannins) do not appear to be oxidized or used during these redox-controlling reactions. Boulton notes, “In wine, it is the hydroxyethyl radical, not oxygen, that is the real villain if you wanted to talk about an oxidizing villain.”
Oxygen in Action: Cellar Techniques
Johnny Brose, the winemaking instructor at Chemeketa Community College and moderator of these sessions, toured several vineyards in Oregon and California to learn how these winemakers dealt with oxygen in their respective wineries. Among his key findings:
Scott Kelley, the owner/winemaker at Paul O’Brien Winery (Roseburg, OR), uses a center stone to inject pure oxygen into his ferments.
Ryan Rech, the senior winemaker, and Dr. Jonathan Cave, an analytical chemist for Berringer Vineyards (Helena, CA), use a low-level nitrogen pressure that prevents oxygen from coming in. All their tanks have a headspace management system that they monitor year-round.
Ryan Hodgins, the winemaker for FEL Wines (Yountville, CA), utilizes a nitrogen generator to flush their tanks.
Jeff Menganhaus, VP and winemaker at Williams Selyem (Healdsburg, CA), uses argon and pressurized tanks in his winemaking process.
Use of DO Meters in the Winery
Finally, Brose demonstrated a range of DO (dissolved oxygen) meters. The first was an Electrochemical (Galvanic and Polargraphic), which is very portable and inexpensive ($500 to $2,000). This requires an electrolyte solution to be inserted into the probe and flushed and rinsed before each measurement. Low temperatures and pressure changes can lead to very inaccurate measurements. An optical DO meter is lower maintenance and offers more precise measurements. But it is relatively more expensive ($1,000 to $4,000) and requires more time to obtain accurate measurements. At the high end of the scale are OxvDot Sensors, which are typically utilized in research or large-scale production sites and are more stationary, with a price point of $20,000 or more. They provide an instant measurement of oxygen in both liquid and gas and can be read in real-time.
In assessing when to use a DO meter, Dr. Sacks recommends focusing on the bottling and packaging process. Once the wine is off the lees, non-enzymatic chemical oxidation is the dominant route for oxygen to be consumed. A DO meter can evaluate the integrity of the tanks and the quality of transport processes to help winemakers understand where the wine is picking up oxygen, how much oxygen and then do something to address it.