There are a lot of recommendations that get thrown around for using inert gases (i.e., nitrogen, argon, and carbon dioxide) in wine production. But what are the best techniques? How do you know when enough gas is used?
In today’s world of high-cost and minimal inputs, these are certainly valid questions.
So let’s dive into some reminders about the use of inert gas in wine processing: the pros, the cons, and not-quite-sure’s.
Let’s Review: Why Do We Use Inert Gas?
Have you ever heard someone say, “Best case scenario: fill your wine tank with wine and eliminate headspace” as an answer to a question about how to best blanket wine with gas?
If so, you’re not alone.
After a wine is fully fermented, it typically is racked off of gross lees and placed into a storage vessel. Today, storage vessels can be made of stainless steel, wood, concrete, and even plastic. Each storage vessel has its own set of pros/cons associated with wine storage:
- Stainless steel is known for keeping wines fresh, potentially has temperature control capabilities, and a winemaker may be able to minimize headspace through use of a variable capacity tank (VCT). (The alternative is a fixed capacity tank in which a set volume of wine is required to minimize headspace in the tank.)
- Wood storage vessels come in many shapes and sizes: from wine barrels to wooden casks. Wooden storage vessels are fixed capacity vessels, requiring a specific volume to be considered “full.” Unlike the stainless steel cousins, wood does “breathe,” meaning there are various levels of oxygen exchange, depending on the type of vessel used and the environmental storage conditions that the wood is held in. However, oxygen exposure is not straightforward. Oxygen exchange and rates of oxidation are variable based on the type of wooden vessel used, environmental storage conditions (i.e., temperature and humidity), and wine chemistry and preservation (i.e., sulfur dioxide levels).
- Concrete offers alternatives to stainless and wooden storage vessels. Like wood, concrete vessels are also fixed capacity and offer some oxygen exchange properties to the wine. Also similar to wood, they require appropriate environmental conditions to maintain wine’s freshness. But unlike wood, they have better thermal insulation, which can manage temperature swings more efficiently. Concrete tanks do typically come with cleaning and sanitation protocols that may differ from traditional stainless and wood use.
- Plastic storage vessels also come in a variety of shapes and sizes. Like wood and concrete, these vessels rely on environmental conditions for wine freshness. Plastic also “breathes,” allowing for oxygen exchange. Some plastic vessel manufacturers declare an oxygen transmission rate (OTR), while others do not. This is true even in relation to the plastic seals in VCT’s in which the remainder of the tank is made of stainless steel.
Most wine storage recommendations are always to fill a vessel. This is the best way to avoid headspace/ullage issues associated with wine storage. Coupled with temperature control and sulfur dioxide management, these three cellar practices can greatly reduce accelerated quality degradations of wine.
If the vessel cannot be filled, then the use of inert gas to blanket the top wine surface may be recommended. While other tools exist, the failure to fill a tank will increase gas exchange at the surface of the wine.
Inert gas can also flush winemaking equipment (e.g., tanks, hoses, and some filtration units), prior to pushing wine into them. This helps reduce the concentration of oxygen that the wine comes in contact with during a wine transfer. A study by D.L. Wilson (1985b) found that DO levels could be reduced with gas-flushing practices prior to wine transfers. The DO pickup was further reduced when gas-flushing and sparging was combined during a transfer operation.
What does this look like in the cellar? Flushing a receiving tank and transfer hoses, while also maintaining a steady stream of inert gas to the tank being emptied can help minimize DO pickup during a transfer. Additionally, the use of “T” connections can introduce inert gas to the delivery line and help maintain a blanket of inert gas on the surface of the wine as it fills a new tank (D.L. Wilson, 1985b). Furthermore, flushing during filtration and bottling operations can also help reduce DO pickup during these final winemaking operations.
And finally, inert gas, especially nitrogen or nitrogen/carbon dioxide mixes, is used to sparge wine to manipulate dissolve gas levels. Specifically, this refers to dissolved carbon dioxide and dissolved oxygen (DO). It is, however, important for winemakers to consider wine temperature. Gas absorption, including oxygen, is more effective with the wine is cold. This is why most experts will encourage certain wine temperatures during transfers and pre-bottling operations.
Some Details About Gas Blanketing
Blanketing practices attempt to create a non-oxygen gas layer directly above the surface of the wine, referred to as “a blanket.” Inert gas blankets attempt to minimize (or eliminate) the oxygen layer above the surface of the wine, which, in turn, minimizes oxygen absorption into the wine through its surface. It’s assumed that by eliminating the oxygen, the wine cannot further oxidize, or support aerobic (oxygen-needing) microbial growth. In old winemaking practices, oil layers were used to separate the wine surface from the oxygen surface for these same reasons. However, the use of oil layers is not commercially recommended today.
Most experts agree that to avoid aerobic microbial growth, oxygen levels must be reduced to 0.5% or less (M. Dharmadhikari). [As a reference, air is made up of about 20.9% oxygen at 20°C at sea level (L. Nel, 1998).]
However, many wineries do not measure the oxygen content in the headspace of the tank, let alone directly above the surface of the wine, to ensure the blanket is effective. D.L. Wilson (1985a) compared three carbon dioxide blanketing techniques with a 1 meter (3.28 feet) ullage space between the wine’s surface and the lid of the tank. The most effective treatment was using carbon dioxide gas directly at the wine’s surface to displace oxygen at the surface of the wine. In this case, oxygen was displaced up and out of the wine tank, showing additional decreases of the air’s oxygen content up to 50 cm away from the wine’s surface. This was compared to a dry ice treatment (that sublimates carbon dioxide) in a bucket or ice bucket with holes that hung from the top of the tank. With these later two treatments, approximately 13% – 19% of oxygen remained at the surface of the wine.
This should remind winemakers just how variable the reduction in oxygen is at the surface of the wine if they are not actively measuring the oxygen content. This indicates that various practices assumed by winemakers may not be as effective as they believe.
Do inert gasses gently push oxygen molecules out of the way?
In short, no. This is a common myth and misunderstanding by winemakers.
While it may make for a nice image in our minds, the inert gases we use in the cellar are not that effective at sinking to the wine’s surface when blanketing.
Nitrogen, which is not heavier than oxygen, gently moves oxygen around and displaces oxygen. Nitrogen can very quickly float above oxygen due to its lighter weight (L. Nel, 1998). Plus, keep in mind, about 78% of the air we breathe is made up of nitrogen gas.
It is assumed carbon dioxide gas can move through air, sinking below any oxygen molecules due to its heavier molecular weight, ultimately resting on the surface of the wine. However, as mentioned above, in practice, this is not the case. Instead, carbon dioxide “uniformly distributes” (M. Dhardmadhikari) throughout the wine tank, diluting the oxygen concentration (L. Nel, 1998) periodically.
Furthermore, it’s important to remember that gas blanket layers are not stagnant. Similar to what we see in liquids, gas molecules are also subject to equilibrium chemistry. They will move from areas of high concentration to low concentration until an equilibrium exists. This is true even with gas molecules of varying molecular weights meaning that heavier molecules can still mix out of the blanket layer into the “lighter” air above it.
Finally, winemakers need to remember that how the inert gas is applied impacts its effectiveness as a blanket. Using minimal, slow flow rates is often recommended. Higher flow rates cause turbulence, which inhibits settling of the gas on the surface of the wine for a short period of time. Additionally, a hose/tube cannot get submerged into the top of a wine tank with the expectation that the gas sinks down to the wine surface layer. It does not quite work that way. Instead, the hose/tube of gas needs to be within about 1” from the surface of the wine, flowing parallel to the surface of the wine. Again, measuring oxygen levels is probably more efficient than guessing how much time it takes to cover the entire surface of the wine, as no standard time periods exist for maintaining a flow.
And finally, do not assume that a once-a-week blanket is enough to maintain a blanket layer. As mentioned above, gas layers are not stagnant.
Carbon Dioxide: Not So Inert
A further problem with carbon dioxide gas use is that it is soluble in wine. Argon and nitrogen are less soluble.
A simple review of gas laws remind us that the exchange of gas over the surface barrier is possible. Therefore, wine can absorb and retain carbon dioxide as dissolved carbon dioxide when it’s used as a blanketing, transfer, or sparging gas. This is certainly an issue with carbon dioxide because it can lead to changes in pressure and sensory perceptions within the wine.
Yes, dissolved carbon dioxide actually has a sensory impact on the wine. The following are dissolved carbon dioxide levels and their associated sensory properties from D.L. Wilson (1985a):
- “<0.2 g/L: wine appears flat and unacceptable
- 0.4 – 0.5 g/L: wine fully develops
- >0.7 g/L: sharp prickling flavor develops, may be acceptable in some white wines
- >1.0 g/L: clear prickling sensation with some bubbles on the glass
- 1.7 g/L: wine is fully saturated at normal temperatures”
Finally, Food Grade Considerations
Expense is another thing that winemakers and winemaking teams need to consider when using inert gases effectively. Because these gases come in direct contact with wine (which will be consumed), the use of food grade inert gas is required. Food grade gases are more expensive, but it’s not something that is negotiable for use when producing wine.
Furthermore, gas cylinders and regulators can be microbial contamination points for wine. Many companies will recommend a sterile filter for the canister/regulator hookup that periodically needs changed to minimize microbial contamination. Of course, this is an additional expense. Given that minimizing the incidence of microbial films is part of the reason why inert gases are used in wine production, it is a good idea to invest in these filters to avoid literally adding a film to the wine through microbial inoculation/contamination.
With that in mind, proper sanitation for the gas hose/tubing ] is also a must as these ends can come in contact with the wine’s surface.
The use of inert gas can be strategic in a winery. It is not the only tool available to maintain a wine’s quality during production. It is a tool. Taking the time to learn how to best use tools effectively usually equates to long run cost savings and better wine quality. While many winemakers get information passed down to them from other winemakers, the DG Winemaking Elite Membership can also provide you with sound directions in improving your winemaking operations. These are affordable options for getting help in your cellar today!
References
Dharmadhikari, M. Use of inert gasses. Available at: https://www.extension.iastate.edu/wine/use-inert-gases
Nel, L. (1998). The use of inert gas in the wine making process. Wynboer, Januarie. Pg. 101 – 103.
Smith, G.K. (1997). Presentation from the 21st Biennial Australian Speleological Federation Conference. Information provided by: Dr. Tom Cottrell, Extension Enologist of the University of Kentucky, 2013.
Wilson, D.L. (1985a). Storage of wine using inert gas for prevention of oxidation. Australian Grapegrower Winemaker, 256: 122-127.
Wilson, D.L. (1985b). Wine transfer using inert gas for prevention of oxidation. Australian Grapegrower Winemaker, 256: 110-111.
The views and opinions expressed through dgwinemaking.com are intended for general informational purposes only. Denise Gardner Winemaking does not assume any responsibility or liability for those winery, cidery, or alcohol-producing operations that choose to use any of the information seen here or within dgwinemaking.com.
