Enology Notes #122, December 20, 2006
To: Regional Wine Producers
From: Bruce Zoecklein, Head, Enology-Grape Chemistry Group, Virginia Tech
1. Carbon Dioxide, Nitrogen, and Argon in Winemaking. While argon is rarely used due to its cost and limited solubility, carbon dioxide and nitrogen are often used to help minimize wine-oxygen contact. The following discusses the use of nitrogen and CO2 gas in winemaking post-fermentation.
Newly fermented wines are saturated with CO2. As a result of outgassing and postfermentation processing (racking, fining, etc.), levels drop to near 1,000 mg/L during the first several months of aging. Depending on intrinsic properties of the wine and cellaring practices, additional decreases of 100 mg/L or more may be seen during the first year, with negligible losses thereafter.
Because solubility is, in part, temperature dependent, fermentation and storage temperatures contribute to retention. The use of slow-fermenting yeasts may also play a role in establishing soluble levels. In wine of 11% alcohol (at atmospheric pressure), increasing the temperature from 0°C-20°C (32-68°F) brings about a CO2 solubility decrease from 2.9 to 1.4 g/L.
Alcohol is also important in retention of CO2. Because its solubility is inversely related to the concentration of alcohol, CO2 retention at equivalent temperatures is greater in wines of lower, compared with higher, alcohol levels.
Other contributing factors include reducing sugar levels, and viscosity arising from polymeric carbohydrates and phenols. Originating from the grape as well as yeast autolysate, polysaccharides exist as polymers of glucose, galactose, arabinose, and rhamnose. The concentration of polysaccharides in white wine ranges from 170 to 970 mg/L.
Cellar processing plays an important role in gas retention. For example, storing wine at low temperatures aids in retention, whereas warming accelerates degassing. Processes such as racking, fining and filtration may reduce the amount present, whereas use of CO2 in purging tanks and lines may contribute to increases.
Nitrogen gas serves as an effective tool in lowering levels of CO2. Compared with CO2, which has a solubility in wine (at atmospheric pressure) of 1,500 mg/L, N2 is relatively insoluble, with a solubility of only 14 mg/L.
Because of this insolubility, N2 in low concentrations can be effectively used to strip other volatile species (including CO2) which are more soluble. The mechanism of action involves the equilibrium between the gaseous form and dissolved carbonic acid:
During nitrogen sparging, N2 molecules collide with and sweep some of the CO2 (gas) molecules to the surface. This, in turn, causes more CO2 • H2O to shift to the gaseous form where it, too, is swept from the system.
Exclusive use of CO2 or N2 may lead to either “spritzy” or “flat” character. Use of formulated CO2/N2 mixtures (“balanced mix”) is reported to more closely simulate natural balance.
Carbon dioxide and protein interactions are partially responsible for maintaining CO2 solubility. The CO2-protein interaction is electrostatic, and not chemical. Due to its symmetry, CO2 doesn't exhibit polarity. Carbonic acid, however, is asymmetric, and its negative dipole is adsorbed to positively-charged protein fractions. Thus, more CO2 is adsorbed at lower, rather than higher, pHs.
Some wineries sparge wines with CO2 before or during bottling. In the United States, CO2 additions are permitted in still wines, provided that not more than 3.92 g/L is present at time of sale. By comparison, OIV levels are set at 1.0 g/L.
Carbon dioxide can provide a tactile sensation, magnify the sense of acidity, and enhance odor intensity. As such, white wines designed for early release may be produced, using residual CO2 to enhance the fruit character and enliven the palate.
Carbon dioxide is perceptible in water at 200 mg/L, and in wine at about 500 mg/L. At levels greater than 700 mg/L, CO2 may be tactilely perceivable, and at greater than 1000 mg/L, CO2 bubble formation is frequently noted. It is the author’s experience that the difference in threshold in wines of 900 to 1300 mg/L is near 300 mg/L.
Use of Gases
Carbon dioxide, nitrogen and argon are used in wine production in three ways: 1) sparging, 2) blanketing, and 3) flushing. Sparging involves the introduction of very fine gas bubbles to help remove dissolved oxygen or CO2, or occasionally to add CO2.
The solubility of a gas in a liquid is proportional to the partial pressure of that gas in the gaseous atmosphere in contact with that liquid. When fine bubbles are dispersed, a partial pressure develops between the sparging gas (usually N2) and the dissolved gas (usually O2). The difference in partial pressures causes the dissolved gas to leave the wine. The effectiveness of sparging is dependent upon the wine, temperature, time, gas volume, and bubble size.
Blanketing attempts to maintain a gas layer above the wine surface in the hopes of minimizing wine-air contact. Both nitrogen and CO2 are used for the purpose, although nitrogen is preferred because it has a very low solubility in wine, 14 mg/L. This is a principal reason why it is also an effective sparging gas. Carbon dioxide, on the other hand, has a solubility of 1,500 mg/L. In order to prevent the growth of aerobic microorganisms on the wine surface, the O2 concentration must be reduced from the 20.9% O2 found in air, to 0.5% or less at the wine surface.
The reduction of O2 to levels low enough to control biological growth for extended periods is difficult, using blanketing gases. Some producers choose to lightly but regularly CO2-sparge partial tanks. As the CO2 comes out of solution, it helps to displace the O2 at the wine surface. Because CO2 is “heavier” than either N2 or air, it may remain on the surface and help protect the wine. It is universally accepted that there is no better substitute for the protection from O2 than storage in completely full containers.
Wines stored in barrels that are properly sealed develop a partial vacuum over time. If the vacuum is maintained, it helps to limit the growth of aerobic microorganisms, even if the barrel is not completely full.
Gassing may be accomplished with N2, CO2, or “balanced mixtures” of the two, as well as with argon. Gassing is best accomplished by introduction at the tanks’ bottom draw. This ensures more complete displacement of oxygen out the top. Properly carried out, O2 levels less than 1% are achievable. Portable oxygen meters are useful for monitoring effectiveness. After movement of a delicate white wine, any headspace should be gassed prior to closing the access port.
If sparkling wine contains dissolved air or nitrogen under pressure, as well as carbon dioxide, gushing can occur. For this reason, nitrogen sparging and excessive aeration of the cuvée wine is undesirable. The solubility of air or nitrogen is very low under pressure. When bottles that contain air or nitrogen are opened, these gasses immediately come out of solution as fine bubbles, that then gather carbon dioxide and gush.
These gases make the system unstable because their escape rates may be higher than that of the carbon dioxide. It is therefore imperative that cuvées not be nitrogen-sparged or undergo excessive aeration. There may be 15 psi or more of air in the wine at cuvée bottling; if too much additional air is dissolved in the wine, it may make the final bottle unstable or “wide” at the time of disgorgement and consumption.
Measurement of Carbon Dioxide
Carbon dioxide in wine can be measured using a variety of analytical techniques and instruments, including the Carbodoseur, titration, blood gas analyzers, and CO2-specific electrodes. The simplest method is the use of Carbodoseur.
The Carbodoseur is a glass tube that measures the amount of CO2 outgassed from a fixed quantity of wine. Comparing this volume to a calibration chart, one can read directly in mg CO2/liter of wine sample. It is relatively easy to use and gives reproducible results. Since the Carbodoseur method doesn’t require the sample to be treated to facilitate “release” of CO2, results may be variable depending upon pH, temperature, etc.
Managing Oxygen During Bottling
There have been some recent concerns about oxygen pick-up during bottling. This is an extremely important issue influencing wine quality, stability, and longevity.
The concentration of molecular oxygen should be measured in the wine before bottling begins, and should be less than 0.5 mg/L. If the concentration of oxygen is greater than 0.5 mg/L, it can generally be lowered by sparging with nitrogen gas.
Just prior to bottling, air should be eliminated from all hoses, filter housing pumps, and the fill bowl by using displacement gas (nitrogen, carbon dioxide, or argon). Feed tanks should be blanketed with nitrogen or CO2, or lightly CO2 sprayed.
Bottles should be completely free of particulate matter, which can occlude oxygen, and flushed with displacement gas just prior to filling. Any oxygen which remains in the bottle will result in an oxygen concentration increase. Any increase above 0.2 mg/L dissolved oxygen indicates excessive pick-up.
The loss of free sulfur dioxide in wine is proportional to the dissolved oxygen content. Producers not using vacuum filters, corkers, or bottle gas flushing can have up to 5 mL of air in the headspace of their bottled wine (750 mL). This amounts to approximately 1 mL (1.4 mg) of oxygen. Four mg of sulfur dioxide are needed to neutralize the effects of one mg of oxygen. Using this relationship, an additional 5-6 mg of free sulfur dioxide is needed to reduce molecular oxygen in the head space.
Monitoring molecular oxygen in wine during any stage of processing is relatively easy. Several portable, handheld meters with probes are available for measuring atmospheric and dissolved oxygen in wine.
2. VVA Meeting. The VVA (Virginia Vineyards Association) annual winter meeting will be held at the Omni Hotel in Charlottesville, Virginia, February 8-10, 2007. The following is the preliminary schedule. There will be two cultivar focus sessions, Petit Manseng and Cabernet Franc.
Winemakers are aware that the flavors in wines have many contributing elements. What is not so well understood is that at harvest the grape carries a flavor quotient that relates to its herbaceous and vegetative origin, and those flavors must be managed to produce a balanced product. This will be explored and discussed at the meeting. For additional information, see the VVA website at http://virginiavineyardsassociation.com.
Thursday, 8 February 2007
1:00 pm Registration
1:30 pm Petit Manseng: a viticultural and enological exploration of the current status and potential for this variety in the mid-Atlantic. Will include wine tastings. Separate registration track.
5:00 pm Adjourn
Friday, 9 February 2007
8:00 am Registration
8:30 am Welcome, Rock Stephens, VVA President
8:45 am Powdery and downy mildew resistance survey.
Anton Baudoin, Virginia Tech
9:15 am Update on Pierce’s Disease in Virginia Tech.
Ashley Myers and Anna Wallingford, Virginia Tech
9:45 am Grape root borer research.
Chris Bergh, Virginia Tech
10:15 am Break
10:30 pm Grower Panel Discussion (Tony Wolf, Virginia Tech, moderator): What worked and what didn’t during the 2006 season in the Mid-Atlantic. Speakers from Pennsylvania, Virginia, and North Carolina will share experiences on crop vs. quality, canopy management, wildlife control, bees, berry splitting, disease control and possibly other topics.
12:00 pm Break for lunch (on your own)
1:30 pm Grape mealy bug: effects on grape quality, leafroll virus transmission, and considerations for management.
Doug Pfeiffer, Virginia Tech
2:00 pm Economic impact of the Virginia Wine Industry (Tentative).
Motto, Kryla, Fisher, LLP (MKF) Research representative
2:30 pm Enology program research updates.
Bruce Zoecklein, Virginia Tech
3:00 pm Break
3:15 pm Viticultural and winemaking options for impacting herbaceous tones in Cabernet franc (Bruce Zoecklein, Virginia Tech, moderator). Program will include wine tasting and discussions of ways to enhance Cabernet franc wine quality.
6:00 pm Reception (6:00 – 8:00 pm)
Saturday, 10 February 2007
8:00 am Registration
8:30 am Assessing the risks of drought and the need for irrigation in the mid-Atlantic.
Tony Wolf, Virginia Tech
9:15 am Late-season disease control options to minimize fermentation problems.
Bruce Zoecklein, Virginia Tech, and others
10:00 am Break
10:15 am VVA Business Meeting
- - Treasurer’s report
- - Legislative update
- - Other business
12:00 pm Buffet Lunch
1:30 pm Guest speaker: Robert Bloxxom, Virginia’s Secretary of Agriculture
2:00 pm Are conservation easements for you? Exploration of benefits of conservation easements to you, your family, and society, including financial benefits, federal and state tax implications.
Roger Holnback, Director, Western Virginia Land Trust
2:50 pm Powdery mildew research (recent research and fungicide updates).
Wayne Wilcox, Cornell University
3:40 pm Break
4:00 pm Grape maturation in a hot climate. How hot is too hot? A discussion of the effects of heat on color formation, flavor and aroma compounds, and how we might modify canopy management, crop load, varieties, etc. to optimize wine potential in a hot environment.
Sara Spayd, North Carolina State University
5:00 pm Adjourn
3. Winery Planning and Design CD Available. The Enology-Grape Chemistry Group’s comprehensive 200-page review covers the essential planning and design considerations of critical importance in winery establishment. The Winery Planning and Design CD is available through Practical Winery and Vineyard magazine for $95. Email them at , or call (415) 479-5819.
4. Enology Service Lab Holiday Closing. The VT Enology Analytical Service Laboratory will be closed over the upcoming Holidays. All samples must be received by Thursday, December 21st to avoid storage by the shipping companies over the holidays. Due to the University’s closing schedule, the service lab will be unable to receive samples until January 3rd, 2007. Samples can be sent starting January 2nd. The results reporting website and the Enology/Grape Chemistry Group website will be accessible during this time. Questions should be directed to .
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Professor and Enology Specialist Head Enology-Grape Chemistry Group
Department of Food Science and Technology, Virginia Tech
Blacksburg VA 24061
Enology-Grape Chemistry Group Web address: http://www.vtwines.info/
Phone: (540) 231-5325
Fax: (540) 231-9293
Cell phone: 540-998-9025