“The winemaker left a bit more CO2 in the wine to help protect it.”
That sentence, referring to the amount of dissolved carbon dioxide (CO2) in a bottled wine, is heard often in the world of artisan and organic winemaking, especially in the case of more “natural”, low-sulfur wines. The logic behind this method is that since the wine has little or no sulfites added, it might need another type of protection from oxidation to stay fresh and drinkable after bottling.
Wines at the end of alcoholic and malolactic fermentation are loaded with CO2 since it is a by-product of the work done by the yeasts and malolactic bacteria. Some of this CO2 will come out of the wines when they are moved around in the cellar, during racking or blending, but most often, some degassing is necessary at the end of the process to make still wines ready for market. There is a balance to be reached: A still red with a slight fizziness can seem strange and a little harsh, while a dry white with too little CO2 can feel flat, less lively (think flat soda pop).
Carbon Dioxide Over the Wine Versus in the Wine
At first glance, a bit of extra CO2 may seem like a logical choice. Blanketing wines with carbon dioxide is a very good practice to protect them in the winery, just like getting neutral gas in the bottle at the moment of bottling is useful to avoid excessive oxygen intake that could affect or even damage the wine.
So does leaving an extra dose of CO2 in a bottled wine give it the same protection? Not really. “Have you ever had a bottle of Champagne that was oxidized? There’s still plenty of CO2 in the bottle—a lot more than in any still wine, but the wine is still affected,” explains Nomacorc enology researcher Maurizio Ugliano.
As opposed to components like ascorbic acid, SO2, phenolics, or glutathione, CO2 doesn’t “intercept” oxygen and its by-products before they attack other, more sensitive molecules. In fact, it doesn’t take part in any way in the reactions that will cause a wine to gradually oxidize. “Under static conditions (like a bottled wine), the CO2 dissolved in the wine doesn’t participate in the oxygen exchange, and its presence in the wine doesn’t affect the presence of O2. Of course, when a wine rich in CO2 is moved around (think of a glass of sparkling wine being swirled), part of the CO2 will be released in the space immediately above it, which will physically remove the oxygen. But when the wine is static, this effect is negligible,” adds Ugliano.
While a blanket of CO2 above the surface of the wine can help prevent oxygen from reaching the wine, whatever oxygen does come in contact with the wine will still interact with the wine. So if a wine that has a relatively high content of dissolved carbon dioxide receives some oxygen, that oxygen will still be consumed by the wine. And if oxygen comes through the closure, it will still react with the wine, no matter how much dissolved carbon dioxide is in the solution.
In short, a bit of extra dissolved CO2 can give an impression of freshness, but it won’t keep your wine fresh in the long run. If a wine that was left with an extra dose of CO2 feels fresh and alive, it’s probably because it was well made in the first place, it was bottled with the right CO2 amount, or because it was under the right closure, but not because of the gas itself.