If you’ve ever left juice in the refrigerator and returned to find it spritzy, you’re acquainted with a key feature of fermentation: carbon dioxide (CO2).
Produced naturally when yeasts convert sugar to alcohol, CO2 is a colorless, odorless gas present in all wines. In sparklers, it’s the bubbles. In still wines, its presence is more subtle, enhancing the perception of acidity and movement on the palate.
The decision of whether to remove CO2 before bottling, and how much to remove via a process called degassing, is critical to the development of a wine’s final profile.
Too much CO2 can prevent a wine from settling properly, resulting in a cloudy appearance.
Such wines can present reductive aromas of varying desirability. (In good cases, this can mean gunflint. In bad ones, eggs or cabbage.)
CO2-laden wines are also sometimes perceived to be “going bad” like that juice in the fridge.
But removing too much of it carries risks.
Wines degassed excessively can seem flat in flavor or flabby. They also become more susceptible to oxidation.
Indeed, says Dominique Hauvette, owner and winemaker of Domaine Hauvette in Provence, France, “leaving it in wine allows us to add much less sulfites because CO2 is a sensational natural preservative.”
So how exactly do winemakers finesse CO2 levels?
Some winemaking processes, like pumping, racking and lees-stirring, have the auxiliary effect of degassing. But before bottling, winemakers can also degas manually with a method called sparging, which involves the introduction of very fine bubbles.
A metal device known as a sparging stone is attached to a canister of an inert gas like nitrogen or argon. Lowered into a tank of wine, it releases microbubbles that flush out CO2 in a controlled manner.
Strictly speaking, there’s no correct CO2 level—it’s winemaker preference.
“You do it by feeling,” says Guy Breton, a winemaker based in Villié-Morgon, in France’s Beaujolais region. “If it’s a high-acid vintage, I can bottle with lower CO2. In years without acidity, I leave more.”
Some, like Christine Pieroth of Piri Naturel winery in Germany, believe a tiny fizz from CO2 brings something positive “In the glass, the first two seconds are a bit bubbly,” she says.
With a swirl, however, it all nicely harmonizes.