Q. I often have a few inches of headspace at the top of carboys when I’m storing wine in them. I add a layer of CO₂ before setting the airlock, which seems to work well. Do you see any downside?
Michael Dickson
via email

A. You’ve touched on one of the most important topics in winemaking, and one that probably doesn’t get enough attention. Headspace is the enemy of finished wine.

Adding a blanket of carbon dioxide before fitting the airlock certainly feels like the right solution, and it’s better than doing nothing. But if your wine has several inches of air space above it, I wouldn’t rely on gas blanketing alone.

For many years, winemakers believed that because carbon dioxide is heavier than air, it would simply settle over the wine and form a protective layer. In reality, gases don’t stay in neat layers for very long — they mix. Every time the carboy is bumped, the temperature changes, or barometric pressure shifts, that protective blanket becomes less reliable. Research over the years has shown that gas blanketing simply isn’t as effective as many of us once hoped.

I’d rather eliminate the headspace altogether. One easy home winemaking trick is to add sanitized glass marbles to the carboy. They displace volume, raising the wine level until it’s at the neck of the vessel. The less air your wine has to live with, the better.

Why am I so concerned about a few inches of air? Because oxygen creates opportunities for all sorts of things you’d rather not have in your wine.

One possibility is the growth of film yeasts on the surface of the wine. These oxygen-loving yeasts can form a thin film and produce oxidative aromas that many people describe as bruised apple, nutty, or Sherry-like. They are entirely different organisms from the bacteria that make vinegar, but they’re another reminder that oxygen changes wine in ways we usually don’t want.

Acetic acid bacteria are a separate concern. These bacteria also require oxygen to thrive, and they convert alcohol into acetic acid. Given enough time and oxygen, they can literally turn your wine into vinegar.

Even before spoilage organisms become established, oxygen alone can slowly rob a wine of its freshness. As oxidation progresses, compounds such as acetaldehyde can accumulate, bringing bruised apple or stale aromas. In more advanced cases, other spoilage compounds, including ethyl acetate, can develop and contribute solvent or nail polish remover smells.

That’s why I think of gas blanketing as insurance, not a substitute for proper storage. If you have a little unavoidable headspace while you’re transferring wine from one vessel to another, a shot of carbon dioxide is certainly worthwhile. But for weeks or months of aging, I’d much rather see the wine physically occupying that space instead of hoping a layer of gas stays exactly where you left it.

Raise the wine level up into the neck by adding sanitized glass marbles, topping with a compatible wine, or transferring into a smaller container, and you’ll be giving your wine much better protection.

When it comes to storing finished wine, less headspace is always better. No headspace is best.