Oxygen is not the Boogie Man in Winemaking

updated 05/22/2024

A common question from new winemakers is about oxygen exposure. It’s commonly believed that even brief exposure to air ruins a wine.

I was once asked, “My wine was exposed to air for 2 minutes. Should I throw it out?”

<face palm> Thankfully the question was asked via email, so I did not have to hide my reaction.

Before answering I considered that I’ve been making wine for decades, I do a lot of research so I know things beyond my own experiences, there is tremendous disinformation about oxidation floating around, and that all beginners (regardless of area) have a lot of fear of failure.

My answer was polite and informative, and I’m pleased to say this person is a successful winemaker well over a decade later.

Side rant: Never be a jerk to a beginner. They are beginners, they don’t know stuff, and they’re afraid of failure and/or looking foolish. Remember that we were all beginners at one point, a few kind words can make a positive difference in someone’s life, and it doesn’t cost anything to be polite.

Not the Boogie Man

Oxygen is NOT the Boogie Man in winemaking. It’s beneficial during the early stages of fermentation, and under proper conditions it is not a real problem during later stages of winemaking.

Unless made totally under a vacuum, every wine is exposed to air. Nothing short of a vacuum will prevent this.

In fact, during fermentation the lack of O2 is counterproductive, as yeast needs O2 for reproduction. This is among the reasons that wine is typically fermented in an open container and stirred at least once daily.

Understanding Oxidation

There is a common misunderstanding of O2 and oxidation. Oxidation is not a rapid process; it’s quite slow. Brief exposure to air doesn’t hurt a wine noticeably, and we add the antioxidant potassium metabisulfite (K-meta) to address O2. K-meta combines with contaminants, such as O2, rendering them harmless.

Note that free SO2 (produced by K-meta) gets used up as it handles problems, so it must be refreshed periodically. [I’m not expounding on that here; it’s a long topic that deserves its own post.]

There is also the consideration of volume of air (O2) vs. volume of wine. Think of it this way, a smaller volume of wine with a larger headspace oxidizes more rapidly than a large volume of wine with a smaller headspace.

Every wine will have some level of oxidation, but as long as good practices are observed, it’s not detectable and therefore not a problem.

Good Practices

None of the above means we should be cavalier about O2 exposure. We can do things to help ensure oxidation is not a problem.

1. Be efficient during racking operations to limit exposure. There’s no need to rush (as rushing often produces errors), but don’t dawdle, either.

However, I’ve been interrupted during a racking and the wine sat in a primary container for two hours. Six years later when the last bottle was consumed, it was fine.

2. Headspace during active degassing is ok. When a wine is actively degassing, it’s emitting enough CO2 to push air out of the secondary container, so a relatively large headspace is not a problem. How big is too big? I cannot provide scientific answer, but my personal guideline is no more than 20% of the container’s volume, although I’ve had 4 liters of wine in a 19 liter carboy for a week without producing a problem.

3. I limit the length of time for a large headspace to between two and four weeks. We cannot see how much CO2 is being emitted, so I tend towards caution and allow less time if the relative headspace volume is larger in proportion to the wine volume. Again, I have no scientific answer for this, just personal guidelines.

One of the problems in winemaking is that when you discover you have significant oxidation, it’s too late to fix it. So err on the side of caution.

4. Dose with K-meta to address O2 plus numerous other things. I add 1/4 tsp K-meta per 19 to 23 liters of wine at each post-fermentation racking, every three months during bulk aging, and at bottling time. The amount of sulfite deemed safe by the USDA is way above my maximum usage, so I’m not concerned about adding too much.

Note that I rack as few times as feasible, which may be as little as three times: pressing of grapes / first racking of juice, racking off the gross lees one to three weeks later, and again at bottling time (three to twelve months later).