You know oxygen is an important part of the winemaking process, but that doesn’t mean you always know how to manage or control it. Oxygen can do wonderful things for a wine, aiding in the development of more complex flavors and improving mouthfeel. But oxygen can also be your wine’s worst enemy.

You want to develop your wine at your own pace, finding that balance between keeping your wine fresh and healthy and letting oxygen work its magic. By understanding your barrels’ relationship to the oxygen in your wine, you can make small adjustments in the cellar that have a big impact on wine quality.

The science behind this topic is overwhelming. For most of us, it’s too complex to even consider the “whys” and “hows.” But I like a good challenge, so I’ve asked all the hard questions. Here are the “CliffsNotes.”

Three Important Questions to Ask

#1 HOW DOES OXYGEN GET IN?

When wine is aging in a barrel, it can be exposed to oxygen through a few different entry points. Only one study has been published that quantifies ALL the possible pathways for oxygen to enter a barrel, and that was done by N. Vivas and Y. Glories in 1997. This is how they broke it down:

• 21% through bung
• 63% between the staves
• 16% through the wood itself

These numbers have been popularly referred to throughout the wine industry, but I’m no longer relying on them for the following reasons:

1. Vivas and Glorias ran their tests on just two barrels.
2. The authors never clarified whether oxygen came in through the bung itself, the joints around the bung or from opening and closing during barrel topping.
3. This testing was done more than 30 years ago, and with new technology, measurements are more accurate now.

Current Research – UVaMOX

The team of researchers responsible for most of the studies done in recent years on oxygen and barrels is from UVaMOX at the University of Valladolid in Spain. Two of the leading professors, Maria del Alamo-Sanza and Ignacio Nevares, PhD, have graciously and patiently responded to all my questions about their studies.

In regard to how oxygen enters wine through a barrel, UVaMOX focused its testing on oxygen coming in through the joints between the staves and through the wood itself. The latter is due to the wood’s permeability, oxygen trapped inside the wood’s cells and the release of oxygen as the cells fill with wine. UVaMOX excluded the bung as a variable by creating an air-tight seal.

• AMERICAN OAK BARRELS:  50% enters through the joints between the staves and 50% through wood.
• FRENCH OAK BARRELS:  25% enters through the joints between the staves and 75% through the wood.

About the bung…

A lot more work needs to be done on how much oxygen comes in through the bung area and exactly how that happens. Here is what we know:

• The highest concentration of dissolved oxygen in the wine is typically found closest to the bung. But if the bung is sealed air-tight, the opposite is true.
• The top of the barrel, near the bung, is the quickest to fill with air as the wine impregnates the wood and water evaporates.
• Topping a barrel adds less oxygen than you would think.

Topping

A few authors have quantified that each topping up of a barrel adds approximately 4 mg/liter to the wine. But UVaMOX has demonstrated it’s actually less than this – topping makes up only 1.4% of the annual Oxygen Transfer Rate (OTR). Based on an average annual OTR of 10 mg/liter (see section #2), this amounts to 0.138 mg/liter.

According to UVaMOX: “When the barrel was opened, depressurization was lost, which caused the air to enter, but when the barrel was immediately filled with deoxygenated wine, it was verified that the DO [dissolved oxygen] level of model wine in the barrel did not vary significantly in all cases and, in some cases, even decreased slightly. Therefore, good management of the barrel topping process does not necessarily involve an increase in wine oxygenation.”

Winemakers…once you remove the bung, top quickly! The theory is that the oxygen doesn’t have time to dissolve.

#2 HOW MUCH OXYGEN COMES IN?

Researchers have been publishing data on this one since 1933 with OTRs up to 45mg/liter per year. With every new study, the numbers change. UVaMOX has performed the most recent testing, and these were the results:

• American oak:  11.3 mg/liter per year
• French oak: 8.18 mg/liter per year

French oak is actually more permeable to oxygen, but American oak barrels have a higher total OTR. According to UVaMOX, this is mostly because of differences in the cutting of the wood and how the barrels are constructed at the cooperage.

These lower OTRs can be explained in the next section and by the fact that UVaMOX measured the OTR of barrels for a whole year, while previous studies only measured for six months and doubled that number to get the annual rate.

#3 WHEN DOES OXYGEN COME IN?

50% of the total oxygen a wine receives from a barrel is transferred in the first 2-4 months.

The reason less oxygen comes in after this point is that the wood’s surface becomes saturated with wine and blocks the oxygen. Dry wood is 20−100 times more permeable to oxygen than wet wood.

UVaMOX, and several others before them, proved that moisture content of the wood is the key factor in decreasing OTR in barrels, discounting past theories that soluble elligitannins in the wood or solids deposited by wine are responsible. “Once the inner 5mm of the staves are saturated with wine, the oxygen has a barrier.”

ARE THERE OTHER VARIABLES? YES!!!

Knowing there are other factors makes it seem impossible to quantify any of this, but we’re dealing with a complicated topic. My role is to provide you with all the information, and you can apply weight where you see fit. Here are some other variables:

• Location of a barrel in the cellar (proximity to the door)
• Cellar humidity
• Toasting 

UVaMOX compared oxygen permeation between raw and toasted wood and determined that wood loses up to three times its OTR during the toasting process. Since very few people use raw wood barrels (yuck), I think most barrels are on an even playing field. There might be slight differences with length of toasting and higher heats, but they have not been quantified.

• Barrel Construction

A study published in February 2018 by Chêne & Cie and others showed that the amount of oxygen coming through the joints depends a lot on the pressure exerted when joining the staves at the cooperage. It’s possible that with enough pressure, almost no oxygen comes through this route.

Equipment and processes vary from from cooperage to cooperage, but word is that all modern joining equipment allows for such pressure. If this is the way of the future, perhaps we need to give more weight to the oxygen coming through the wood itself.

• Grain tightness

While tighter grain oak is more permeable to oxygen, there have NOT been significant differences in the annual OTR between grain types.

• Stave thickness

While stave thickness does affect permeability, it doesn’t seem to be a significant factor in the OTR of barrels with staves in the 22mm-27mm range. Perhaps since the wine only penetrates the wood 5mm and at that point reduces oxygen flow, the additional thickness on the outside of the stave is inconsequential.

IN CONCLUSION

Once you emerge from this deep dive into the science behind barrels and oxygen, here are a few key takeaways:

• Not only does French oak differ from American oak in wood flavor, there’s a definite difference in the oxygen introduced as well.
• Topping only makes up a small portion of the oxygen introduced by a barrel, as long as you top quickly after removing the bung.
• Half of the oxygen a new barrel brings to wine is introduced in the first 2-4 months. This is due to the increasing moisture content of the wood, which happens fairly quickly.

As testing methods get more sophisticated, the old data we’ve relied on can change. A good way to stay on top of this is to keep in touch with me!

Want more information? You can see my sources here.