Cork, Oxygen & OTR: What Actually Happens After Bottling?

Once a wine is bottled, the winemaking process is not necessarily finished.

The closure you choose can influence how a wine develops over time, particularly through its interaction with oxygen. This is where oxygen transfer rate, or OTR, becomes important.

You may have heard that cork allows wine to “breathe”, or that screw caps completely prevent oxygen from entering the bottle. Neither explanation is quite that simple.

Different closures have different oxygen transmission characteristics, and understanding those differences can help explain why a winemaker might choose one closure over another.

What is OTR?

OTR stands for Oxygen Transfer Rate.

It describes the rate at which oxygen passes through a closure and into the bottle.

The important word here is rate.

A closure with a higher OTR generally allows more oxygen transmission over a given period than a closure with a lower OTR. This can influence the rate at which a wine develops in bottle.

However, the closure is only one part of the equation.

There can also be oxygen present in the bottle headspace, dissolved oxygen in the wine at bottling, and oxygen introduced during the bottling process itself.

So when we talk about oxygen and wine closures, we are really talking about the entire packaging system rather than simply whether a closure “lets oxygen in”.

Which cork is it?

Natural-

Natural cork has been used to close wine for centuries and remains an important closure for wines intended for bottle ageing.

Natural cork is a biological material, so individual pieces can vary in their physical properties and oxygen transmission characteristics.

One of the other considerations with natural cork is TCA, the compound associated with cork taint.

Modern cork production has made significant advances in reducing TCA risk, but natural cork can still have more natural variability than an engineered closure.

That does not make natural cork inherently better or worse. It simply means it has different characteristics that need to be considered when selecting a closure.

Technical cork-

Technical corks are engineered closures made using cork material that has been processed and manufactured to achieve more consistent physical characteristics.

They are not simply pieces of natural cork compressed into a bottle.

Technical closures can be manufactured with different physical properties and oxygen transmission characteristics, allowing them to be selected according to the intended style and ageing period of the wine.

This is where OTR becomes particularly useful.

Rather than thinking about cork as either “breathing” or “not breathing”, we can think about how much oxygen transmission is appropriate for the wine and its intended lifespan.

DIAM closures-

DIAM closures take this approach even further.

DIAM closures are manufactured using cork granules that undergo a process designed to remove the compounds responsible for TCA before the material is reconstituted into a technically engineered closure.

There are also different DIAM closures available with different oxygen transmission characteristics.

You may have seen closures such as DIAM 2, DIAM 3, DIAM 5, DIAM 10 and DIAM 30.

The number refers to the manufacturer's intended ageing period in years. It does not simply mean that DIAM 30 allows thirty times more or less oxygen than DIAM 2.

The different closures are designed with different oxygen transmission and mechanical characteristics, giving winemakers the ability to select a closure according to the wine and its intended ageing period.

For me, that makes closure selection another part of winemaking rather than simply a packaging decision.

So What Now?

Does more oxygen mean mean better wine? Not necessarily.

Oxygen plays an important role in wine development, but too much oxygen can accelerate oxidation.

Less oxygen is not automatically better either.

The appropriate oxygen exposure depends on the wine, its composition, its packaging and the intended drinking window.

A fresh aromatic white intended to be enjoyed young has different requirements from a structured red designed to develop over several years.

This is why there is no single closure that is right for every wine.

The aim is to match the closure and its oxygen transmission characteristics to the wine.

Should you lay a cork sealed bottle down?

This is where the traditional advice to store cork sealed bottles on their side comes from.

Keeping the wine in contact with the cork helps maintain cork hydration, which can help the cork retain its elasticity and sealing properties over long periods.

A dry natural cork can become less elastic over time and potentially compromise the seal.

However, modern technical closures are not necessarily dependent on wine contact in exactly the same way as traditional natural cork.

Engineered closures such as DIAM are designed to provide a consistent seal and do not rely on the same behaviour as a traditional natural cork.

For wine intended to be consumed relatively soon, storing a quality cork sealed bottle upright is not going to suddenly cause it to oxidise.

For long term storage, however, storing natural cork sealed bottles on their side remains sensible, particularly when the wine is intended to age for several years.

Does upright storage automatically mean oxidation?

No.

It is easy to think about wine as either being completely sealed or exposed to air, but the reality is much more complicated.

Closures create a barrier to oxygen, but they are not necessarily an absolute barrier.

With cork closures, oxygen transmission occurs gradually over time. Different technical closures can provide more controlled and consistent oxygen transmission.

There can also be differences in the amount of oxygen already present at bottling.

So if you find a bottle that has been stored upright, it does not automatically mean the wine has oxidised.

The closure, bottling conditions, storage temperature, storage duration and the wine itself all play a role.

The closure is part of the winemaking process

For me, choosing a closure is not simply about deciding whether a bottle should have cork or screw cap.

It is about thinking about what I want the wine to become.

A closure with a particular OTR can influence the pace of bottle development, so the decision starts well before the bottle reaches the shelf.

Natural cork, technical cork and engineered closures such as DIAM all have different characteristics.

The most appropriate closure depends on the wine, the intended drinking window and how the wine is going to be stored.

Because once the bottle is sealed, the wine keeps evolving.

The winemaking is not quite over yet.