Cork Closures and Oxygenation

Natural cork closures have been around for hundreds of years. At first they were merely a more convenient way to seal a bottle of wine. Over time, however, we have come to understand that corks offer an incredible benefit over synthetic closures: micro-oxygenation.

That’s right, natural corks allow your wine to get oxidized. While this may seem scary at first it is actually very good for wine in very small doses spread out over very long periods of time.

The Key Feature of a Cork Closure

Natural corks have pores that wine is able to seep into. This is what causes a cork to swell up and provide a tighter seal against the glass neck of the bottle.

Oxygen too is able to penetrate these pores. Thus there is an interface for oxygen and wine to interact.

Take a look at the picture on the right. You can see that the Shiraz I made almost a year ago has seeped about a third of way through to the outside world. After that the cork looks pretty much like it did the day I inserted it.

It is where the red stops that my wine and oxygen from outside of the bottle are allowed to interact. The rest of the cork pores are full of oxygen. The wine has not seeped uniformly through the cork but it is making its way toward the end.

It took nearly a year for the wine to get that far through the cork. In another year it may very well be approaching the end of the cork.

As wine makers we try very hard to limit the exposure of our wine to oxygen. After all an oxidized wine turns colors and can pick up off flavors. However, in order for a wine to age properly it does need a very small amount of oxygen. Because the wine and oxygen are only permitted to interact within the very small pores the surface area of the wine is actually very small. This is why red wines are able to be stored for years or even decades and still improve.

Storing Wine On Its Side

In this second picture you can see how wine affects the swelling of the cork. I’ve just removed this cork from the bottle and the red portion of the cork where the wine has seeped in is a bit larger in diameter than the dry end where the cork screw entered the cork.

By storing my wine on its side this cork was kept swollen for a tight seal. It also allowed for some interaction between the wine and oxygen. This is why I prefer natural corks and why wines that are aged any length of time in the bottle will have natural cork closures.

There are some synthetic closures that allow for micro-oxygenation but in my opinion there’s no replacement for a natural cork.

If you do not store a wine on its side the cork may dry out over time causing it to shrink. As it shrinks the amount of oxygen that is able to pass into your wine increases. Sometimes the cork may shrink enough that the bottle is practically open to the outside world.