It's Not What It's All Corked Up To Be

Recently, two colleagues provided me with inspiration to revitalize an article I wrote some years ago.  One joint consensus is that a bottle of wine is a living, breathing biosphere.  Since all living things need oxygen, a very small amount of oxygen is required for wines to survive and develop over time whether it’s one year or decades.  Too much oxygen will oxidize wine destroying a wines vibrant color, aromatics and taste.  A wines survivability is largely dependent on the presence of sulfur dioxide, which neutralizes the effects of oxygen.

Corks are an important part in the development of wines that are being cellared allowing the wine to reach a point where the flavors are in harmony between the fruits and acids.  This unique complexity is the Holy Grail to wine connoisseurs.  Cork is a very dense material that is made of many tiny cells which contain air.  Richard Grant Peterson, PhD wrote,” Remarkably, each cork cell is tetrakaidecahedral (14 sided).  The math majors among us realize that it takes 14 sided bodies to exactly fill a space with uniform bodies of minimal surface dimensions and without interstices.” His belief is that a cork is a perfect seal thus preventing a cork from breathing.  One piece of evidence used to support this position is Champagne.  Champagne maintains a constant positive pressure, preventing the carbon dioxide gas that makes it bubbly from diffusing, or passing through, the cork and becoming flat.  Even aged Champagnes with many years in the cellar maintain the distinct carbonation that they are known for.  However, the one flaw with this view is related to the size of the molecules involved.  Carbon dioxide has 1 carbon atom attached to 2 oxygen atoms and is much larger than oxygen (O2) which only has 2 oxygen atoms. Though Dr. Peterson states that the 14 sided cells of cork have minimum surface interstices, does the cork have the ability to filter gasses from diffusion and infusion?  Simply stated, does cork filter molecules of a certain size?

Another piece of evidence that supports oxygen is present in wine can be assessed by the amount of sulfur dioxide that is present. Shown by tests and documented in various literature that normal wines after bottling typically have a loss of 5-6ppm of sulfur dioxide.  Remember that the cause for sulfur dioxide loss is through direct contact and reaction with oxygen.  Sulfur dioxide is the chemical that helps preserve wines and protect them from oxidation.  Experiments have been run through careful processing, verifying that oxygen was completely removed from a wine when bottled, yet the results still showed a 5-6ppm sulfur dioxide drop found in numerous wineries. So where did the so called nonexistent oxygen come from to cause the sulfur dioxide to drop?

If cork is such a good seal that gas cannot pass through it, why do old wines that are properly maintained in cellars decrease in volume with age?  One possibility is that water and ethanol, like oxygen, are also small molecules. Another challenge to cork permeability of wine was shown years ago when lead was used as wine foils over the cork. Testing the wines, it was discovered that traces of lead was being found in the wine.  Where did the wine come from to cause the lead cover to corrode? Did the wine leak directly through the cork or around the sides (poor seal)? All of these facts show that cork is not a hermetic seal (100%).

Maybe saying that a cork breathes is a misnomer and should rather be referred to as transfer.  Saying that a cork breathes implies there is a vacuum in the bottle. The transfer of oxygen from the cork or cork/glass interfaces would be a very slow process as oxygen would need to migrate to the wet surfaces of the cork and diffuse into the wine.  However, the dilemma is the sealing properties of the cork and how oxygen gets to the wine.  The sulfur dioxide test showed that oxygen does indeed get into the wine providing evidence that the minor exchange could be the reason for allowing wines to age to splendor through a very slow process.

Evidence of gas transfer/breathing in cork is further reinforced with plastic corks that are raised or protruding from wine bottles.  Obviously, cork is not an absolute barrier where plastic synthetic closures are.  Is the cork working like a selective filter allowing gas molecules of only a certain size to penetrate and not others?  Champagne appears to provide evidence that this is occurring by maintaining carbonation and developing with age which requires oxygen.

Another piece of evidence that oxygen is finding its way into wines using cork was recently discovered where some screw capped wines have been found to contain trace amounts of hydrogen sulfide.  Hydrogen sulfide gives the odor of rotten eggs and has been one of the negative drawbacks noted with screw caps.  Detection of hydrogen sulfide in bottles using cork closures becomes undetectable almost immediately after the bottle is opened and oxygen is introduced.  This is not the case with screw capped wine, where hydrogen sulfide has been found more frequently and can remain after the bottle has been opened.  Hydrogen sulfide plus oxygen reacts producing sulfur dioxide and water, which, has no negative impacts to the wine.  This process does not happen since screw caps prevent oxygen from reaching the wine.  If cork is impermeable, why does cork not show signs of hydrogen sulfide like screw caps?  The amount of hydrogen sulfide noted in screw caps is very small and will probably not be noticeable by the average consumer.  

Looking at the evidence it is hard to dispute that gas exchange is occurring with cork closures and it is also being filtered to some molecular size, otherwise, oxygen exchange would not occur.  It can also be noted that corks are not hermetic, or 100% seals.  The Cork Council and industry have mitigated a lot of the concerns of Trichloroanisole (TCA) which has plagued the wine industry.  Cork is not the only source of TCA in wine, but is the most common source.  The beauty of development of fine wines such as Bordeaux, Burgundy, Barolo, Brunello, etc over decades of time will be lost with the new screw caps.  Maybe this is why we are not seeing a French Chateau Lafitte Rothschild or Chateau Figeac Bordeaux getting on the bandwagon to use screw caps.  Even Domaine Laroche went back to cork after seeing the impacts on development and loss of complexity in his wines.  Generally, you see very few old world wine producers using screw caps.  This is a very controversial topic with many variables.  The economics of small wineries needing 100% effectivity on quality and to remove the risk of TCA or other cork related impacts is very crucial.  For such situations where the wines are not meant to be aged for 10 years plus, this is a perfect answer.  Development has continued with screw caps and let’s face it, we have the technology to do anything.  But for wine collectors maybe it's a “Tradition” you just don’t screw with.


Rusty Sly