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CORKS DO NOT BREATHE -- PART II

The Adventures of Mr. Cork, Part 1: No Air Allowed: "Corks never inhale, and they exhale only once (right after bottling)."

America (Country Appellation)

Corks Do NOT Breathe - Part 2

by Richard Grant Peterson, PhD
October 13, 2008

Editor's Note: We received many great questions in response to guest columnist Richard Peterson's first plea to wine writers and cork promoters, asking them to cease telling people that wine corks “breathe” (Please Stop Telling People That Corks ‘Breathe’ 02/10/08). In this follow-up piece, Dr. Peterson hopes to answer those questions and give you the full scoop on how corks interact with wine in the bottle.

Cork Class is now in session. Read on…


DropCap Development of beautiful “Bottle Bouquet” is a primary goal of long term wine cellaring. It’s well established that bottle bouquet requires the absence of oxygen to develop. Wine would die early and fail to age properly in bottle (being unable to develop bottle bouquet) if corks were to breathe air, so it’s a good thing they don’t. Although sound wine corks don’t breathe, I admit they have a confusing way of showing it. Just imagine: corks never inhale, and they exhale only once (right after bottling). The exhale is slow, lasts a few weeks and is only a partial one. To understand this, look at the cork structure.

Corks are cut from the thick, non-living bark of cork oak trees, Quercus Suber, which grow naturally on land around the Mediterranean Sea. The bark of cork differs from that of other cork-stack-225.jpgtrees in that most tree bark contains fibers running lengthwise like the wires in a cable. Cork bark does not. Rather, cork bark is made up completely of myriads of tiny cells, each imprisoning within its walls a microscopic bit of air. Those bits of air are natural, having been imprisoned there as the bark cells grew on the tree. There are about 200 million minute cells in a one inch cube of natural cork, the cells averaging 1/1000 inch in diameter. Each cell is separated from its neighbors by a thin, thread-like but extremely strong membrane of resinous materials which binds the cork cells together. Just over 50 percent of the volume of a piece of cork is this captive air within the tiny cells.

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. The cells snug together perfectly to fill the whole space without leaving any voids at all. A piece of cork is completely cellular with no “empty” spaces between the cells. This seems to me a major reason why corks don’t transmit air: the path through a cork is just too tortuous for significant numbers of gas molecules to work their way through, even if pressure is applied to one end of the cork.

Champagne Proof Positive

Champagne people have known since Dom Perignon that carbon dioxide gas doesn’t escape from a bottle of bubbly during many years’ storage even though the pressure inside a bottle of Champagne or Sparkling Wine is as much as 5 atmospheres (75 psi.) If CO2 gas can’t get through a cork and out of a bottle with 75 psi of pressure pushing on it, how could O2 get into a bottle with only a few ounces of outside pressure pushing it? CO2 molecules are physically larger than O2 molecules - but only a little larger, and that factor doesn’t explain it. If cork contained longitudinal gaps, voids or fibers, the escape route for gases would be easier. But that only happens with damaged corks (read: cheap) which might contain cracks. Since cracks in corks are usually obvious, proper inspection and grading eliminates defective corks prior to their shipment to a winery.

Several Other Properties Of Cork Need To Be Kept In Mind

With a specific gravity of only .25, cork is one of the lightest of all solid substances.

***

The Cork Institute reported (1941) that a 1” cube of cork had been compressed at 14,000 psi without breaking. After release of the pressure, the cube returned to 90 percent of its original 1” height and showed no appreciable change in its other dimensions. Some of the cellular “cork air” escaped during compression but most remained trapped and compressed within the cork’s cells, even at this extreme pressure. This helps us understand the myth of corks breathing.

***

Cork does not harden or deteriorate under ordinary pressures. This partially explains its facility for use as stoppers and flooring materials.

***

Cork is not absolutely impervious to moisture penetration, but its cellular structure gives it high resistance to penetration by water and wine. I have noticed many wine corks that were “wine stained” less than one inch of their length after residing in a bottle of wine for twenty years. Wine corks are mostly 1 ¾ inches long, but sometimes as long as 2 ¼ inches, which provides a sufficient length of sound cork to preserve a solid seal over time.

***

Cork’s coefficient of friction is much greater than that of rubber or leather, because the exposed surface cells act like tiny suction cups. Because of this, it is necessary to lubricate the outsides of new wine corks with paraffin and/or silicone prior to use. We’d never get them out of a bottle otherwise.

***

Next to vacuum, a “dead” air space minutely divided is one of the most efficient nonconductors of heat. Cork cells, since they contain air, also tend to absorb impact or vibration which helps keep the corks well seated in wine bottles during shipping.

Let’s suppose a newly bottled wine contains a trace of air or oxygen (O2). Fortunately, wine contains a few parts per million (ppm) of SO2, which protects wine from oxidation by the O2. The SO2 and O2 destroy each other as the two react whenever they make contact. The basic-cork-125.jpgchemical reaction between them in wine is complex but this simplification doesn’t change the gist of the story. Winemakers have long noticed that the free SO2 content of newly bottled table wine diminishes by 5 or 6 ppm within the first month after bottling, then by only 1 ppm or less in the months after that. The accepted explanation is that this initial loss of SO2 is due to the pickup of O2 by the wine as it passes through pumps and hoses while moving from the bottling tank through the bottling operation. The SO2 is lost from the wine by reacting with, and removing, any O2 that gets in.

What if the wine was oxygen-free at the time of bottling? There should be no loss of SO2 since there was no O2 to remove. At Beaulieu Vineyard, I ran into a stone wall that was hard to believe at first: wine lost this SO2 following bottling even when we carefully excluded air from pumps, hoses and equipment. That is, virtually no air had entered the wine prior to bottling, yet we still had that same loss of SO2 in the first month after bottling. Somehow oxygen was entering the wine in the first weeks after bottling to the extent that 5-6 ppm of SO2 was used up in getting rid of that O2. Where the !@&# did that O2 come from?

Another mystery: many wines containing traces of H2S (“rotten egg” smell) tend to lose the H2S immediately after bottling with corks but not with screw caps. Obviously, O2 enters the bottle and oxidizes the H2S, getting rid of it. But where did the O2 come from? How could O

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Reader Feedback

Reader Comments... [19]

[1]
Robert Rex , winemaker
Deerfield Ranch Winery, Kenwood, CA
So, now we learn that corks do breath but just at first and not after a long while. I can go along with this explanation. If you examine the most recent permeability studies you will find, according to the graphs, that all closures allow some oxygen uptake. Whether it is from the time it takes for the closure to completely seal or that fact that there is more head space under the screw cap than the cork, or other mechanical difference, all closures release some air/O2 into the wine.

I don't find the reduction in SO2 such a mystery or hard to understand. The chemistry is relatively simple. SO2 is an unstable molecule. The stable form is SO3. SO2 is an oxygen scavenger. It takes up oxygen, changing from SO2 to SO3, thereby keeping the wine from oxidizing and also reducing measurable SO2. If we are careful and know how much dissolved oxygen our bottled wine has in it (easy enough to measure) we can add just enough SO2 to use up most of the O2 and not only leave the wine protected but not poison our customers with too much SO2. If the wine is clean and stable it will last a long time with very low or no SO2. SO2 is not a preservative at any rate.


[2]
Paul Koehler , Assistant Winemaker
Green Bay, WI
Please tell me that I'm not the only one who understands the difference between Micro-Ox and Oxidization! More over the difference between aeration and the ever so slow influx of dissolved oxygenation into wine through the cork.

A cork is porous, absorbs moisture (ex. bottom of the cork in contact with wine swells) absorbs gasses, and can fail if allowed to dry out and shrink (your described instance of aeration and oxidization is an example of cork failure not success)!

For most wines – i.e. those lacking tannins and other antioxidant polyphenols – Micro-ox does nothing but deteriorate the antioxidant qualities of the limited S02 added, eventually leading to oxidized wine. However, for tannin and polyphenol rich wine (any age worthy wine in general) slow diffusion of oxygen in parts million over time helps to polymerize tannins. This happens at a faster rate when available SO2 levels decline.

On a recent visit to a winery in Michigan I spoke with the owner/vintner of some 30+ years. He was using a micro-ox system on some of his wines. He describes the process just as I have described. Duplicating what happens in a barrel which is exponentially faster than what we later see in a cork sealed bottle due to the difference in the surface to volume ratios of the wood sealing both vessels; the whole barrel is wood (greater O2 diffusion) and just the cork wood in the bottle (lesser O2 diffusion).

It is obvious that over time the development of ullage or headspace in both barrels and bottles proves that there is movement of at the very least liquid across the membranes of the so called "hermetically" sealed vessels. If liquid is moving through these membranes certainly gas is moving as well!

You need to accept that there is movement across the wood/cork barriers and then decide what is happening to the wine because of it. Certainly that last 1976 Chateau Montrose I had from my father's cellar last spring was not oxidized! I felt it still had significant cellar life to it! However the ullage was obvious vs. the 1996 Chateau Montrose we had side by side with the 1976! What replaced the liquid in the 1976 Montrose? Was it a vacuum? If so what effect would negative pressure have on your studies regarding the transmission (breathing) of O2 across the cork barrier? Did your study of decades ago take this into consideration!!!


[3]
Phillip Hart , owner/winemaker
AmByth Estate, Templeton, CA
A tremendously interesting article (I read the previous one also), but there seems to be something missing. I can't put my finger on it but I feel there's a lot more going on with this wondrous beverage than meets mine or the Dr.'s eye. One thing is for sure "Absolutes" have a tendency to become a little "Porous" with time.


[4]
Tanya
Sprucewood Shores Estate Winery, Harrow, ON, Canada
Why would we age our wine in barrels if not for the micro oxygenation that occurs with oak versus stainless steel? If you say that the best way to age a wine is in an anaerobic state, then why expose any oxygen at all to it with barrels/micro oxygenation?


[5]
Steve Felten , GM/Winemaker
Norman Vineyards, Paso Robles, CA
Peterson's latest diatribe is no more convincing than before, and the repeated use of the terms "absolute" (truth or untruth) and "idiot" still does nothing to strengthen his argument.


[6]
Keith Pritchard , owner
Slate Run Vineyard, Canal Winchester, OH
Sure, a champagne cork holds the pressure. But why does a wine that is freshly corked (without vacuum) in the bottle have the pressure equalize over a few days? Many plastic corks won't equalize and will push out. They supposedly don't have as airtight a seal between the cork and bottle. An oxygen molecule is smaller than a CO2 molecule and much less molecular weight. I also don't buy the interstices between the cork cells being so tight as to be impassable over time. We're talking a natural material here, not stainless steel. I still don't buy it, and much of Dr. Peterson’s quoted research is quite old and done with less advanced equipment that couldn't measure accurately. I agree much may have to do with the seal between the cork and the bottle, but I would think there is more to it than that.


[7]
Keith Pritchard , owner
Slate Run Vineyard, Canal Winchester, OH
On further thought I would like to suggest maybe a cork is like a depth filter (with a nominal and absolute rating). It’s tight enough to stop most oxygen molecules, but not all of them; but is tight enough to stop all CO2 molecules, which are just slightly larger. Just as a filter may be tight enough to block all yeast, but not tight enough to stop all bacteria, but will filter out most of them.


[8]
Suzanne Groth , PR
Groth Vineyards & Winery, Oakville, CA
Ok Dick,
So what is your recommendation for closure? Are you arguing for Noma or Stelvin? Or do you still think that cork is a viable closure?


[9]
Jeff Del Nin , Winemaker
BC
The AWRI did a study where they sealed wine in an ampoule. Such an ampoule is an absolute guarantee of zero oxygen transfer to the wine. I believe they also de-gassed the wine prior to filling so that it contained negligible oxygen. Many months later, they opened it and found SO2 had dropped significantly. The SO2 was only slightly higher than what was present with a tin-lined screw cap. This showed conclusively that there is some process that consumes SO2 in the absence of oxygen.


[10]
Dr. Richard Grant Peterson
Richard Grant Wines, Napa, CA
RE: [comment #2]…What replaced the liquid in the 1976 Montrose? Was it a vaccum?

Absolutely not. The liquid leaked out along the interface between the outside edge of the cork and the inside of the glass bottle. It happened after the cork got older because of the minuscule movement of the cork in and out just enough to allow wine to seep out and air to dissolve in the liquid and be sucked back in. A number of years ago we were worried about the use of lead capsules on wine bottles to dress up the bottle by covering the cork. We wondered whether it could be possible to contaminate the wine with a lead capsule outside the bottle. It was checked and, sure enough, a trace of lead actually got into wine (though the cork was unopened) over long storage.

There was corrosion under the lead capsule as a bit of lead dissolved in the tiny bit of wine that seeped out between the cork and the glass. When the cellar temperature dropped a degree or two the reduced pressure inside the bottle sucked the wine back in bringing traces of lead with it. That's the way your bottle got its ullage and it was certainly oxidized somewhat and had lost some bottle bouquet because of that. Bottles with significant ullage are never as well preserved as bottles of the same wine without ullage. Unfortunately all corks aren't identical. When I see a bottle in the cellar getting more than normal ullage, I use it up shortly after that because it is often still OK, though it wouldn't have lasted much longer. That should be normal procedure for any wine cellar, I believe.

I hope this helps.
~ Richard Grant Peterson

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