evelopment 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 trees 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.

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 chemical 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 O₂ enter the bottle with a cork closure and not the screw cap? Some made the obvious guess that the cork must be “breathing,” especially since that made a glamorous story, mysterious and almost apocalyptic. Just the thing to elevate the mystical attraction of wine and (ahem) sell lots more cases of it.

Unpublished experiments in 1958 showed that we were unable to force O₂ through a 1¾ inch cork, even at high pressure levels. We were left with a question: since corks weren’t porous to O₂, how could small amounts of O₂ show up in the wine immediately after bottling? The answer was suggested by early literature, though none of us recognized it at the time.

Remember that corks are made up of miniscule cells each of which contains a tiny bit of air. At bottling, corks are compressed by the corker jaws for a fraction of a second to make the corks small enough for a plunger to punch them into the bottle. This squeezing raises the air pressure inside those tiny cork cells to 3 – 4 atmospheres. That’s enough to force microscopic bits of air out of the cork cells closest to the ends of the cork. It “exhales” slowly from the cork ends over a few weeks of time – faster at first, but slowing steadily to zero after some weeks as the pressure equalizes.

Professor Jean Ribereau-Gayon reported back in 1933 that 0.1 to 0.38 mls of O₂ diffused out of the inner end of a cork into the wine bottle in the first three weeks after bottling, but less than 0.07 mls over the next four months. How could that dissipating rate be explained? It would make no sense to believe that corks “breathe” air at a high rate for 21 days, and then slowly taper off and stop. That sounds to me more like a dying gasp than breathing. Clearly, the initial air has come out of the compressed cork cells near the inside end of the cork and not “through” the cork. As this compressed air escapes (from both ends of the cork) the pressure inside the cork cells diminishes, returning to near normal in two, three or four weeks. It probably tails off to extremely low levels for a few months more before stopping entirely, but that wasn’t measured.

Ribereau-Gayon’s suggestion that oxygen diffuses out of corks into wine due to the high pressure in cork cells immediately after bottling was correct, but no one seemed to take notice immediately. He was a superb scientist and we should have kept that information in the front of our minds; somehow, nobody did. I visited him in Bordeaux three decades later and, unfortunately, I think we discussed everything except compressed corks.

Why Don’t You Chill Out – Corks Do

The subject of cork closures regained importance in recent years, and Caloghiris (1997) suggested that air contained within the cork cells might account for cork’s oxidative capacities. That is true, and it is easy to calculate. He reported that only a negligible volume of atmospheric O₂ enters the bottle along the boundary between the edge of the cork and the glass. That is certainly true in the first few years after bottling. But over longtime storage, especially with rising and falling temperatures in a wine cellar, corks “relax” in the bottle and it appears that most of the air leakage into a bottle after many years storage is along the edge of the cork, between the cork and the glass.

Waters, et al (2001) reported that the main route for O₂ entry into wine bottles (ignoring the first short time period after bottling) is along this cork-glass interface. I have seen that very thing many times as older bottles show increasingly more wine stain all along the cork to the point that liquid wine droplets finally appear outside the (unopened) cork, causing corrosion under the capsule. When those corks are pulled, the cork is stained along the complete outside of the cork showing that wine has indeed seeped between the cork and the glass wall. When cut with a razor blade, these same corks were clean and unstained inside at the cut cork surface, so we know the wine did not seep through the cork but only along the outside edge next to the glass.

We also notice that corks that were “unmovable” a year or two after bottling become easier to push a few millimeters into the bottle (by thumb pressure) after the cork has gotten considerable stain along its sides. The explanation, widely recognized, is that a daily, monthly or yearly rise and fall of temperature in the cellar of even a few degrees can cause a slight movement of the cork both in and out due to pressure changes as the wine bottle storage temperature fluctuates.

Cork movement is imperceptible at first but it becomes more obvious with time. The wine in a warmer bottle expands, causing an increase in pressure inside the bottle and miniscule movement of the cork outwards. As the bottle cools the liquid contracts, and this reduces pressure in the bottle, sucking the cork back inwards ever so slightly. Each time the cork is moved a bit by these pressure changes, a tiny amount of wine moves out along the glass/cork interface. After a few years of this, the staining along the edge of the cork is extensive and the slight wine seepage between cork and glass acts as a lubricant to allow even easier cork movement. The home remedy for this is to make sure your wine cellar holds an absolutely constant temperature of storage. That isn’t easy to do in most houses and, probably, none of us has done that yet.

Reasons to Re-cork Wine

The major problem wine collectors live with is that corks lose their ability to continue pushing against the glass over time. I’ve heard wine people in France say that the cork seems to “die” after several years in a bottle. They mean the cork’s ability to continue pushing outwards against the glass relaxes over time until the seal isn’t good enough to protect the wine any longer. To preserve their finest wines as long as possible, first-growth wineries in France and elsewhere “re-cork” fine wines about every twenty to twenty-five years. This removes the old cork and immediately replaces it with a new cork. Andre Tchelistcheff brought this practice to California and I remember doing it at Beaulieu Vineyard in 1968 to preserve his best wines of the 1940s and 50s. I did it again for my older Beaulieu wines in 1990, as they were truly irreplaceable wines.

Beaulieu was one of only three or four American wineries of world class in the late 1960s and early 70s and we were routinely visited by winemakers and owners of first-growth wineries of Bordeaux in those years. Today, of course, there are many wineries of that quality level in California but that wasn’t true back then. I believe all of today’s quality wineries should adhere to that type of “re-corking” regime. Also, in view of what we now know about cork exhalation at bottling time, I think they might be wise to add 15 or 20 ppm of SO₂ to each bottle at the re-corking session.

I still believe that sound corks are the preferred closure for fine wines. The TCA problem is diminishing as cork suppliers continue to improve their production methods. My hat is off to them for doing so. Modern screw caps may be OK, provided the liner seals well and continues to keep the air completely out for a long enough time. Screw caps didn’t do that thirty years ago and screw capped wines were never bottle-aged back then.

---

I hope this answers most of the questions engendered by the first article. Corks are one of the more interesting and unique products of nature. What else doesn’t breathe but can exhale without inhaling? Come to think of it, I guess any of us would exhale a little air if squeezed hard enough. Space limits me from adding a full Bibliography but I’d be happy to send that to readers who want to study it. Contact me at APPELLATION AMERICA or www.richardgrantwine.com