Corks and/or cork treatments clearly play a role in the premature oxidation being experienced. As most tasters who have experienced the premature oxidation problem recognize, bottles from a single case of ten year old perfectly cellared white burgundy may vary widely – with some perfectly fine, some partly oxidized and perhaps some fully oxidized. The oxygen permeability of corks varies and as has been demonstrated in the scientific literature to represent somewhat of a bell curve.
The problem of cork seal failure has been with us as long as corks have been used to seal wine bottles. However, the oxidation rate experienced on seven to ten-year old premier cru and grand cru white burgundy from decent vintages has gone from something on the order of 5% on decent vintages in the pre-1995 period to a range of 9% to 23% (and in 1995 as high as 50%) for the 1995 through 2002 white burgundy vintages. The question is why is this happening?
There are three widely-discussed problems involving cork to which various burgundy producers ascribe some or all of the blame for premature oxidation: (1) a general decline in the quality of the cork used for wine corks; (2) substitution of hydrogen peroxide for chlorine (hyperchlorite) as a bleaching agent for corks starting circa 1995 and (3) the substitution of silicone for paraffin as coating on some wine corks.
A General Decline In The Quality Of Wine Corks
Etienne de Montille is one of several burgundy winemakers who attribute at least some of the jump in premature oxidation since the 1994 vintage to a continuing decline in the overall quality of corks. This is said to be both a function of global warming, as a result of which cork oaks supposedly produce bark which is less thick and perhaps more porous, and a huge increase in demand for wine corks from burgeoning wine production in California, Australia, South America and Italy, which has also allegedly caused some cork oaks to be stripped at a younger age when the cork is more porous. The preferred standard is that a cork oak be stripped of its bark once every 15 years, but apparently it is becoming commonplace to strip the trees every 12 years, and in some cases, even more frequently.
At least one producer, Benoit Ente, believes that the increasing use of irrigation on cork oak trees has caused a problem. Ente purchases cork from a producer who does not irrigate and farms organically. According to a friend who has visited and compared corks, the dry-farmed corks which Benoit Ente uses had five or six growth rings, and sounded like wood when he tapped them on a kitchen counter, while the corks from irrigated producers had two or perhaps three rings, and were quite soft. Similarly, both Bouchard and Fourrier purchase their corks from dry farmed producers in Corsica who strip their trees only once every 15 years as was the traditional custom.
Olivier Lamy (of Domaine Hubert Lamy) recently reported that he has observed significant differences in the percentage of oxidized bottles between batches of similarly treated corks provided by different cork producers.
Philippe Prost of Bouchard states that he has noted considerable variation in the weight of corks contained in the same bag of corks allegedly all of equal high grade. As he notes, those corks with noticeably lower weight are less dense than they ought to be and therefore have a far greater propensity to allow the migration of air into the bottle. Bouchard has therefore recently undertaken to weigh all of the corks on its premium wines and to reject any corks found to have substandard weights. The presumption is that by rejecting those corks with the lowest weights (as a surrogate for lower cork density), Bouchard can eliminate those corks most likely to permit excess oxygen into the bottle. Bravo Philippe!! While this step might at first seem difficult to achieve, it is actually fairly simple to accomplish from an engineering perspective. One pauses to wonder why it is necessary for a winery to do this and why the cork producers are not doing this themselves.
Change of Bleaching Agents from Hyperchlorite to Hydrogen Peroxide
Many burgundy producers, including Etienne de Montille and Gerard Boudot (Sauzet), believe that one of the principal culprits in the premature oxidation problem was an unannounced change made by the cork producers circa 1995. Because of widespread complaints about corked bottles, and the discovery of the role of hyperchlorite (chlorine) used in bleaching wine corks in inducing the production of 2,4,6 trichloroanisole, or “TCA” as it is commonly called, (which is the chemical compound responsible for about 80% of what we call “corked” wines or wines having “cork taint,”1/) the cork industry began substituting hydrogen peroxide for hyperchlorite as a cork bleaching agent beginning about 1995.
In making this change, the cork industry appears to have reduced the incidence of cork taint. However, it may have triggered an even greater percentage of wine failures due to premature oxidation. Hydrogen peroxide, or H202, is an oxidant. It is a fairly unstable chemical which rapidly breaks down into water and oxygen or which interacts with other chemicals so that one of the atoms of oxygen is stripped off and combines with (“oxidizes”) another compound. Peroxide rapidly oxidizes the sulphites which are added to wine to prevent the formation of acetaldehyde, which we detect as “oxidation.” Peroxide will rapidly convert sulphites (SO3) to sulphates (SO4), and once that conversion takes place, the remaining sulphate (SO4) will not “protect” the wine from oxidation.
The argument is that peroxide from the bleaching process becomes trapped in the pores of the cork and, over a period of time, migrates through capillary action into the wine with which the cork is in contact. While some have argued that the peroxide trapped in corks from the bleaching process would quickly break down into water and oxygen long before the corks could be used in wine, others have argued that peroxide trapped within the pores of cork from the bleaching process would be quite stable, perhaps for a period of years. Since cork manufacturers are allowed to leave 0.2 mg of peroxide in the corks by law, that definitely suggests that some peroxide is inevitably trapped within the pores of the corks subject to the bleaching process.
An alternative hypothesis for peroxide-induced oxidation is that peroxide trapped in the corks breaks down into water (H20) and oxygen, and that the expanding gas produces increased pressure on the capillaries and other cell walls of the cork which degrades the cork structure resulting in increased gas permeability. While I personally find this hypothesis less compelling as an explanation, a couple of wine chemists have argued that it is theoretically possible and could be verified or refuted in the laboratory. There is some suggestion that this issue is being examined by the BIVB, a burgundy trade organization based in Beaune.
On the surface, the peroxide problem sounds like a smoking gun cause of premature oxidation, but at this point there is no scientific evidence available to demonstrate that either the residual amounts of peroxide trapped within bleached corks or the hypothetically increased gas permeability that might result from peroxide use could be sufficient to cause the huge percentage increase in oxidation that has been experienced starting with the 1995 vintage. Some wine producers are true believers that peroxide is the primary source of the problem, while some wine chemists are highly dubious that peroxide alone could be the cause of the problem.
Assuming that trapped peroxide is the problem, what is the solution? For many it is to use unbleached corks. While it is claimed that bleaching of corks is antiseptic and kills unwanted bacteria and fungi which may be in the cork, some producers, among them the Domaine Romanee Conti, have for many years used unbleached corks and claim that that they have had no problems with such corks. According to John Gilman (quoting Jean-Marie Fourrier), as of the 2005 vintage, only 20% of the top corks are unbleached while the remaining 80% are still bleached using peroxide (or, according to Kevin Harvey of Rhys Vineyards, in a few cases another oxidant, ozone). Since the 2005 vintage, a number of producers have changed to unbleached corks.
Substitution of Silicone for Paraffin as Cork Coating
Another potential culprit sometimes pointed to as a cause of premature oxidation is the claimed substitution of silicone coatings (more accurately called polymerized siloxanes or polysiloxanes 2/) for paraffin coatings on some wine corks beginning in 1995. A majority of the corks produced today have a combination paraffin/silicone coating. A layer of paraffin is sprayed-on first, followed by a layer of sprayed-on silcone.
The asserted reasons for the change seem somewhat dubious. It is claimed that the change was made in response to complaints from restaurants about the difficulty in removing corks from bottles, and that paraffin/silicone coatings rather than paraffin only made it easier to remove the corks from the bottles. It is also claimed by some that using praffin/silicone coatings made the corks less resilient or pliable and that, as a result, the cork’s ability to seal the bottle was reduced (which of course seems highly logical if the cork is easier to extract from the bottle when coated with silicone.)
Allegedly, the use of paraffin/silicone coatings began with less expensive bottlings. According to one wine collector I know who has investigated the problem, there is a huge difference in the feel of a cork which has been coated with paraffin/silicone and one coated with paraffin only and it would be clear and unmistakable from even casual contact with the corks that a change had been made. Assuming this is true, it seems highly unlikely that one day the burgundy producers found out that paraffin/silicone had been quietly switched for paraffin on their corks. It is also far less than clear how widespread the use of paraffin/silicone-coated corks is among better burgundy producers.
The precise mechanism by which paraffin/silicone coatings may be causing premature oxidation is seldom explained. However, it it is clear that silicone compounds have “High gas permeability: at room temperature (25 °C) the permeability of silicone rubber for gases like oxygen is approximately 400 times that of butyl rubber, making silicone useful for medical applications (though precluding it from applications where gas-tight seals are necessary).” (Emphasis added). 3/ One of the most noteworthy uses of silicone (as silicone hydroxide gel) is soft contact lenses. The material is preferred because it is highly permeable to oxygen and allows oxygen to pass through the lens membrane to the cornea.
Paraffin, on the other hand is a group of hydrocarbons, which depending on their chemical formulas can be a gas, a liquid or a solid. The simplest paraffin is methane gas. Again according to Wikipedia, the solid forms of paraffin, called paraffin wax, are from the heaviest molecules from C20 to C40. Paraffin is also widely described to be gas permeable, but I could never quite find out how gas permeable the solid (wax) forms are.
Also I have spent a good bit of time trying to find a comparison of the gas permeability of paraffin vs. silicone vs. cork, but have had no luck. I did, however, see one reference indicating that silicone oil was much much more gas permeable than paraffin oil, though the article did not quantify the differences. Assuming that the same applies to the wax forms of silicone and paraffin, paraffin/silicone coatings could be much more gas permeable than paraffin alone – but how much difference would it make given that cork is tightly embedded in the neck of the bottle? Corks are ordinarily 24 mm wide. They are compressed into a bottle opening measuring 18mm. Apparently the cork is compressed to 16mm when it is slipped into the bottle by the corking machine and “expands back” after being inserted into the bottle.
The literature – both promotional (from people selling corks) and quasi-scientific – describes the purpose of using the paraffin or silicone coatings as permitting the corks to be inserted in the bottles more easily (a factor universally mentioned) and facilitating a “better seal” (a factor mentioned in about 75% of the cases.) It is commonly acknowledged that silicone coated corks are easier to extract than paraffin coated corks.
So maybe silicone coatings are more gas permeable than paraffin coatings, but how much more permeable are they? And what is the ultimate significance to the oxidation debate? At this point it is still unclear. Nevertheless, several burgundy producers, including Hubert Lamy, Montille and Sauzet, believe that that the silicone coatings play a role in the premature oxidation problem and have thus eliminated the use of such corks.
Here, the “fix” for the silicone coatings problem, if it is indeed a problem, is for producers to refuse to buy silicone-coated corks and to insist on a return to paraffin-only coatings (as Bouchard, Drouhin, Fevre, Lafon and Sauzet have recently done) or to use no coatings at all – the corks are simply boiled, bagged and later inserted (as Fourrier and some others are now doing). However, some producers maintain complete silence about this issue.
[Text and opinions by Editor Don Cornwell]
1/ See “Cork Taint in Wine” by Richard Gawel http://www.aromadictionary.com/articles/corktaint_article.html “Complex chemical mechanisms underlie the production of TCA. The one of most importance is the conversion of chlorophenols to chloroanisole by common microscopic fungi such as Aspergillus sp. and Pennicilium sp., in the presence of moisture. … Cork bleaching with hyperchlorite (less frequently used now, peroxide bleaching is now favoured), also provide a ready source of chlorophenols for use by these micro-organisms.”
2/ According to Wikipedia (http://en.wikipedia.org/wiki/Silicone): “Silicones … are mixed inorganic-organic polymers with the chemical formula [R2SiO]n, where R = organic groups such as methyl, ethyl, and phenyl. These materials consist of an inorganic silicon-oxygen backbone (…-Si-O-Si-O-Si-O-…) with organic side groups attached to the silicon atoms, which are four-coordinate.”
3/ Quoting from http://en.wikipedia.org/wiki/Silicone: