Roger C. Bohmrich, Master of Wine
June 2019

Many wine drinkers probably do not pay close attention to the closure on a bottle of wine. Undoubtedly, even fewer – a tiny fraction – realize that how a bottle is sealed can actually influence the taste. In some cases, the visible portion of a closure, in particular a twist-off cap, may trigger a negative reaction. These perceptions are not universal: they vary by individual and country. U.S. consumers, by and large, exhibit a bias in favor of a cork, whether entirely natural or a facsimile. Of course, such stoppers require a tool to extract, yet wine buffs seem to take this in stride. The method of sealing a wine bottle can be a persuasive and sometimes decisive purchasing criterion for more serious aficionados. In this, wine is an exception, as consumers insist on ease of opening for other beverages and comestibles. The belief that a wine is not “fine” unless sealed with cork is a long-standing convention. As The Economist wrote in an article about corks, “a tradition is only an innovation that worked” (The Economist, 2003).

There are four main closure types: natural cork, technical cork, synthetic stopper and screw cap. Stoppers from cork in all formats account for approximately two-thirds of the global market (Schmitt, 2017). There are glass stoppers as well, though they represent a very small share. Winemakers and marketers must answer several critical questions before deciding which option is best suited to their brand and to individual wines within their range. They involve both technical considerations as well as likely consumer perceptions. In some cases, the closure may vary substantially from one wine to the next in the same family of wines (Fig. 1).

Figure 1. Three Provence rosés of Château Sainte Roseline, each in a different bottle sealed with a different closure type. Photo courtesy of Karyne Duplessis Piché.

As Patrick Schmitt MW wrote in a comprehensive article about closures, “it is the choice of stopper that gives producers one of the greatest headaches” (Schmitt, 2017). In recent years, the focus has shifted from cork-related contamination to the OTR or Oxygen Transmission Rate. It might seem counterintuitive, but every stopper allows oxygen ingress, and a winemaker must come to a judgement as to the benefits or drawbacks of any given transmission level. Another acronym, OIR or Oxygen Initial Release, has now been added to the lexicon. This refers to the “big shot of oxygen” contained within any porous stopper which is released when it is compressed and inserted into the bottle neck (Gordon, 2019). This is yet another detail for winemakers to work into their closure calculations.

Natural cork might be called the heritage stopper. It is an integral, single piece punched in a cylindrical form. The Cork Quality Council, a coalition of cork producers, says that: “Ancient Egyptians, Greeks and Romans referenced cork as a preferred material for stoppers used with wine and olive oil” ( The historical record does evidence that cork was introduced as a means of sealing amphorae in the 1 st century BCE. The authors of Divine Vintage claim that “the oldest use of a cork stopper in a wine decanter jug” dates from the 5 th century BCE (Butler & Heskett, 2012).

Cork boasts properties which make it ideal for sealing containers holding liquids, such as the ability to be compressed and remain impermeable. While it does eventually deteriorate, high-quality cork has a remarkably long useful life – several decades if not more. The wood barrel gradually replaced the amphora as the principal vessel in which to store and ship wine in the early centuries of our era. The cork did not play an important role again until the 17 th century CE, when glass bottles came into wide use. According to the oft-told story, Dom Pérignon opted for corks to replace the wood stoppers of the day. The cork has enjoyed a symbiotic connection with wine ever since. In fact, maturing wine in bottle would not have been possible without the complementary characteristics of cork.

It is an interesting fact that both cork and nearly all wood employed for wine barrels come from oak trees: the former from the exterior or bark and the latter, the trunk itself. The preferred oak species for the finest barrels include Quercus sessiliflora in Europe and the American Quercus alba whereas Quercus suber is harvested for its cork bark (Fig. 2 & 3).

Figure 2. Cork oak forest.

Figure 3. Corks punched from bark.

Photos © Amorim.

Cork may have remarkable properties, but it also has an Achilles Heel, as Jamie Goode explains in The Science of Wine (Goode, 2005): “As a natural substance it is variable, and is prone to failure.” It is an ideal host for contaminants, of which TCA (2, 4, 6-trichloroanisole) is the best known for its moldy, musty odor. It can be detected by humans at extremely low levels, 5 nanograms per liter. Other chloroanisoles may also cause unpleasant contamination. Cork taint gained attention in the 1980s and became a serious concern in the 1990s. Notable figures such as New York Times columnist Frank J. Prial sounded the alarm in an article entitled “Popping Corks: A Sound for Oblivion?” (Prial, 2003). It is also now known that natural cork suffers from “extreme variability in both OIR and OTR” (Gordon, 2019). This amounts to a wild card in assessing the impact of oxygen on wines under cork, possibly the primary reason collectors find variation from one bottle to the next when they open old vintages.

The universe of closures has changed dramatically in the past decade. Cork sterilizing and quality control procedures have made tremendous advances. The reported incidence of “corked” bottles has been reduced from an estimated peak of 5% – it is impossible to know for certain – to a lower or even negligible level, perhaps less than 1%. Cork Supply has developed their DS100 protocol employing both machines and human screening; they can now offer assurances of taint-free corks never before possible. Amorim, the world’s largest cork supplier, offers NDtech natural corks (Fig. 4) which are backed by two global insurance companies, such is their reliability (Adams, 2018). The company claims that, if any TCA remains, it is below the detection threshold of 0.5 ng/l (

Figure 4. NDtech cork.
Photo courtesy of Amorim.

Then there are the so-called technical corks made from cork granules which, to the unknowing eye, can look very much like their natural counterpart. Some variations have single-section natural cork covers or discs at one or both ends. This type of cork has also undergone rapid evolution and improvement in just a few years. The worldwide leader is a French company, Diam Bouchage. They have gained a reputation for high-quality “technological” corks purified using supercritical carbon dioxide. Their proprietary procedure, which the company says assures “incomparable sensorial neutrality,” is known as Diamant ® (www.diam- Diam claims annual sales of 1.5 billion closures which are, they maintain, TCA-free down to a level of 0.3 ng/l. Their brand names are Diam for still, and Mytik Diam, for sparkling wines (Fig. 5 & 6).

Figure 5. Diam cork for still wines.

Figure 6. Diam cork for sparkling wines.

Photos courtesy of Diam Bouchage SAS.

Another example is Amorim’s innovative Helix® design, made of granulated cork. It has grooves to match threads inside the neck of the bottle, allowing for easy removal without a corkscrew (Fig. 7). At present, this closure has a shelf life of two years, the maker advises.

Figure 7. The Helix® cork and bottle.
Photo courtesy of Amorim.

The third main type of closure is the synthetic stopper manufactured from plastic. When it was introduced, this imitation “cork” was derided by Prince Charles, a passionate environmentalist, who famously said: “Quite why anyone would want to encounter a nasty plastic plug in the neck of a wine bottle is beyond me” (Lechmere, 2002). Once again, meaningful advances have been made with cork substitutes, led by Nomacorc®, which is owned by Vinventions. Their range of stoppers actually belong in a separate category. They launched PlantCorc™ in 2013, a material made from sugarcane-based polymers ( The company claims that their Select Green stopper is the “world’s first closure with zero carbon footprint.” The wood-grained skin could easily fool many drinkers, who do not associate the Reserva stopper with low-quality wines (Briscoe, 2019). This particular model is intended for wines to be aged up to 25 years (Fig. 8).

Figure 8. Nomacorc’s Reserva cork from plant-based polymers.
Photo courtesy of Vinventions.

The fourth main seal is the screw cap, which is made of aluminum. Two considerations have driven adoption of these caps for wine. The overriding factor is that they are free of cork taint. There is as well the undeniable simplicity of a closure which can be twisted off – and then back on – by hand. But screw caps have a major drawback: they leave the largest carbon footprint. Amorim estimates that one-fourth of all wine bottles worldwide are closed with screw caps (Schmitt, 2017). They come in two basic versions, one lined with tin (saratin) and the other with plastic (saranex). The tin lining provides a nearly airtight seal – still not 100% – yet more than any other type of closure.

When Frank Prial wrote about the possible demise of the cork in 2003, he also captured a prevailing sentiment when he said: “The cork is sacred, the screwtop profane.” This may have been hyperbole, yet U.S. consumers still seem to embrace cork in one form or another far more readily than metal caps. (That said, screw caps have not held back the burgeoning U.S. imports of New Zealand Sauvignon Blanc!) A study by The Freedonia Group in 2015 indicated that there is a “stronger cultural preference for corks in the U.S. than in other countries” (Lasky, 2019). In the 2018 Closure Survey by Wine Business Monthly, 36% of U.S. winemakers reported using screw caps on one product at least. The Stelvin ® cap has become synonymous with this type of closure even though it is proprietary to Amcor, a French company ( Even this perfunctory closure has been perfected and is available in several formats including the Inside model allowing four levels of OTR (Fig. 9) and the sleek, stylish Lux with concealed threading (Fig. 10).

Figure 9. Stelvin ® Inside cap

Figure 10. Stelvin ® Lux cap without available in four liners with different external threading. Oxygen Transmission Rates.

Photos courtesy of Amcor.

There are still other closures for wine, notably glass stoppers, which are the most costly alternative. Some studies show U.S. consumers place glass on par with natural corks – when asked to rate closures by themselves – with respect to the “perceived intrinsic attributes of a wine” (Briscoe, 2019). Vinolok, a Czech Republic company, manufactures the best-known glass closure (

Figure 11. Vinolok glass stopper.
Photo courtesy of Vinolok.

It turns out that decisions concerning which closure to utilize are unexpectedly complex. Potential damage from taint has largely been brought under control, and the focus has shifted to the extent and rate of exposure to oxygen. How much O₂ is captured at bottling and gains entry after the closure is applied can have a meaningful impact on the wine’s taste and longevity. While a cork still seems to possess potent imagery for many wine lovers, convenience is not its strong suit. And now more wine of decent quality is coming in cans and boxes; as with screw-capped bottles, no tool is needed. In other words, there is good news all around for wine drinkers in terms of closures and containers, whatever the preference and budget.



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Briscoe, S (April 2019), “Two Studies Measure Consumer Preferences for Wine Closures,” Wine Business Monthly.

Butler, J; Heskett, R (2012), Divine Vintage: Following the Wine Trail from Genesis to the Modern Age, St. Martin's Press, Kindle Edition.

Goode, J (2005), The Science of Wine, University of California Press, Berkeley and Los Angeles, California, ©Octopus Publishing Group Ltd.

Goode, J (August 2014), “Using Closures to Customize Oxygen Transmission,” Wines & Vines.

Gordon, J (May 2019), “New Study Measures the Dramatic Release of O₂ from Stoppers Post- bottling,” Wine Business Monthly.

Lasky, MS (March 2019), “Winemakers Discuss Why They Moved to Screw Caps,” Wine Business Monthly.

Lechmere, A (12 June 2002), “Prince Charles slams ‘nasty’ synthetic corks,”

Phillips, C (June 2010), “2010 Closure Survey Report-Increasing Number of Wineries Turn to Multiple Closures,” Wine Business Monthly.

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The Economist (April 12, 2003), “Wine-A corking idea?”