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Wine closures: the facts

Whether it be cork, plastic or aluminium, winemakers are spoilt for choice when it comes to stoppers. Patrick Schmitt MW considers the performance of each closure type.

Picture credit: Sparflex

It may seem extraordinary, but one of the hardest decisions for any winemaker doesn’t concern practices in the vineyard or cellar. Although the implications of picking dates, fermentation temperatures, extraction regimes, barrel selections and so on are all crucial for wine style, it is the choice of stopper that gives producers one of the greatest headaches.

That’s because so many factors are involved in the selection, while the options continue to expand, and the science behind closure innovations is always evolving.

Furthermore, stoppers do affect the way a wine can taste, as well as a producer’s costs and carbon footprint.

With this is mind, we have decided to look at the key differences between closure types from three materials – cork, plastic and aluminium – according to their performance, environmental impact, and price. The idea is not to say which is best or worst, but to look at the facts, after many years reporting on these closures. This, it is hoped, will help you decide which type of stopper is the most suitable for your wine.

Before considering the options, it should be stated that if this analysis seems to feature more on the cork industry, that is not because of the bias of the author, but simply because the cork producers, who were heavily criticised by the wine industry in the nineties and early noughties for a poor-quality product, have, in the past eight to 10 years, been extremely active in technical innovations to defend their position. This means that there’s much to say about cork. It has also ensured that cork closures are now slowly regaining market share of the total stopper business, above all from plastics.

The market

Picture credit: Amcor Capsules

To consider the market for the different closure types first, the exact proportions vary according to who one asks, but according to Antonio Amorim – chairman of Amorim, the world’s largest cork producer – the number of wine bottles produced annually totals 18 billion, of which 11.5bn are sealed with cork, 4.5bn are closed with screwcaps and 2bn use plastic stoppers. (However, Guala Closures claims the screwcap business is bigger, and currently amounts to 6.5bn units).

Antonio believes that the trends favour the cork stopper, and, as a result, in the long term, he sees this closure gaining market share. Indeed, he sees his business growing by another 1bn stoppers within the next three years.

Helping the cork industry to expand is rising wine consumption in countries such as the US and China – two markets that favour cork-stoppered bottles – as well as expanding fine wine sales, and a surging sparkling wine business (two categories that use cork seals).

Amorim says: “World consumption for still wine is growing by around 1.3%-1.4%, and 2%-2.5% in sparkling wine – mainly thanks to Prosecco – and the cork industry is growing at 6%, and Amorim is growing more than that: we grew by 10% in value in 2015 and this year, till the end of March [2016], we are up by 8%.”

Turning his attention to competitor closures, he adds: “The market for plastics is declining, it has fallen to 2bn stoppers from 4.5bn in 2006/2007.”

He adds: “The 4.5bn screwcap market is growing very slowly now because Australia is not growing anymore [in production] and New Zealand is the same,” picking out two markets where screwcaps are dominant (although New Zealand has since picked up in volumes following a large 2016 harvest).

Meanwhile, he mentions the strategic importance of the UK.

“We just have one market to get back and that is the UK, because then we can get back South Africa and Chile, and that’s because they use screwcaps because they are told to by the UK – the UK is a decisive market,” he states.

However, with the UK consumer now accepting screwcaps for aromatic whites and lighter styles of red wine, despite quality improvements from leading cork producers, it is unlikely the major buyers in Britain will start to specify a return to cork closures for wines sealed with a screwcap.

Environmental impact

With a growth in demand for sustainably sourced products, the environmental impact of wine is under increased scrutiny, meaning that the eco-friendliness of the different closure types has become a factor in which stopper to select.

Consequently, closure manufacturers have undertaken carbon footprint and other analyses to see which is the greenest stopper. When it comes to CO2 emissions alone, Nomacorc was the first to release a comparative study when it published the results of a Greenloop audit in April 2008. Having analysed the CO2 released during the production of the raw material for the closures and their packaging, then the manufacture of the stoppers, and distribution, as well as disposal, it showed that the cork had the lowest carbon footprint and screwcaps the highest (see bar chart), with the Nomacorc synthetic stoppers producing double the amount of carbon dioxide equivalent (CO2e) on average than a natural cork, although the plastic stopper was shown to be 30% less polluting than micro-agglomerated corks.

A few months later, in November 2008, Amorim released its peer-reviewed PriceWaterhouseCooper/Ecobilan study, called Analysis of the life cycle of Cork, Aluminium and Plastic Wine Closures. This incorporated seven indicators (see table, bottom). In terms of Carbon Dioxide emissions, the results were similar to the Nomacorc study – cork was the least polluting, screwcaps the most – although natural cork was shown to have a much lower output, under 2 grams of CO2 per unit, compared with 8g/CO2 in the former producer’s analysis. The Amorim study also showed that, in comparison to the aluminium and plastic closures, the cork stopper is the best alternative in terms of non-renewable energy consumption, contribution to atmospheric acidification, and production of solid waste, although screwcaps are the best alternative in terms of consumption of water, followed by cork stoppers.

Finally, the Cork Quality Council has also pointed out that the Portuguese cork forest, aside from being a diverse natural habitat, acts as a carbon sink for 4.8m tons of CO2, representing an offset of 113.2g of CO2 per cork.

As for recycling rates among the different closure types, it is assumed that 100% of the corks produced go to landfill, where they will slowly biodegrade, while around one third of aluminium closures are recycled and less than one quarter of plastic stoppers (see table, right).

Aware of the higher environmental impact of screwcaps, Alupro (Aluminium Packaging Recycling Organisation) in the UK has launched the ‘Leave Your Cap On’ campaign, part-funded by Guala Closures, encouraging residents to recycle aluminium screwcaps along with the glass bottles on which they came.

Meanwhile, Nomacorc created “the world’s first zero-carbon-footprint wine closure” in 2013. Called Select Bio, it is partly made with plant-based polymers derived from sugar cane, and, according to the manufacturer, the bio-plastics contribute a negative carbon footprint value that fully offsets the positive emissions originating from conventional raw materials in the product.

Little has been said so far on the topic of energy use in micro-agglomerated corks, although this closure type did feature in the initial Nomacorc study, and came out with a higher Carbon Footprint than the synthetic closure. This is no doubt due to the processing of the cork granules – Amorim for example wash them using water at 70ºC and then put them through a steam distillation process called ROSA. This is to remove any potential wine spoilage compounds, above all trichloroanisole (TCA). Diam treats cork granules for the same reason, but uses the Diamant procedure, in which “supercritical CO2” extracts the unwanted compounds. Needless to say, both these processes are energy-intensive.

The performance: TCA

Whatever the environmental implications of the different closure types, the most important single factor for winemakers concerns the stopper’s role in a wine’s development, both positively and negatively. While all brand owners would seek to remove any adverse affects on a wine’s taste, it has also been shown that closures can play a beneficial role in the character of the product.

The most emotive issue in closure selection is TCA incidence in a wine, a musty smell detectable by humans at concentrations of 5 nanograms/litre (ng/l). Although TCA can be present in packing materials, and winery equipment, it is most commonly believed to enter wine from corks (although it must be present in the part of the cork in contact with the wine, as it can’t migrate through the closure). The number of wines sealed with corks contaminated with TCA is a subject of speculation, but it is widely felt to be at a relatively low level today, having fallen from more than 5% at its peak to below 3%. Indeed, during the course of the last decade, California’s Cork Quality Council (CQC) reported an 80% decrease in the incidence of TCA contamination, while Amorim’s marketing director, Carlos de Jesus, says that his company’s data on TCA incidence in cork stoppers shows a present level of 0.7%.

As for an independent view, in 2012, Peter Gago, head winemaker at Penfolds, admitted that levels of TCA in his wines were down to 1%, which he added was the same percentage that are prematurely oxidised as a result of mechanical damage to screwcaps.


Standing for non-detectable, NDtech uses gas chromatography to screen each natural cork for TCA

But, over the past 12 months, cork producers have developed techniques to offer winemakers the reassurance they could once only dream off – a natural cork with a guarantee that TCA will be non-detectable, meaning that if there is any remnant of TCA in the stopper, it is below the detection threshold of 0.5ng/l. Such is the breakthrough that former poster boy for screwcaps – Domaine Laroche in Chablis – has now gone back to natural cork for its premiers and grands crus wines.

Last year, the Burgundian producer, which famously switched to screwcaps for all its wines in 2001, adopted Amorim’s new ‘NDtech’ corks. Standing for non-detectable, NDtech uses gas chromatography to screen each natural cork for TCA. Currently, the patented machines can scan each cork in 20 seconds, which is significantly less than the 14 minutes for standard gas chromatography technology.

In 2016, Amorim screened and sold 10m NDTech corks, but has the capacity to process up to 50m corks annually, although Antonio Amorim says that the demand for the new guarantee means that he could already sell five times that number. Such a development has also put demand for the natural whole cork stopper back into growth, says Amorim’s de Jesus, noting that he now has an order for NDtech corks from New Zealand, the most anti-cork nation in the world, and founder of the New Zealand Screwcap Initiative – an affiliation of producers who united to promote the benefits of the aluminium closure back in 2002.

But it’s not only Amorim that is offering such reassurance when it comes to TCA. Both MASilva Portugal and Cork Supply have invested in machines that check each individual cork for traces of TCA. In January 2016, MaSilva unveiled its One by One service, which, like Amorim’s NDTech, comes with a non-detectable TCA guarantee. Six months later, Cork Supply launched the DS100+ “dry soak” detection system for TCA in natural cork. With this system, Cork Supply says it will eliminate the risk of TCA in natural cork at levels as low as one part per trillion. Cork Supply expects to triple its capacity to 60 million corks tested by the end of this year.

Of course, there is an added cost for such guarantees, with, for example, Amorim saying that NDTech adds between E0.12-E0.15 to the price of each stopper. The fact that demand is outstripping supply suggests that people are willing to spend a premium to seal a bottle with a natural cork. “There are more than 800m cells in each natural stopper; it is an incredibly sophisticated material. Cork was a great closure 10 years ago, it’s just that we needed to make it better,” says de Jesus, explaining the comeback for natural cork.

Nevertheless, it should be said that, at the moment, and despite the guarantees for non-detection, there is no such thing as a natural cork closure that claims to be 100% TCA-free, says Riccardo Tiso, sales and marketing manager at Labrenta. For that, he says, you must choose a synthetic stopper or screwcap, having, of course, already established that TCA isn’t present in the barrel, or the bottling line.

The performance: OTR

With the advent of non-detectable TCA guarantees in natural cork, the issue of closure performance is now more than ever centred on the ability of different stoppers to manage the passage of oxygen into a wine. And, while a hermetic seal may be the ultimate goal for a white wine made from aromatic grapes that is designed to be drunk young, low levels of exposure to oxygen may actually benefit other types of wine, particularly reds made from naturally reductive grape varieties such as Pinot Noir or Syrah.

In general terms, tests have shown that standard plastic stoppers offer the worst protection against oxygen over time, as the synthetic material struggles to fill imperfections in the neck of the bottle (and plastics can become brittle with age), allowing air to pass between the closure and the glass. Tin-lined screwcaps appear to offer the closest thing to an air-tight seal on the market today, closely followed by a high-quality natural cork, which forms a better seal than plastic-lined aluminium or glass closures. Figures from a range of sources show that oxygen transmission rates (OTRs) vary for each closure type (see table, right, and graph, bottom).

Complicating this topic further is the fact that closure manufacturers offer stoppers with different OTRs, most notably Diam, which makes agglomerated corks with a range of “permeability values”.

For instance, it offers five types of corks for still wines, from Diam 2 to Diam 30, with the latter offering a 30-year guarantee, because the OTR is so low.

Meanwhile, screwcap manufacturers using a tin liner – as opposed to more permeable plastic one – are now able to supply versions with microperforations in the metal layer of the liner. Using a mathematical formula, they are able to predict the level of OTR depending on how many perforations are made in the tin and where they are located.

As for the role of oxygen in a wine’s development, for those products that are made to develop greater complexity as they slowly mature in the bottle, Miguel Cabral, who is director of research and development at Amorim, has said that cork’s unique properties may be beneficial.

“There is an initial bit of oxygen released into the wine from a cork in the first six to eight months that comes from within the cork; it is expelled into the wine when the cork is inserted into the bottle, but the cork is not permeable, the oxygen’s origin is from the interior of the cork.

“And this initial bit of oxygen is not damaging the wine, and later on [in the wine’s development], it may facilitate oxidative reactions,” he says.

Oxygen transmission kinetics of different types of closures. Results are expressed as the accumulated quantity oxygen transmission over 60 months of storage. Results obtained using the method developed by Lopes, P.; Saucier, C.; Glories, Y. Nondestructive colorimetric method to determine the oxygen diffusion rate through closures used in winemaking. J. Agric. Food Chem. 2005, 53, 6967-6973. Main routes by which O2 enters into bottles. Lopes, P.; Saucier, C.; Teissedre, P.L., Glories, Y. Main routes of oxygen ingress through different closures into wine bottles. J. Agric. Food Chem. 2007,55, 5167-5170.

Positive effect

Amorim’s Dr. Miguel Cabral says that cork-derived phenolics “allow for a balanced wine ageing”

Cabral’s latest research centres not on the role of low levels of oxygen in the maturation of wine, but extractible compounds in a natural cork, which, he says, may have a positive effect on wine development.

“A cork in a bottle will slowly release compounds into a wine that will react with it and produce complex compounds that probably have a role in colour stabilisation and reducing bitterness and other roles we don’t know,” he says.

He continues: “You put wine in a barrel because you want these phenolics, and when you put wine in a bottle, it receives some phenolics from the cork – data collected over 36 months shows that the cork will release a small amount of phenolics, and this is a linear release.”

Notably, Cabral said that these cork-derived phenolics “allow for a balanced wine ageing”, which, he said, was in contrast to other closures, commenting that “synthetics accelerate ageing,” while the almost hermetic seal formed by a Sarantin screwcap, which is tin-lined, “accelerates a reductive wine ageing” – referring to the production of volatile sulphur compounds that, it is believed, are formed as a wine ages in bottle without sufficient oxygen (although these characters are more commonly believed to result from reductive winemaking).

The next stage for Cabral is to see if he can select a cork for its higher phenolic content, so it can be used for wines destined for extended ageing in bottle.

“Phenolics are variable in cork, and some corks release more than others, so our idea is trying to get a technique that will determine the amount of phenolics in the corks using infrared technology,” he says.

Such an approach is already used by barrel manufacturers, observes Cabral, who says a cooper such as Radoux is using near-infrared spectrometry to grade oak staves according to the phenolics, specifically tannins, that will be released into the wine. Cabral says: “Our objective is that in the future, a wine producer could specify a cork with a high concentration of phenolics.”


Of course, no packaging decision can be made without reference to cost, closure type included. And with the huge number of options now available, the prices range widely (and depend on the size of the order too). The broadest range comes within cork, where prices start from around 1 penny per unit to more than £1 for a whole natural cork – with a further fee of around 10p-15p per stopper for the TCA screening technology now available. The top-of-the-range synthetic, such as the Ardea Seal AS Elite, costs around 35p per stopper, similar to a Sarantin-lined screwcap, which is double the cost of a plastic-lined one. As for the glass solutions from a company such as Vinolok, these cost around 50 pence each.

Of course, if one does change from a cork or synthetic stopper to a screwcap, it will be necessary to change your bottle as well as your closure.

Other factors

The Vino-lok stopper uses glass and eythylene-vinyl acetate

Finally, it should be stated there are many other factors that affect a winemaker’s decision to choose a particular closure type. These include, critically, market preference, as well as issues such as convenience – screwcaps, for instance, remove the need for a corkscrew (although Amorim has developed the Helix twist-open natural cork).

Indeed, Gago has identified a reason to use cork for wines shipped over long distances – and that’s because a screwcap masks the impact of heat damage.

“A red wine under screwcap looks perfect. It might have been at 50ºC for four weeks but you would never know. On the other hand, a weeping cork suggests exposure to intense heat, as does a lifted capsule due to the cork being pushed up with internal thermal expansion,” he says.

But it should be stressed that winemakers today are unlikely to choose one type of closure for all their wines, choosing different stoppers depending on the wine style and target consumer.

And more innovations will ensure that the battle between different closure types continues. Indeed, Penfolds’ Gago believes the ultimate stopper of the future won’t be made from cork, plastic or aluminium, but sintered glass – a type of breathable glass used for filtering bacteria and fine precipates in scientific analyses. Not only that, but he’s trialling this as a glass-to-glass solution.

While there are glass stoppers on the market already, these contain plastic to hold the closure in place and prevent wine leaking down the sides of the bottleneck.

While they look good, they suffer from the same potential problems as any other closure containing plastics: a longterm concern the material may become brittle and fail, or potentially react with the acids in wine.

So, it’s only with Gago’s innovation that a wine could be housed in an entirely inert container – should that be the ultimate aim. However, this has its own problem: such a solution currently costs far more than the wine itself.

And finally…

Although synthetic closures receive a bad press when it comes to oxygen transmission rates, the best grades are designed to provide a minimal but controlled exposure to air.

For example, Nomarcorc’s co-extruded plastic corks have an outer flexible skin to provide an airtight seal and an inner breathable air filter to allow for a controlled oxygen exposure over time.

Meanwhile, the Ardea Seal AS-Elite, a synthetic stopper famously used by Laurent Ponsot from Domaine Ponsot in Morey-Saint-Denis, similarly promises a controlled level of oxygen ingress over extended periods.

Following sampling, Ponsot found in 2008 that his 10 year-old wines sealed with natural cork contained a level of oxygen that ranged from 10-350 parts per million (ppm), but found that the optimum in terms of taste was a wine containing 100ppm of oxygen – a quantity he hopes to achieve in a decade’s time for all his bottles sealed with the synthetic closure today.

Read more:





And finally, you can read a five-part series on the closure debate from the perspective of five different winemakers by clicking on the following links:

Part 1 – The world’s most high profile experiment?

Part 2 – In praise of screwcaps

Part 3 – Moving back to cork

Part 4 – Synthetic solution for Grand Cru Burgundy 

Part 5 ­– The Perfect Solution?

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