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Cork Myths and Curiosities

Did you know that a single cork stopper can show a balance of up to -562g of CO2?

And that cork was used in Ancient Egypt and can be used to produce energy? Did you know that scientific research consistently proves that consumers associate cork with high-quality wines?

Find out everything you’ve always wanted to know about cork.

Cork is the bark of the Cork Oak tree (Quercus suber L). It is an all natural raw material, with unique properties which give it an unparalleled character. It is light, impermeable to liquids and gases, elastic, compressible, provides thermal and acoustic insulation, a fire retardant and highly abrasion-resistant. Furthermore, it is completely biodegradable, renewable and recyclable.

Cork is stripped from the trunk of the Cork Oak every nine years, without damaging the tree. The largest areas of cork oak forest are in Western Mediterranean countries: Portugal, Spain, Italy, France, Morocco, Tunisia and Algeria.

The world's production of cork is 340,000 tonnes per year, 55% of which is from Portugal.

No. The extraction of cork is a controlled process and does not required the cork oaks to be felled - on the contrary, it contributes to their regeneration. It is the cork industry which makes the continuity of the cork oak forest viable, by contributing to the maintenance of forests and the populations that depend on them. A recent estimate forecasts that only in Portugal, where there is the world's largest cork oak forest area, shall the harvestable cork be enough to meet market demand for the next 100 years.

Cells were discovered in 1665 by Robert Hooke. On observing a sliver of cork under a rudimentary microscope, the English scientist discovered that it was made up of multifaceted cavities, which he called cells (from the Latin cellula, small room).

Yes. According to the Lifecycle Analysis of Cork Stoppers, commissioned by Corticeira Amorim to PriceWaterhouseCoopers, each cork stopper is responsible for capturing 112 g of CO2. Comparing to artificial closures - aluminium and synthetic - these emit 37,2 g and 14,8 g, respectively.

Each tonne of thick cork planks can provide, on average, 66,700 cork stoppers.

Yes. Being a 100% natural product, cork stoppers are biodegradable and do not pollute the atmosphere if they are thrown in the rubbish bin. However, they are completely recyclable and reusable. Although recycled cork shall never be used in stoppers again, it can be used in coverings, insulation, memo boards, high competition kayaks, badminton rackets, tennis and cricket balls, car and aircraft components, design and fashion items and a multitude of other uses.

Environmental motivation is one of the most important factors in recycling. Cork stoppers absorb CO2 particles that have been retained by the bark of the cork oak. If they are decomposed or incinerated, they release the CO2 into the atmosphere, thus contributing to global warming. Recycling enables the CO2 retention capacity of cork to be extended. In each tonne of cork stoppers, around 1.07 tonnes of CO2 is retained, which, once they have been recycled, this capacity is ensured forever, as the reuse of this raw material is unlimited.

On the other hand, by recycling used cork stoppers you are also contributing to enabling the reuse of a raw material and the decrease of the costs associated with the production of other high added-value products.

Amorim is a pioneer in promoting the recycling of cork stoppers, developing collection programmes in Portugal, the USA and Canada, France, Italy, the United Kingdom, South Africa and Australia.

Learn about the cork stopper collection projects in:

Cork is a very light raw material, weighing just 0.16 grams per cubic centimetre, and can float.

Each cork stopper is made up of around 800 million watertight cells. Among them is a gaseous mixture which allows it to be compressed to around half its thickness, without losing any flexibility, and to be decompressed and return to its original shape. This is what is called an elastic memory. Cork is the only solid which when compressed on one side, does not increase in volume on the other. This feature enables it to adapt to variations in temperature and pressure, without compromising its integrity as a stopper.

Thanks to the suberin and ceroids, it is practically impermeable to liquids and gases.

Decay resistant
Cork is highly resistant to moisture, and therefore to subsequent oxidation and decay.

Cork is an excellent thermal, acoustic and vibration insulator. In relation to wine, the insulating properties of cork contribute to cork stoppers being the best protection against variations in temperature.

Biodegradable, recyclable and renewable
Cork is a natural raw material which is 100% biodegradable, recyclable and renewable. A recycled cork stopper is never used to manufacture a new stopper, but its recycling has endless uses, from materials for construction, fashion, sport, art, the aeronautics industry, among others.

Cork consists of suberin cells in the shape of tiny pentagonal or hexagonal honeycombs, a complex fatty acid and is filled with an air-like gas, which makes up 90% of its volume. It possesses an average density of around 200 kg/m3 and low thermal conductivity.

Each cubic centimetre of cork may contain around 40 million cells. There are around 800 million cells in a single cork stopper.

All over the world, 13 billion cork stoppers are produced annually. If we were to join them together, they would circle the Earth 15 times.

There are some remnants of the use of cork by the people of Ancient Egypt and in Roman civilisation. In France, amphorae from the 3rd century BC were found full of wine considered to still be in good condition. The use of cork at pre-industrial level dates back to the end of the 17th century.

Stripping is the ancient process of extracting the bark of the cork oak - the cork. This work is done by specialised professionals, with absolute precision, who use just a single tool: the axe.

This delicate operation takes place between May and August, when the tree is at its most active time of growth and it is easier to remove the bark from the trunk. Harvesting cork is the world’s best paid seasonal agricultural job.

Over the course of its lifetime, a cork oak may be stripped around 17 times, at intervals of at least nine years, which means that the harvesting of the cork will last 150 years, on average.

The first stripping is called "desbóia" from which the virgin cork is obtained, which has a highly irregular structure and hardness that make it difficult to process.

Nine years later, when the second stripping takes place, the cork, known as "secundeira", has a regular structure which is not as hard.

The cork from these first two harvests is not fit for the manufacture of stoppers and thus used in other applications for insulation, flooring, decorative items, among others.

From the third and following strippings the "amadia" or reproduction cork is obtained. This cork has a regular structure, with a flat front and back and the ideal characteristics for the production of natural, quality cork stoppers.

The first stripping takes place when the cork oak is 25 years old and the trunk has reached a perimeter of 70 centimetres, measured 1.5 metres from the ground. Subsequent strippings take place at intervals of at least nine years.

No. Stripping is carried out manually and the trees do not have to be cut down. In fact, the cork oak undergoes a self-regeneration process of the bark, which gives the activity of cork harvesting a uniquely sustainable nature.

No. After stripping, the planks are stacked into piles in structures and shall remain outdoors for at least six months for the cork to stabilise. This process is governed by the strict compliance of the Code of Cork Stopper Manufacturing Practices.

A cork oak has an average lifespan of between 170 and 200 years.

Nothing is wasted from the cork oak, all its components have a useful ecological or economic purpose:

  • The acorn, which is the fruit of the cork oak, is used to propagate the species, as animal fodder and in the manufacture of cooking oils;
  • The leaves are used as fodder and a natural fertiliser;
  • The material from tree pruning and decrepit trees provides firewood and charcoal;
  • The tannins and natural acids contained within the wood from the tree are used in chemical and beauty products.

The oldest and most productive cork oak in the world is the Whistler Tree, in Águas de Moura, in the Alentejo. The cork oak was planted in 1783, stands over 14 metres tall and the perimeter of its trunk is 4.15 metres. Its name comes from the noise made by the numerous songbirds that shelter among its branches. Since 1820, it has been harvested over twenty times. Its 1991 harvest produced 1,200 kg of cork, more than most cork oaks yield in a lifetime. This single harvest produced over one hundred thousand cork stoppers.

It is estimated that there are over 2.3 million hectares of cork oak forest. About 716,000 hectares are situated in Portugal, which represents 22,5% of the national forest area. Half of the world cork production is Portuguese. The rest is situated in Spain, Italy, France, Morocco, Tunisia and Algeria.

Besides constituting a natural ecosystem which is unique in the world, cork oak forests make a wide range of agricultural, forestry, forest grazing, hunting and economic activities viable: the harvesting of medicinal plants and mushrooms, honey and beeswax production, coal production, hunting, cattle breeding, birdwatching, tourism and horse riding. It also gives rise to the creation of indigenous food products which are certified by the European Union.

In the seven Mediterranean cork-producing countries, over 100,000 people directly or indirectly depend on the economy provided by cork oak forests.

Thanks to the thermal and weak combustion properties of cork, cork oaks are more fire-resistant than other trees. The slow combustion of cork makes it a natural fire retardant, forming a barrier against fires. Its combustion does not release smoke or toxic gases.

The cork oak is an evergreen tree, of the Fagaceae family (Quercus suber), to which the chestnut and oak tree also belong. There are 465 species of Quercus, mainly found in temperate and subtropical regions of the Northern Hemisphere. Cork is harvested from the Quercus suber L species.

The cork oak may be sown, planted or propagate spontaneously, which is often the case in cork oak forests, thanks to the acorns that fall on the ground.

The cork oak is native to the Western Mediterranean Basin, where there are ideal growing conditions:

  • Sandy, chalk-free soils with low nitrogen and phosphorus, high potassium and a pH from 4.8 to 7.0;
  • Rainfall from 400-800 mm per year;
  • Temperature from -5º C to 40º C;
  • Altitude from 100-300 m.

In Ancient Greece, cork oaks were revered as the symbol of Freedom and Honour. Thus, only priests had permission to cut them down.

This is due to the high level of expertise necessary to harvest the cork without damaging this precious resource.

No. The Quercus suber L genome is the same, therefore there are no significant differences according to origin. There are, however, individual differences from tree to tree.

When in contact with wine, the cork stopper forms antioxidant and anti-carcinogenic compounds that may reduce the risk of heart and degenerative disease. Furthermore, waste from the cork industry gives rise to composites which are used in vaccine adjuvants to enhance immune system response.

Yes. Cork dust can be used in the cogeneration of electricity, making a valuable contribution to improving energy efficiency. Amorim meets over 60% of its energy needs by using cork dust (biomass), which is a CO2 neutral source of energy.

Due to the lightness and acoustic and thermal insulation capacity of cork, it is also used in wind turbines.

The name Quercus suber L. derives from Linnaeus, who was the first botanist to describe the species.

Cork’s success over centuries lies in its unique physical properties that no man-made closure has been able to replicate: lightness, compressibility, elastic memory, gradual recovery, impermeability to liquids and resistance to wear, heat and rot.

Cork also allows a minute amount of oxygen to permeate into the wine after sealing. This appears to have a beneficial impact, but more research is needed to understand the full contribution of the cork to wine development.

In addition, the introduction of technical corks such as Twin Top, champagne corks and Neutrocork has extended the range of cork products available to suit each wine style and market segment.

With proper handling during and after bottling to ensure the best performance, cork is unsurpassed as a wine closure.

Cork remains as relevant today as it has ever been. Producers such as Amorim have effectively defeated the problem of TCA and are working constantly to improve performance.

New research is helping us better understand the unique contribution that the cork makes to wine development. We will be able to use that new knowledge to further improve our products.

We do not accept that a single closure can cover every single wine segment. Amorim is developing new corks products to suit specific wine segments, responding to a specific need in the wine market. Our closures range from natural corks to Twin Top corks for premium wines and Neutrocork for the popular premium segment. All meet high standards of sensory performance and consistency.

The cork industry has made great improvements in sensory performance, especially in the last five years, and new initiatives such as the communal cork processing facility will allow smaller producers to take advantage of the most recent advances in cork processing technology.

However, it is true that not all producers meet the standards achieved by companies such as Amorim.

By buying from an accredited supplier such as Amorim and by implementing their own quality control procedures, winemakers can buy cork confident there will be minimal risk of TCA.

Cork stopper production is at the heart of the cork economy; without it other uses of cork would become unviable. The forests would fall into neglect. It would be harder for farmers to resist pressures to clear the forests for more profitable uses such as eucalyptus.

It is true that the cork industry as a whole was too slow in responding to the problem of TCA. That has all changed and a huge R&D effort is devoted to improving cork quality.

The emergence of alternative closures was a wake-up call to the cork industry. Competition has been good for quality.

The problem of TCA in cork has been effectively addressed, while problems with alternative closures persist; in fact, screwcap proponents are in denial about the problems being encountered.

There is enough cork in Portugal to meet demand for the next 100 years. Under a large-scale re-forestation program funded by the EU the forests are growing by 4 per cent a year. High quality cork forests in North Africa (Algeria, Tunisia and Morocco) are being brought into commercial production.

Most importantly, the introduction of high quality technical corks is allowing much better utilisation of the cork resource.

Amorim produces high quality closures to suit each market segment and price point and believe cork-based closures are very competitive. In fact, in some markets cork closures such as Neutrocork can cost half of the current prices for leading oil-derived stoppers

Given the importance of the closure to the wine after bottling, winemakers are encouraged to ensure they purchase a high quality closure. Amorim commercial personnel can advise on the most appropriate closure for each wine.

Winemakers may make recommendations, but they cannot control when a wine is to be drunk. They must be confident that the closure will last the distance, regardless of when the wine is to be drunk. Cork has proven itself to be an effective seal on wine stored for decades. Technical corks such as Twin Top, which are designed for short-to medium-term cellaring, are still performing well in comparative trials, even after 5 years.

Winemakers can use cork confident in the knowledge that it will perform well, regardless of when the wine is to be drunk, without the risk of premature oxidation or sulphide-like odours associated with other closures.

Wines may not need oxygen to develop, but it appears that they benefit from the minute amount of oxygen ingress allowed by cork stoppers.

Where wines have been totally deprived of oxygen, i.e. in glass ampoules, they have also displayed a marked tendency to develop suphide-like odours. So evolution may continue to occur, it just so that it leads to results that are far from what the winemaker originally intended.

The findings of researchers at University of Bordeaux 2 suggest that oxygen diffusion through the natural cork is more important than reported by J. Ribéreau-Gayon in 1933.

"Ribéreau-Gayon used a similar colorimetric method, but did not record the characteristics of the closures, the bottles or the storage conditions, factors that Lopes et al. believe would help to partially explain some of the differences between the results of the 1933 and 2005 studies."
Nancy Mills, "Sealing Themes And Variations", Article in Aust. Wine Industry Journal, October 2005

Post-bottling wine chemistry is a very complex issue and we are still a long way from understanding all the factors at play let alone controlling them for each wine style, region and vintage. Cork, on the other hand, has been doing a remarkably good job for centuries. As we understand it better we are able to further improve its performance.

"To try and make one extremely low permeability closure to suit all wines is going to lead to variability of results not between individual bottles, but wine to wine.… To require wines to be completely free of any sulphide precursors will place demands which at this point in time we are unable to (and may not wish to) consistently fulfil as winemakers."
A. Limmer, "Do Corks Breathe? Or the Origin of Slo", Aust. & Nz Grapegrower and Winemaker, Annual Technical Issue 2005

It’s good that all closure manufacturers seek to improve their product. However, according to recent research, synthetic closures still have some way to go, especially in relation to oxidation and flavour ‘scalping’.

Cork on the other hand also continues to improve and retains all the benefits of a natural wine closure. So even if parallel advances are achieved by the plastic stoppers, we will be able to retain cork’s advantageous departing position.

Winemakers may make recommendations, but they cannot control when a wine is to be drunk. They must be confident that the closure will last the distance, regardless of when the wine is to be drunk.

Winemakers can use cork confident in the knowledge that it will perform well, regardless of when the wine is to be drunk, without the risk of premature oxidation or sulphide like odours associated with other closures.

Cork is not the only source of TCA contamination, so changing closures will not eliminate the risk of TCA taint. There are also other taints that have nothing to do with cork, such as TBA (Tribromoanisole) and Brettanomycaes, and using synthetic closures will not protect against these.

For example, some manufacturers of synthetic closures were forced to review their production processes when it was found that talc used in the manufacturing process caused the stoppers to smell of cow or horse stables and deadened the wine.

Independent market research studies show that consumers do indeed care about the type of closure that is used and the majority prefer cork. Even the producers of alternative closures have not been able to publish a single consumer poll where cork is not the preferred closure.

They associate synthetic closures with cheaper wines.

As cork quality continues to improve and problems with alternatives become more apparent, we are confident that cork will retain its strong support among customers.

Unlike cork, synthetic closures are not natural, renewable or biodegradable, and their production adds to greenhouse emissions.

The synthetic materials may be recyclable but efforts to establish recycling of synthetic closures have failed because it is uneconomic.

No closure can match cork as a sustainable form of packaging, a matter of increasing importance to many consumers.

There are high quality cork closures at all price points to suit all wine styles and market segments. These bring with them all the advantages that winemakers and consumers have come to appreciate in cork.

Scientific trials such as the AWRI’s comparative closure trial have concluded that there is no such thing as a perfect closure.

"No one closure tested in this study could be considered entirely suitable by all criteria assessed, for the long term storage of wine."
Awri Media Release, "First Results of the Awri Wine Bottle Closure trial published 12 July 2001

Corks, especially Twin Top, performed at or near the top on all key parameters, without the presence of sulphide-like odours or premature oxidation. Even after 63 months, Twin Top continues to perform well.

According to the former director of the AWRI, Professor Peter Hoj, all closures can be improved:

“All could perform better: cork producers needing to go further in reducing taint; screwcap producers having ‘a long way to go’ to improve liners and gas exchange, and also in preventing damage to caps when knocked; and synthetics are still considered to ‘scalp’ flavour from wine”
Prof. Peter Hoj, quoted in "Australia pulls out the stoppers", Chris Snow, Herald Sun, 11 September 2004, P94

No closure is perfect, as the AWRI comparative closure trial concluded, and it would be irresponsible of any supplier to claim perfection for their product.

Cork has been doing a remarkably good job for centuries. In fact, as we learn more about post-bottling wine chemistry, we can only marvel at the unique contribution cork makes to wine preservation and development.

As we understand it better we are able to further improve its performance.

The incidence and intensity of TCA continues to fall and the latest research shows that cork is remarkable consistent in terms of oxygen ingress.

That does not mean that cork producers can relax. Amorim will remain ever vigilant against TCA. Our R&D effort will help us better understand and control oxygen ingress through corks and we will work with winemakers to minimise the risk of ‘random oxidation’ associated with bottling line operations.

While the problem curve for cork is coming down, the problem curve for screwcaps is going up.

Screwcap producers and wine researchers are a long way from knowing how to avoid the sulphide-like odours (SLOs).

More and more questions are being raised about the effect of the screwcap on wine development.

While there may have been some occurrences of SLOs in wines sealed with corks, several studies by the AWRI have confirmed these are not nearly so frequent as they are in wines sealed with screwcaps.

Feedback from winemakers suggests that screwcaps make even greater demands on bottling line management than cork closures. They are vulnerable to bottle finish variability, capping faults and bottling line faults, all problems that screwcap proponents acknowledge.

Such problems have existed on bottling lines for years and apply equally to cork closures. It is unrealistic to hope they will be overcome simply with a change of closure.

“Mr Eggins [chief winemaker, Taylors Wines] said screwcap seals could break during storage, which led to wine going off through oxidation. If a stack of wine was stored on an uneven floor, pressure from bottles above could break the seals on bottles below”
Adam Eggins, Chief Winemaker of Taylors Wines (Who use screwcaps exclusively) quoted by Geoff Strong in the Melbourne Age

Post-bottling wine chemistry is a very complex issue and the closure is only one of a number of factors affecting wine development (others include the chemical composition of the individual wine, headspace volume and gas composition, free SO2, bottling conditions, etc).

Research is improving our understanding but wine researchers don’t yet know enough about how all the factors affect wine development to predict let alone control what will happen in the bottle.

Experts such as Dr Alan Limmer believe that the likelihood of reduced characters forming after bottling varies from wine to wine, vintage to vintage, placing severe demands on the winemaker wanting to control them.

Meanwhile, cork has been proven over centuries

It is a mistake to equate the best quality corks with screwcaps, which have a much stronger tendency to impart reduced characters to wine and restrict wine development.

While screwcaps may be suitable for some wines intended for quick drinking, very little is yet known about the impact of screwcaps on more complex wines intended for long-term cellaring.

Current thinking suggests that wines will develop more slowly under screwcap and may develop into different wines. They may never develop the characters winemakers and consumers appreciate in cork-sealed wines.

In those markets with greatest experience of screwcap, there is growing recognition that the issue of cork vs screwcap is far less clear-cut than screwcap advocates claim.

No. Research and practical experience have shown all closures are subject to some degree of variability. In fact, they show that when properly handled cork performs remarkably consistently.

The high degree of consistency claimed for screwcaps depends to a large extent on eliminating all other sources of variation, including bottle finish variability, capping faults and bottling line faults, all problems that screwcap proponents acknowledge.

These same factors affect the performance of cork, although feedback from winemakers suggests that screwcaps make even greater demands on bottling line management than cork closures.

Amorim staff can advise on correct bottling line procedures to help avoid random oxidation (see Focus, ‘Choosing and Handling Cork’).

Cork is not the only source of TCA contamination of wine, so changing closures will not eliminate the risk of TCA taint. There are also other taints that have nothing to do with cork, such as TBA (triboromoanisole) and brettanomycaes, and using screwcaps will not protect against these.

“That venerable and venerated doyen of wine, Len Evans AO and OBE has been selecting those Qantas wines for 42 years and chairman of the panel for 37. At a recent tasting some of his fellow judges rejected some of the wines because of the dreaded and oft mentioned ‘cork taint’. Trouble was that the ‘tainted’ wines were under screwcap!”
Sydney Daily Telegraph, 11 March 2006

Winemakers and researchers are still learning about the interactions between the wine and the closure, including the ability of the closure to absorb certain flavour compounds from wine.

At this stage it appears that synthetic closures have the greatest tendency to scalp flavours, whereas screwcaps scalp little if any flavour. Cork appears to lie somewhere in between.

It may be that in some cases the ability to modify flavour has a beneficial effect, for example in modifying high concentrations of TDN, which imparts a kerosene-like flavour to Riesling.

Some researchers also claim that cork has the ability to remove sulphide compounds from wine, but this is unproven.

Winemakers have begun to use screwcaps, especially on fresh aromatic and quick drinking wines, but most also use cork.

As well as concern about suphide like odours, many winemakers have expressed caution about their suitability for wines with greater complexity.

They have concern that screwcaps will slow down bottle ageing, creating ‘Peter Pan’ wines, ‘frozen in time’, which never develop the same character as they do under cork.

Some winemakers have returned to cork, disappointed with the effect on wine quality.

We are confident that with further experience winemakers will recognise the value a cork closure can add to their wine.

It would be surprising if good winemakers chose to completely ignore new products available in the market. However, as Penfolds Chief Winemaker Peter Gago has said, screwcaps must prove they can perform well over decades and still allow the wine to develop the characters wine-lovers value.

Winemakers should do due diligence and when they do we are confident they will recognise the value cork can add to their wines, a value that has been proven over centuries.

Not at all. The overwhelming majority of consumers in all markets prefer cork to screwcaps (and synthetic closures).

Even where consumers accept alternatives, they still prefer cork. In US four out of ten markets and in the UK three out of ten consumers say they dislike buying a wine sealed with a screwcap.

Research consistently shows that consumers value cork as a natural and environmentally friendly closure. They appreciate its traditional association with wine and that fact that it allows the wine to develop interesting and complex characters over time.

Some customers may value the convenience of screwcaps, but most still prefer cork. Amorim is investigating the potential to develop a technical cork that can match the convenience of screwcaps while retaining all the benefits of natural cork.

Market research has consistently shown that consumers prefer cork. They value it as a natural and environmentally friendly closure. They appreciate its traditional association with wine and the fact that it allows the wine to develop interesting and complex characters over time.

Even in Australia were screwcaps are more common consumers still overwhelming associate cork with wine for special occasions and associate screwcaps with cheaper wines.

Winemakers should take care before risking that special association with cork and the rituals of wine that set wine apart from other beverages.

Not at all. Amorim technical corks (Twin Top and Neutrocork), which compete directly with synthetic closures, have performed just as consistently as synthetic closures without the problems of premature oxidation and scalping that these closures incur.

Moreover, synthetic closures are just as vulnerable as cork and screwcaps to faulty product application, which can lead to sporadic post-bottling oxidation (‘random oxidation’).

One challenge for synthetic closures is that manufacturing changes that reduce oxygen permeability also increase the required extraction force, making it almost impossible to remove them from the bottle or the corkscrew.

Meanwhile, cork producers are working constantly to improve the consistency and overall performance of their product.

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