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Cork in winemaking: history, production, taint and Diam

As early as the fourth century Egyptians used cork in the production of fishing buoys. However, there is no consensus as to when the first cork was used to stopper a bottle of wine. Corks have been found in Roman shipwrecks dating from the fifth century BC, though it does not appear to have been the usual method of closure. After the fall of the Roman empire, global trade vastly decreased. Between 500 and 1500 cork farmers from the Iberian Peninsula struggled to their products and cork gradually disappeared. In the 17th century cork reemerged and for almost the last four centuries virtually every bottle of wine has been sealed using a cork. However, since the 1970s alternative solutions began to emerge and the cork monopoly looked to be in question. The cause of the onslaught, amongst other things, was a chemical compound known as TCA, otherwise known as cork taint. Despite the growing presence of alternative closures, millions of winemakers drinkers around the world refuse to budge. In this article, I will explore the history of cork in winemaking, it’s production, faults and the future.

A brief history of cork

In 1956, 20 bottles of vintage 1789 wine were found with corks in them, most notably the corks were in fine condition and the wine well-preserved. Around 9000 years ago, the Chinese were consuming a product made of wine, beer, honey and other products. Western wine was produced around a millennium later somewhere in the mountainous regions from eastern Turkey to Northern Iran. The discovery of pottery around 6000 BC made it possible for people to store wine, as well as to trade it. Winemakers soon learned that air is the enemy of wine, whilst some air is crucial, the wine will become vinegar if it stays in contact with air.

For millennia, winemaking was an endless struggle to halt the natural process of wine turning into vinegar. Soon enough winemakers developed containers not only intended to carry wine but also to protect it from the air, the most popular of these were amphoras, they remained in use for around 6000 years. Early amphoras were said to be sealed by putting a glob of wet clay around the top of the neck.

Around 3000 BC, Egypt was the centre of winemaking, having first imported wine they soon planted their own vines and took the rudimentary craft to new levels. Around 1500 BC, during the Egyptian golden ages, winemaking was somewhat sophisticated by ancient standards. Frescos in tombs show that the Egyptians had refined the craft of keeping air out of their amphora, organic material such as leaves and reeds were stuffed into their necks, a piece of pottery sat on top, and clay applied to create a sealed container.

The rise of the Greek city-states as major political powers begun around 800 BC, with this came innovation in product and practise. A number of Greek scholars left detailed accounts of advances in viticulture under the Greek empire, including ways of dealing with diseases. Scholars also wrote of how cork trees were harvested for various practical purposes, amongst these were bungs for casks.

Corks have subsequently been found in Roman shipwrecks from the fifth century BC to the fourth century AD. Amphoras dating from 500 BC show cork being used to seal containers, despite them not being the common closure. These corks differed a great deal from the corks of today, they were sealed with resin whilst today they are punched plugs which form a tight seal. Even as corks become more prominent, they were sealed with resin, archaeologists have found evidence of this in a number of amphorae found across the globe.

As was the case with many of their innovations, corks fell into disuse with the fall of the Roman empire as the Dark Ages fell on Europe. Trade shrunk dramatically and cork farmers struggled to find buyers for their produce. With this decline in trade came a change in political rule, the Moors were forbidden from drinking alcohol, wherever they went, viticulture did not. The Moors were not expelled from these lands for a further seven centuries. For almost a millennium wine went without a cork, many different solutions were adopted to avoid the dangers of oxidation. These solutions ranged from a small amount of olive oil being poured on top of the wine, acting as a barrier to oxidation, to stoppers held in place by pieces of string.

Whilst Cork was first imported to England in the 1300s, it was not until the 1500s that it was used as a stopper for wine. In the 1600s wine was mostly shipped in barrels and served to the customer in decanters. However, in 1632, Kenelm Digby introduced a bottle-making technology that was able to produce strong and inexpensive glass containers. Early bottles were hand blown and size was mostly determined by the blowers individual lung capacity. At this time corks were stuck in the top of the bottle and left sticking out, this was because a corkscrew had not yet been invented.

In the 17th century inexpensive bottles appeared in France and in 1703 a treaty between England and Portugal gave a major boost to cork production. In the 18th century bottle design changed, original glass bottles had been pear-shaped, making them difficult to store. With this change in shape came the fact that corks could now be kept moist during storage. In 1795 a patent was filed for what is thought to be the first corkscrew and by the end of 18th century the bottle, cork and corkscrew had brought about a new age in wine history. For the first time since the Romans, it was possible to enjoy aged wine, it is no surprise that Hugh Johnsons calls the cork the most important event in the history of fine wine.

The little-discussed decade

By all accounts, it appears there has always been a problem with cork taint, albeit at one point too small to be considered anything more than a passing nuisance. In 1957, author Charles Pouillaude spoke of ‘cork tastes’, citing the findings of a study undertaken by a leading Champagne house between 1933 and 1956, which proposed cork taint was present at a rate of around 0.3%. There are a number of other references to cork taint as a present but negligible problem throughout these years. However, in the 1980s the wine industry began running into more corked bottles. The important question, one which frames later discussion, is why did the problem get so much worse during this period?

No single factor can be blamed entirely and one must consider the lag of problems on the ground appearing in the market. It takes nine or ten years to grow raw material, another couple of years until that cork is put in to a bottle and any number of years before the cork is pulled.

The first of the causes worth considering is the lingering effect of drastic sociopolitical upheaval in Portugal. In 1974, a coup was staged by junior officers in the Portuguese armed forces, they overthrow the fascist regimes. Over the next two years Portugal underwent a major shift in political paradigm, for months Portugal barely functioned, the people were too busy protesting and the world too worried the nation would fall to communism. Just as the Soviets had strived for collectivism of the nations farms, there were those in Portugal who ‘liberated’ the cork forests from their owners and instated labor committees to run them, all of this whilst the owners, those with the knowledge of the land, fled abroad. An industry with a profound, complex and long view was being run by amateurs in deprived conditions who wanted yield not quality. Use of fertiliser and insecticide containing ingredients not entirely understood were used indiscriminately. Areas of the trees which were previously untouched were hacked at by untrained workers and gross damage done.

Following the April 1976 election, Portugal began a long, slow return to some form of stability. Even prior to the leftist regime, the cork industry was somewhat backwards, much of Portugal had been sheltered from the revolutions taking place in Western Europe. George Taber notes that in 1982, Terry Lee visited Portugal for the first time and was ‘appalled by what he found’. He wrote that chlorine was still being used to wash cork, raw bark was left out in the open for weeks and mould grew freely.

Counterintuitively, at a time where the industry was battling desperately to recover from sociopolitical trauma, and quality was perhaps most questionable, demand was increasing. New wine regions, namely Australia and New Zealand, created new demand; however, there are recordings of entire shipments of cork arriving contaminated. Wood preservatives painted on the deck of ships contained chlorine, this would overact the mouldy corks and cause TCA. Given the increase in demand, there were reports that Portuguese producers were boiling cork in dirty water, sometimes used four or five times without replacing, there were simply no quality controls in place.

By the late 1980s, there was much greater public awareness of TCA, winemakers and wine drinkers alike were conscious. For many years there had been no alternative to cork, the industry had a monopoly, they had no incentive to compete, no incentive to improve quality, for there was no replacement. Most of the concerns of winemakers were shaken off by producers of cork, winemakers were told it was down to their storage opposed to the production, or from products in their wineries.

By the end of the 1980s, a number of winemakers and drinkers had had enough, work had begun on a number of alternative closures, and competition kicked the cork industry into action, the time had come for change. It is important to understand the factors which defined this decade as there can be little doubt to the extent at which they shaped the future closure industry.

Growing, harvesting and production

Cork is formed from the bark of Quercus suber. These trees are predominantly found in Mediterranean countries such as Spain and Portugal (80% of the global population), where there are around 730,000ha and 300 landowners.  Although attempts have been made to cultivate cork elsewhere, it still mainly growers around the western-Mediterranean. The tree reaches maturity after around 25 years of growing. Once maturity has been reached, specially trained cork harvesters will begin to strip the bark (the outer layer is dead whilst the inner alive) using an axe. The whole process of removing the bark is done skillfully by hand, usually by a team of two, one senior and one apprentice. Together they can strip a tree in 15-30 minutes, totalling 15-30kg of bark worth around 25 euros.

The year the cork is harvested from a particular tree will be marked on the tree so that it isn’t harvested again too soon. The tree needs a minimum of nine years to regrow enough bark to be harvestable again. The best cork comes from forests which are maintained under the Forestry Stewarding Council which ensures sustainable and environmentally-friendly methods are employed.

Typically, trees will be harvested every nine to twelve years. This process does not harm the tree, making it a sustainable and renewable resource for the wine industry. With a lifespan of between 150 and 250 years, each tree is able to provide around 12 harvests worth of cork throughout its lifetime. Of the 340,000 tonnes of cork that is gathered each year, only 15% of that gets made into wine corks; however, the wine industry brings over 66% of the cork industry’s revenue from that amount!

Overview of cork manufacturing process courtesy of Ronan Sayburn MS

Before anything happens with the bark, it will be left outside for a few months to cure. The most diligent of cork producers will ensure the bark is kept on concrete, not natural material to avoid contamination, the bottom plank is usually discarded to further reduce risk of contamination. The planks of cork will be loaded up on palettes and taken to a processing facility to be cleaned. Firstly they will be boiled, which helps to both clean the cork and soften it up. In years gone by planks would be boiled in murky pits without the water being changed very often. Now, to avoid cross-contamination, the water is cleaned, filtered and replenished regularly, with volatiles being removed on a continuous basis.

Boiled planks are also usually flatter, making them easier to work with and turn into corks. The water it is soaked in will also contain a fungicide to ensure that the cork is free of any bad fungus or mould. At this time, any bad, or low-quality bark will be stripped away. The remaining planks will be stacked up and left in a humidity-controlled space for a few weeks. 

Each plank is then graded based on quality and cut into smaller strips. The best cork is sent to be hand punched into corks, while the rest will be ground up to make technical corks. They may then be further cleaned and sterilised. Next, the corks are sorted both optically by a machine and then again by eye to ensure that the corks are organised by the correct grading. The top grade cork will be the most expensive, and will be used to make the best wine bottles complete! 

Many pieces of bark will be too thin to form a high-quality natural cork out of, as it needs to be thick enough to have no gaps at all to stop wine spilling out! There is also leftover cork between the strips where corks are punched out that can be used. To stop bark going to waste, the remaining pieces of cork are ground up and then fused together to form a ‘technical cork’. At this stage, discs are also cut to make the end pieces for sparkling closures and technical closures.

Jamie Goode has a useful illustrated guide to cork production on his website Wine Anorak.

Cork Taint

Cork taint is an absolute wine fault (by which I mean almost any level is considered undesirable) and is characterised by a set of undesirable smells and/or tastes. Though modern studies have shown that other factors can also be responsible for taint including wooden barrels, storage conditions and the transport of corks and wine, the cork itself is commonly considered to be responsible.

The primary compounds responsible for cork taint are 2,4,6-trichloroanisole (TCA) or 2,4,6-tribromoanisole (TBA), the compounds form through the interaction of plant phenols (from the breakdown of lignin) chlorine and mould. TCA and TBA are responsible for the vast majority of cases of cork taint, but other less common and less known compounds that can cause different varieties including guaiacolgeosmin2-methylisoborneol (MIB), octen-3-ol and also octen-3-one, each has its own aroma, all of them considered objectionable in wine.

Most incidents of TCA occur when naturally occurring fungi are presented with chlorophenol compounds, which they then convert into chlorinated anisole derivatives, such as TCA. Chlorophenols taken up by cork trees are an industrial pollutant found in many pesticides and wood preservatives, which may mean that the incidence of cork taint has risen in modern times. As Jamie Goode notes in Flawless, it is widely accepted that the major source of environmental chlorine in cork oak forests is organochlorine insecticides that were widely used from the 1950s to 1980s. Chlorophenols can also be a product of the chlorine bleaching process used to sterilise corks (not in use anymore); this has led to the increasing adoption of methods such as peroxide bleaching. Furthermore, taint can originate elsewhere in the winery, where damp surfaces and chlorine-based cleaning products are commonplace. There is also a controversial discussion around the role of cooperages in the incidence of TCA.

Whilst the human threshold for detecting TCA is measured in the single-digit parts per trillion, this can vary by several orders of magnitude depending on an individual’s sensitivity. Detection is also complicated by the olfactory system’s particularly quick habituation to TCA, making the smell less obvious on each subsequent sniff.


Whilst Diam is in a sense an ‘alternative’ closure, it fundamentally remains a cork product. That being said, Diam is particularly interesting from the perspective of category innovation and so, in my opinion, deserves its own subheading. Diam is currently run by Dominique Tourneix, a former food processing engineer who was responsible for assessing the best and most efficient way to manage food. Having worked for Mars for 12 years, Dominique joined the Diam company (not at that time named Diam) in 2003. He joined to work on an interesting patent, one which used supercritical Co2, the same process as is used to decaffeinate coffee.

Diam starts with raw cork which they purchase directly, once they have received the cork they break it down into tiny particles and treat it using their patented supercritical Co2 DIAMANT technology. At a certain temperature and pressure, Co2 lives in a state between liquid and gas, this is known as the “supercritical” state. when in this supercritical state, Co2 can penetrate deeply into the cork and remove any compounds of TCA. Although supercritical Co2 technology was already being used in some industrial applications (decaffeination, nicotine extraction from tobacco, etc.), it took 7 years of joint research before it could be applied to cork.

As a result of the DIAMANT technology, the cork Diam uses is free of flavour, void of TCA, and many other substances causing flavour modification known as scalping. Stringent testing at every stage of production means that Diam is the only cork individually guaranteed free of releasable TCA.

After this treatment, the TCA-free particles are put back together using another patented technology which uses food-grade binding agents and food-grade microspheres to provide elasticity and consistency in the finished corks.

Diam has also focused a great deal on sustainability, most notably it is resource-efficient. Consider a rectangular piece of cork bark, once small cylinders are punched from that piece there remains excess cork, or wastage, from that original piece of bark, cork which that didn’t get punched out into a Traditionally, this wastage has been discarded; however, because Diam grinds down cork into small particles, they are able to use the entire piece of bark without any wastage at all.

The wine industry appears particularly resilient to change, perhaps even more so than many others, no matter what the topic you will find staunch opposition. However, if one is to consider the most common arguments against Diam, it is strikingly obvious that they do not include cork taint. Even if Diam were to be subject to some of the challenges of regular cork, most notably oxidation, it would remain a net benefit in that Diam eradicates the possibility of cork taint. Albeit anecdotal, I am yet to drink a wine stored under Diam which has shown any sign of storage-related faults or flaws. Interestingly, I believe Diam’s patent is near expiration, meaning theoretically competitors will be able to enter the market, that being said there are significant barriers to entry (namely cost and established trust) and as such it is unlikely Diam will be dethroned anytime soon.

The future

According to Harpers, Portuguese closure manufacturer Cork Supply has unveiled new TCA extraction technology that it claims can achieve a success rate of 99.85% in eliminating the risk of cork taint in cork-sealed wines. If this claim proves to be true, this could, whilst still not ticking all of the Diam’s boxes, provide an alternative for those who question the efficacy of Diam and go further toward fault-free wine closures.

For anybody observing, it seems evidentially obvious that alternative closures will continue to grow in prevalence in the near future; however, that being said I am absolutely certain that corks will remain a fundamental form of closure for many years to come. Many consumers, rightly or wrongly, associate cork with quality, there is also the romantic notion of opening a bottle under cork, and additionally, producers show a preference for ageing under high-quality cork.

George M. Taber ‘To Cork or Not To Cork‘ was an invaluable resource in producing this article.

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