What is a cork? Each cylinder of bark comprises millions of gas and wax-filled impermeable cells which have the properties of elasticity and the ability to be compressed in one direction without increasing in another. Unlike my stomach. So says Jamie Goode (though he fails to mention my tummy) in Wine Science - an invaluable book. Cork is used for many other applications, from the table mat to the lining of protective headgear, but its longest association is with wine. Romans used it to seal amphorae, for instance. Before and since Roman times we have wandered off piste a bit when it comes to wine storage and transport mediums, as evidenced by biblical injunctions not to put new wine into old skins, for instance. And there’s much mention of jars in old history: cork would, when available, have been the best sort of wood to tamp into the top to make a decent seal.
There is, confusingly, more than one book called Wine Science and in his, Ron Jackson says Portugal is by far the largest producing country, but others include Spain, Algeria, Morocco, Italy, France and Greece. Dry, rocky soils at altitude make the best corks, he reckons, because in these conditions the bark is more resilient. Those grown at lower altitudes aren’t as firm or resilient. Jackson also confirms that it is only from the third harvest that wine bottle quality corks are produced.
The tree that produces cork is of course our old friend the oak tree, but a very particular one. The Quercus Suber is not unique, but certainly rare in that its bark can be harvested for the cork material. It is not until it has reached 25 years of growth that the Quercus Suber can tolerate the removal of its bark without causing damage to the tree. It is then at nine-year intervals thereafter that the cork harvest takes place, to allow enough time for the regeneration of its bark. As stated by Jackson it is not until the third harvest, and at more than 40 years old, that the oak tree produces the dense cork needed for wine stoppers. The suber part of Quercus Suber gives the name to the complex multi-molecular properties, which enables cork to do its job. Jamie Goode reports that suberinization is still somewhat of a mystery and involves a lot of very complex biochemistry based around the honeycomb structure.
That flexibility of cork can make a fine closure for a bottle. At its best its permeability to oxygen can aid a wine’s development over a 50 year and longer period. It is a satisfying ritual of removing the cork from the bottle that consumers like and miss from other closures... In terms of aroma and flavour, no cork is neutral. Ask almost any producer of aromatic whites and they will tell you that the decision to use cork is never taken lightly. You’ll find alternatives all over the new world as well as Germany and Austria for these wines. Mostly though it is a quality judgement on the part of the wine producer; do they like the aromas imparted by the cork, or do they want a more neutral stopper?
As an example, we have Dog Point from New Zealand; a country where it is difficult to find a bottle under cork. Yet three out of the four that we stock are sealed with cork, including two white wines. Dog Point very much believes that good cork is unbeatable for wines that have already had contact with oak (barrels), though they use the screw cap for unoaked wine.
What is the process cork undergoes?
Experienced men armed with special axes climb the trees and precisely strip the bark to the correct depth, without damaging the tree. The first extraction can only take place once the tree reaches 1.3 meters in height and a 0.70cm perimeter. After collection, the bark is taken to the factory where it is sorted and trimmed prior to the first wash. For this the whole slabs are boiled or steamed for around 20 minutes. After this it’s normal for the bark to be dried for a couple of weeks. Then the bark is cut into strips and natural corks are punched through the sides of those strips, so that the cork length is parallel to the tree rings. This means that the open cells are pressed up against the side of the bottle neck, which is a natural way of sealing them. Now that the cork looks like a cork it passes through further washes and tests that should ensure total hygiene and structural integrity. After all this washing the corks are air-dried and sorted for size and quality. Many corks are waxed or coated with silicon prior to shipping to ease insertion and removal. Ron Jackson’s Wine Science has more details of this than one could possibly need.
As well as natural cork there are a variety of other types of cork available:
- Natural stoppers are made from one single piece of punched cork and is considered the crème de la crème of corks. These are most commonly used in premium wines that mature in the bottle for longer
- Technical stoppers are made from cork granules pressed into an agglomerated cork body, with natural cork disks glued to one or both ends. These corks are designed for wines that are consumed within two to three years
- Champagne stoppers are a technical cork but with a larger diameter agglomerated body and two or three natural cork disks at one end. The larger diameter supports the high pressure of the bubbles in the bottle
- Agglomerated stoppers are made entirely from the by-product cork granules from the production of natural cork stoppers. A cost-effective solution that offers the perfect seal for wines consumed within one or two years after bottling
Why do we use cork?
If you have wine in a glass bottle and you would like that wine to mature gracefully, then cork bark can be a decent medium. Its origin mirrors that of the vine and the olive. They are all trees that grow well in areas where cultivation of more essential crops would be difficult. Quercus Suber has the additional advantage of growing more naturally, generally in a biodiverse environment. The cork oak forest is one of 35 global biodiversity hotspots in the world, and in large tracts of countryside that go largely undisturbed by humans between the nine-yearly harvest programme. Cork is as natural a product as wine and is sustainable and recyclable.
In her Corks Fight Back article published recently in August 2017, Jancis Robinson discussed much of the current cork-based thinking. I’d urge you to read this article (link below) and it would therefore be redundant to quote much of it, but let us look at the price of stoppers, since that is a not insignificant element – especially of cheaper wines.
Generally, natural cork stoppers are charged at anywhere between 3 cents – €2 each, whereas plastic stoppers are charged at between 2 – 4.5 cents and screwcaps at 6 – 10 cents.
What don’t we like about cork?
Like all products that rely on a controlled and predictable set of behaviours – that elasticity is crucial. Some of our winegrowers have noted that over the years many corks have become softer, so initially more elastic, but with a shorter life in terms of that elasticity. Jean-Marie Fourrier told us that when he was young, all corks had to be soaked in hot water for half an hour to get them pliable enough to get them into the bottle. To counter this, many of our growers are buying wider diameter corks than they used to, so that they are more compressed. It is also possible to buy bottles with longer, straighter necks so that the corks can’t flare at the base. Egress of wine and ingress of oxygen is the potential problem here.
I need to mention cork taint. This is usually credited to 2,4,6-trichloroanisole (TCA), though other anisoles are involved and not all of them are cork derived, including TCA. Taint can be transmitted from barrels, pallets and other wood components in a winery. Nor is TCA restricted to wine. Marks and Spencer food technologists have found ‘corked’ chickens and bagged salads with their chlorine washes are very susceptible. Sometimes the cork cleaning treatments can be to blame at least anecdotally. Fourrier is very sceptical of treatments that involve peroxide and peracetic, for instance. These are two things anathema to wine production.
Of course, as with most things in life, we only notice the few problem corks and not the vast majority that do their duty properly.
And finally, as a natural product, cork is eminently recyclable. If you have a load you want to be rid of and you haven’t yet made a framed cork board out of your finest drinking, just drop them into us and into our cork recycling box in association with Recorked UK.
This month we have collaborated with APCOR to bring you a specially designed seminar and wine tasting with cork ambassador Sam Caporn MW.
The ‘Need closure? This is the tasting for you’ tasting will take place on Thursday 28th September 2017 between 7-9pm. Sam Caporn MW will seek to separate science from mythology for you.
We all know that cork is a fascinating topic, controversial and conventional in equal measure. On one hand reassuring and utterly traditional; on the other a source of flavour and faults, the bit of tree bark that wine producers have been banging into the tops of our bottles for what seems like time immemorial has fans and detractors. /CW September 2017
Wine Science: Jamie Goode
Wine Science: Ron S Jackson
The Wines and Vineyards of Portugal: Richard Mayson