All this time we’ve been together and we haven’t once talked about cork. Can you believe it? No, I can’t either. The truth is, I planned to write about cork 11 months ago, and then something else popped into my head and drove it out. I really must buy a larger-capacity memory stick.

I’ve been pulling corks out of wine bottles for the last half-century, without giving a moment’s thought to the interesting questions: Why do they use cork to seal wine bottles? Where does the cork come from?

I’m guessing you erudite lot don’t think cork is off-topic; just in case you do, let me tell you that Portugal is the world’s largest producer of cork, being responsible for 49.5% of annual production worldwide. Another 30.5% comes from Spain, and the remaining 20% comes from other European and North African countries in the western half of the Mediterranean basin, whose temperate climate is ideal for these trees.

You don’t have to be in Portugal long to discover the cork trees, or cork oaks, as they are properly known, or quercus suber (which is ‘cork oak’ in Latin and therefore posher), or, in Portuguese, sobreiro. Although the main concentration is in the South, the trees are cultivated as far north as Penamacor, where they can easily be spotted in the mixed-tree forest. 25% of Portugal’s total forest area consists of cork trees.

In the south, over a million hectares are covered by the montado system, an ecologically balanced multi-functional agro-silvo-pastoral system, which has existed for over 1300 years; during all that time, the oak trees have been protected by law, and a Government permit is needed to fell any cork oak.

This three-function ecology, supporting agricultural crops, fruit-bearing trees (the oaks’ acorns) and also grazing land, ensures the continued viability of the soil, acts as a barrier to the desertification encroaching from the south, and produces over a billion dollars in export value of natural and agglomerated cork, making cork Portugal’s sixth most valuable export.

Being an evergreen, the cork oak also photosynthesises throughout the year, and captures an estimated 73 tonnes of carbon dioxide for every tonne of cork harvested. As if that were not enough, the cork forests provide a habitat for more than 130 different species of vertebrates and about 95% of all land mammals in Portugal, including the Iberian lynx that I encountered, stuffed, in Penamacor’s museum.  

As a result of all this, the montados are recognized as one of the 36 biodiversity hotspots in the world and have UNESCO-protected status. In addition, the cork oak was officially designated Portugal’s national tree in 2011.

There are two final environmental boxes that cork ticks.

The harvested cork retains the ability to absorb carbon dioxide, so your unvarnished cork noticeboard is actually helping the planet.

Because of its cell structure, cork does not burn easily. In addition, it produces no flame. It is, therefore, a very good fire retardant, which, in Portugal, is a significant advantage. You may remember I wrote many months ago about Portugal’s recent spate of forest fires.

Now, I’m the kind of person who can recognize a Christmas tree, even without fairy lights (although I can’t tell my spruce from my pine), a horse-chestnut (after a youth heavily invested in conkers), an olive (you can’t walk 50 yards in Maale Adumim without tripping over the roots of an olive tree), a loquat (we have one in our garden) and a cypress (we used to have one in our garden till it started encroaching on the flight-path of passing planes)….and…and…and that’s about it. However, I quickly learnt to recognize the cork trees around Penamacor, at least the mature ones, because of the extraordinary nature of the tree and the way the cork is harvested.

Cork is basically an outer bark of the tree. It can be stripped away, leaving the inner bark, without damaging the tree, and the tree then produces new cork, growing a new outer skin. The stripping of the cork is strictly regulated by law; only a mature tree (at least 25 years old) can be stripped; the width of the band stripped from around the tree cannot be more than three times the diameter of the tree; a tree cannot be stripped more often than once every nine years.

Since the trees have a lifespan of under 200 years, on average a tree will be stripped 17 times in its lifetime. The first and second strippings of virgin cork do not have a sufficiently regular structure, and are too hard, to be used for bottle corks. They instead are used for tiling and decorative items.

So that’s the ‘Where from’; what about the ‘Why’?

Cork has a honeycomb cell structure which gives it remarkable insulating properties.  It’s flexible, compressible and elastic as well as lightweight, impermeable, durable and hypoallergenic. All of this makes it an ideal material for sealing a bottle of wine. And yet….

For white wines, and other wines designed to be drunk young, a completely airtight seal is ideal. This can be provided by an aluminium cap (screwtop). To the best of my understanding, if you keep your white wine for any length of time before opening, a screwtop might be the most reliable way of retaining the full flavour of the wine.

However, many reds are bottled in the expectation that they will mature in the bottle, and they are only ready to drink some time later, and may indeed reach their prime several years later. For this maturation in the bottle to occur, the wine needs to be exposed to some air…but not too much. If the right cork (and I believe this is a function of the exact degree of elasticity and compressibility of the cork, the tightness of the honeycomb ‘weave’) is married to the right wine, then natural cork does a better job than any other material of helping the wine achieve its full flavour.

The big disadvantage of cork is that it can (in a small percentage of cases) taint the wine, so that the wine becomes ‘corked’. How it does that is interesting. Cork, like any wood material, attracts fungal micro-organisms. If the cork has been exposed to certain chlorinated compounds (sometimes used in pesticides), these compounds can combine with the fungal micro-organisms to create TCA, which can be retained in the cork and later dissolved into the wine. While completely harmless and odourless in itself, this TCA can then distort the way our brain perceives smell, and make the wine taste, to us, like mouldy newspaper or wet dog. ‘I’m picking up notes of underripe strawberry and previously flooded basement.’ Remarkably, some people can detect TCA at one part per trillion, which is the equivalent of one thousandth of a teaspoon in an Olympic swimming pool.

However, just as chemistry created this problem, chemistry can resolve it. If you open a bottle at home and find it corked, pour the wine into a bowl lined with cling film. Leave for a few minutes while the non-polar TCA molecules are drawn to the chemically-similar plastic. Decant the wine from the bowl and you will find the essence of soggy doggy gone. (In a restaurant, of course, you simply summon the sommelier and inform him that the wine is corked. If the restaurant does not have a sommelier, you call over the waiter and tell him it stinks.)

Synthetic ‘corks’ avoid this problem, but do not have the same subtly imperfect hermeticity, and therefore cannot be matched as well with great reds.

So, that’s more than you may have wanted to know about cork.

There’s just one more piece of business today. A couple of weeks ago, some of you were impressed by Tao’s skills at breaking into kitchen cabinets. You will be interested to know that he has, since then, upped his game considerably.

To forestall any comments:

1. Tao is entirely self-taught; he learnt this skill purely through observation.
2. There are no caustic products in that cupboard. The washing powder and other products are home-made and not hazardous.

Nevertheless, I think we’ll be returning those child-proof locks and asking for our money back.