The story of a cork stopper

Christmas festivities are not only an occasion to meet family and friends, but in some cases, especially in my food-loving home country – Italy, it represents a demanding seven-days gastronomic marathon. During last year season holidays, Italians have been eating a hundred million kilos of pandoro and panettone, twenty thousand tons of pasta and 6.5 million kilos of cotechini and pig’s trotters. To go with the meal and to welcome the new year Italians popped fifty million sparkling wine cork stoppers.

Corks are simple objects that have an age-old tradition and an interesting story. The first references to cork dates back to the third millennium BC, when in Egypt and Persia it was used by fishermen for its flotation properties. Its most ancient use with wine storage purposes is documented by some two thousand years old cork-sealed amphorae found in Ephesus and Pompeii.
Cork has entered history once again in more recent times thanks to the work of the British scientist Robert Hooke. In his book Micrographia, he described the appearance of a small piece of cork when he observed it with a pioneering microscope and coined one of the most used terms in modern biology: cell.

I could exceeding plainly perceive it to be all perforated and porous, much like a Honey-comb, but that the pores of it were not regular. These pores, or cells, […] were indeed the first microscopical pores I ever saw, and perhaps, that were ever seen.

cork microscope
On the left, a drawing of cork structure as Hook observed it with his microscope. Modern microscopes reach much higer magnifications (350X in the picture on the right). Wikimedia Commons, www.realcork.orgPortuguese Cork Association

Today, we know that the structures that Hook observed that looked like monastic cells are the result of a biological process called ‘Programmed Cell Death’. In fact, during the maturation of the bark the cells that compose it die and are emptied of their contents to leave only the cell walls that are mostly made of cellulose, suberin and waxes. This layer of dead cells has the function of protecting the plant against external agents and is only partially permeable to water and gases.

This process occurs in all tree species, but in the cork oak, Quercus suber, it produces a particularly light, elastic and waterproof material, suitable for sealing liquid containers. The air-tightness could however be a problem for the inner part of the plant stem which, just like any metabolically active plat tissue, needs to exchange gases with the surrounding air. This need is met by the existence of lenticels, specialized bark structures where cells are less closely packed and are visible in cork stoppers as dark streaks.

Quercia da sughero
A recently peeled cork oak in a Portuguese forest. The frequent bark removal that is usually done every ten years shortens the tree life to 100-150 years. www.realcork.orgPortuguese Cork Association

The cork oak is a tree that cannot survive winter frosts and therefore is only grown in areas with a mild climate along western Mediterranean coast and in Portugal. The latter produces more than a hundred thousand tons of cork every year, half the total global production, and is followed by Spain, Morocco, Tunisia, Algeria and Italy, which accounts for nearly 3% of world production.

Area quercia da sughero
Natural distribution of Quercus suberWikimedia Commons

Corks production therefore begins in the forest, where oaks are grown for 15-20 years before producing the first cork layer. This first stripping consist of a poor-quality cork called “virgin cork”, which has little commercial value because of its inhomogeneity and sponginess. Even though cork oaks are monoecious trees which carry both male and female flowers on the same plant, this type of cork from young plants is called male cork to be distinguished from the female cork which is produced later. In fact, after the removal of the first layer, the cells surrounding the stem divide rapidly to generate new layers of higher-quality cork. In order to obtain a cork layer of the proper thickness for the production of stoppers, a ten year interval of undisturbed growth is required between each round of decortication.

Sughero decorticato
Immediately after cork harvesting, the underlying tissues display a reddish color, but they rapidly turn gray within a few weeks. Wikimedia Commons

Following the harvesting, cork planks are boiled and seasoned to make them softer and more elastic, to increase their thickness and to extract the water-soluble compounds. The most important function of this step is however the sterilization and polishing, and often it also requires some type of chemical treatment. The removal of microorganisms such as the parasitic fungus Armillaria mellea which grows on cork oaks is in fact crucial for the production of stoppers that will not compromise the taste and quality of wine. If spores are not completely eliminated during cork processing, the fungus can proliferate in the stoppers and produce a compound called trichloroanisole. This is the substance responsible for the “corky smell” which affects 1% to 15% of the wine bottles.

Tappi sughero fustellati Portuguese Cork Association

After cleaning, cork planks are first pressed to obtain a smooth texture and later cut into the desired shape. The most valuable natural stoppers consist of a single piece of cork directly cut from the plank. However this type of processing produces up to the 70% of waste of the initial raw material. This waste is normally used in the production of the so called “technical” cork stoppers. The cork waste is fragmented into small granules which are then agglomerated using food-grade glues. The bodies are then either individually moulded or extruded, and for special preparation, such as for sparkling white wine bottles, cork disks are glued to the end of the agglomerated cylinder to the side that will be in contact with the liquid.

Ready to tell your friends the story of the cork stopper you will pop tonight?

Tappi sughero tecnici e naturali
On the left, a one-piece stopper directly cut from the a cork plank. On the right, a technical stopper for sparkling wine formed by a granulated cork body and two cork disks. www.realcork.orgPortuguese Cork Association

– Raven P. et al., Biology of Plants

Cover: Pixabay

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