Natural Cork - Raw Material and Production Process

Perfect by nature

It is from the cork oak (Quercus Suber L.) that one of nature's most extraordinary products is harvested: cork. A raw material which is so perfect that no industrial or technological processes have yet been able to replicate. Only such a material could give rise to the demanding and sophisticated device that is the cork stopper.

Cork is the bark of the cork oak tree which is stripped every nine years, always between May and August, when the tree is at its most active phase of growth and it is easier to strip without damaging the trunk. The work is done by specialised professionals, who, with absolute precision, are able to handle the only tool used in this delicate process: an axe.

At dawn, the sound of the axe can already be heard making small groves in the brown, leathery trunk of the cork oaks. The cork is removed in large bulging planks, which are very light and still damp from the tree's sap. An average of 40 to 60 kg of cork is harvested from each cork oak. The thinner planks are to be used in the production of disks for technical stoppers or other materials. Only the best quality planks, which are thicker and softer, after resting for six months in the factory's yard and being subjected to state-of-the-art treatments, shall give rise to the most distinguished stoppers - the natural cork stopper, the preferred choice to seal the most prestigious wines.

Those who strip the cork from the trees feel that it is a raw material unlike any other. Although, perhaps they are not aware of its surprising intrinsic characteristics. It consists of a honeycomb of microscopic suberin cells, filled with an air-like gas. This makes it very light, easy to compress, impermeable to liquids and gases, an insulator against moisture and noise, and resilient. It has an «elastic memory» which enables it to adapt to variations in temperature and pressure. It is resistant to fire, abrasion and wear and tear and does not absorb dust. It is one of the most versatile materials available to Man. Furthermore, it is completely natural, biodegradable, renewable and recyclable.

Centuries of experience

The removal of the planks makes a dry noise, reminiscent of the gentle creaking of a door, the sound of a cork passing through the bottleneck. A particularly characteristic aroma fills the air, which is slightly sweet, while the men move on to the next tree, for the process to be repeated over and over again. Due to the precision, seasonality and experience it requires, it is the best paid agricultural work in the world.

Contrary to what happens in the following stages in the cork stopper production process, which is carried out with state-of-the-art technology, stripping is the only process which has been used over the centuries and practically remained unchanged, since Man began to commercially exploit cork to seal wine.

The most noble of products needs time

The recently stripped trunk is orange in colour and trees take on a curious and unusual appearance, only seen in cork oak forests, which only feature in some Mediterranean countries - Portugal, Spain, Italy, France, Morocco, Tunisia and Algeria.

No trees are cut down, which makes it a prime example of sustainability and interdependence between the cork industry and preserving the environment. Stripped from the base up to where the branches grow, they are marked with white paint with a number that indicates the year the cork was harvested. Over the following nine years, they shall continue to flourish and regenerate until they are ready to repeat the ritual.

It takes each cork oak 25 years before it can be stripped for the first time and it is only from the third harvest that the cork will have reached the high standard of quality required for producing cork stoppers. The first two harvests yield the raw material for insulation, flooring and other purposes. This means that to produce the high quality cork for stoppers, it takes each cork oak over 40 years. It is an unhurried tree, with a lifespan that can reach 200 years. In its lifetime, a cork oak may be harvested around 17 times, although there are extremely rare cases which have exceeded this target.

A purpose for each plank

The cork leaves the cork oak forest in trucks bound for the yard where, for a minimum of six months, it shall stabilise and develop an even moisture content. Each load is labelled and registered so that it is possible to trace the origin of an individual batch to the forest - traceability is very important to guarantee quality and a key factor in maintaining integrity in the chain-of-custody, which is essential to obtaining certification by the Forest Stewardship Council® (FSC).

The planks are stacked onto stainless steel structures in large concrete areas, built at a slight angle. This procedure prevents the cork from being in contact with the ground and shows significant results in reducing microbial contamination. The stacks are piled in such a way that they maximise water drainage and air circulation. Cork which has recently arrived is conspicuous in the yard. It is brighter in colour, next to the grey tones of the cork placed there longer ago.

After the resting period, the planks are sorted. Skilled hands select the high quality cork to be transformed into stoppers. The thick cork, with the height needed to extract a whole long single stopper, is to be used for natural stoppers. The thin cork shall be transformed into disks, to be used in the technical corks, Twin Top and Spark.

The cork from the first and second harvest, which has not yet reached the standard of quality required for manufacturing stoppers, as well as that which is harvested from the base of the tree, shall be ground and used to make other products for a wide range of areas such as construction, aeronautics, fashion and design.

Prevention and treatment at the highest level

From the plank to the final product, cork goes through a series of stages which differ according to the type of stopper to be produced. However, in regard to prevention and treatment, all planks for stoppers are subject to the same level of care.

The planks are boiled in stainless steel closed and filtered tanks. The goal is to remove organic objects embedded in the pores and enable the cork to reach the ideal moisture content for processing. The boiling process especially implemented by Amorim called CONVEX, also captures and removes volatile organic compounds from the boiling water.

Each two tonne batch is boiled for an hour in a high technology treatment tank. This process causes the inside of the cork cells to expand, improves the internal structure and contributes to reducing microflora. When the planks are taken out, their volume has increased by about 20%. Besides becoming thicker, they are more regular in structure and easier to handle. However, they are too damp and therefore shall be stacked and stabilised to rest in a sterile area with good ventilation. After two days, they are ready to be processed - a clear advantage of the effectiveness of the CONVEX system, compared with the three weeks required with the old process.

The edges of the planks are prepared and the corners trimmed before an initial manual assessment takes place. The planks are separated into quality categories, based on their thickness, porosity and appearance. Those with defects are sent to be ground and shall be used in other products. Although only good quality planks are selected for producing cork stoppers, none of the cork is wasted.

Natural Cork Stoppers

The planks to be used for top of the range - natural - cork stoppers, are cut into strips and punched with a machine to extract the cylindrical stoppers. Each cylinder is a whole stopper, which results from a highly precise manual or semi-automatic process. Skilled workers can punch up to twenty thousand stoppers a day. The manual punching process results in greater consistency in quality, since each worker can choose the best segments of the strips of cork. In this way, an ideal balance is achieved between quantity and quality.

The stoppers are separated mechanically and photographed by sophisticated computers which classify them on the basis of visual quality. Many by-products from this phase are used to manufacture cork granulate and used in technical stoppers. Only around 25% of the cork is used in the manufacture of natural stoppers.

After the punching process, a statistically significant sample from each batch is subject to analysis by gas chromatography, a highly sophisticated and effective process which aims to detect undesirable components, namely TCA precursors. The batches undergo treatment with the ROSA Evolution system, an innovative system patented by Amorim, which is unique in the market.

Each batch of stoppers is submitted for analysis. Whenever the minimum percentage of TCA is shown, it is immediately returned to the source, for new corrective measures to be applied. Irrespective of the results of the analysis, all the stoppers in the high quality batches are submitted to a steam distillation process, which provides an additional guarantee with huge advantages for wine producers.

Near the end of the final stage of the process, the natural stoppers are polished, resulting in a clean, smooth finish. They are then washed in an aqueous hydrogen peroxide (H2O2) solution, dried in industrial ovens and stabilised. All these processes aim to minimise the risk of microbial contamination.

After confirming that the stoppers meet the client's specifications, they are branded, with heat or ink approved by the Food and Drug Administration (FDA) and covered with a fine film of paraffin or silicone to facilitate their insertion into and extraction from the bottle.

Finally, the batches are packed into polyethylene bags which contain a widely-used preservative in the wine industry (SO2) and the packages are sealed.

Technical stoppers

Technical stoppers are made from granules produced from the by-products of high quality cork, from the narrow planks and punching of natural cork stoppers, and may or may not include natural cork disks.

The planks for manufacturing disks are selected and boiled using the same process as that used for the planks for natural stoppers. After stabilising, both sides of the planks are trimmed to produce smooth uniform sheets about 6.5 mm thick. These sheets are mechanically punched along the growth lines of the cork, which shall enable the disks to exert strong uniform pressure inside the bottleneck of the bottle of wine. The disks are then electronically sorted into categories.

The disks are the part of the technical stopper which come into contact with the wine. To ensure deep cleaning, Amorim has patented the INOS II washing process, which pumps purified, chlorine-free water at 70ºC. This washing process causes the cork cells to contract and expand forcing contaminants out.

The granules to be used in the body of technical stoppers are pulverised and screened before agglomeration. Those with a diameter between 3 mm and 7 mm are selected, as they provide the greatest strength and elasticity to the stopper. They all undergo the ROSA Evolution steam distillation process.

Subsequently, the body is moulded individually or obtained by extrusion and cut to size. Both agglomeration processes use glue approved by the FDA.

The stoppers are assembled mechanically. The disks and bodies are inserted from silos. On the way, the disks pass through a sorter that uses cameras to determine the upper side of the disk which shall be in contact with the wine. The other side is glued to the body of the stopper. After assembly, the stopper is oven-dried for an hour before being stabilised. Technical stoppers undergo an identical polishing, washing and finishing process to natural stoppers.

After the packages have been sealed, the cork stoppers are ready to discover the world's finest wine.

Production Flowchart

Floor and Cork Coverings

Floor and Cork Coverings
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Composite Cork
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Insulation Cork Board
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