The bark is extracted in the summer, when the cork's cells growth reaches its peak. After extraction, a new protective layer is formed at an annual growth rate of 2-6 mm according to the tree and the local conditions.
The type of cork that is obtained after the third extraction is known as amadia. This cork will be used in the production of cork stoppers. During the tree's lifespan (approximately 150 years), extraction will occur yet another 12 to 15 times. The cork is highly used in industry because of its major distinctive properties:
After being burnt, the residues (cork powder) are used in the production of thermal energy, namely heating boilers and other appliances.
In the following 6 months after extraction, a period of time in which the plank releases its saps and moisture contents, the cork will be boiled in water.
At this stage, an aqueous extraction at boiling temperature takes place thus identifying compounds with long carbonated chains, polyphenols (the tannin content is reduced to more than 50%) and sugars in batches. The cork will also increase in size and improve its visual properties.
In 45 minutes time, the cork will increase in length and height and, similarly, a linear decrease in width will be observed. Water will be used on different time schedules according to each factory, after which it is emptied and replaced by clean water.
3.2. Stabilization
The cork displays a 25% moisture rate. During the stabilization stage, the cork will be stacked for two week so as to obtain a decrease in moisture and the adequate mechanical stabilization. The stabilization room must be well-ventilated and have specifically controlled environment conditions (RH<70%) in order to prevent the disordered growth of microorganisms.
The cork's microflora is virtually killed during the boiling procedure and very few thermophilae species manage to survive. A further recontamination of the planks by the factory's microflora will then take place.
A study that was carried out in 3 factories by performing several microbiological analysis to the planks and the corresponding environment shows that there is already a major growth of microorganisms directly from the 2nd week onwards.
3.3. Punching and rectification
The planks are cut into cross-strips whose lenght will be slightly higher than the cork that will be manufacured. The corks are punched with a metal hollow cylinder whose diameter corresponds to the desired cork's.
The process of rectification consists of trimming the ends and controlling the surface's oval shape.
3.4. Washing/Drying
There are several washing procedures that can be performed in tanks or automatic rotatory machines. The traditional method consists of a wash with chlorinated lime solution, followed by neutralization with oxalic acid. As a result, the cork stopper will display a white look by deposition of a solid crystal. This process improves the cork's visual quality and it is highly spread by commercial reasons.
The hypochlorite washing is similar, although the neutralization product is the sodium oxalate (non crystalline) and the cork stopper's look is not so white.
The washing method with peroxides is done inside a drum or tank followed by neutralization with citric acid.
As to the washing with metabisulphite, the corks are immersed in a 1-2% potassium metabisulphite solution, which allows to maintain or even darken the cork's natural colour.
It is extremely important to control the several washing baths according to their adequate chemical parameters as well as controlling the cork's residues. After washing, the cork should be immediately dried at a 6-8% moisture rate. Storage of high moisture corks may lead to the disordered development of microorganisms and the appearance of mouldy odours.
3.5. Marking and Surface Treatment
The process of marking the cork's surface can be made in ink or fire (hot metal). The adequate surface treatment permits to easily insert and extract the cork in the bottle, thus improving its sealing ability, as well.
Basically, 2 types of products are used:
3.6. Package
There are 2 main types of package, depending on whether or not the cork is sold as finished or semi-finished product. Consequently, packaging material will be made of raffia or jute bags with a maximum capacity of 5.000 to 20.000 semi-finished corks. As to the marked and treated corks, they are packed (1.000 or 1.500) in airtight bags under vacuum and SO2 (used as sterilizing agent), whose action will preserve their aseptic conditions for several months. Finally, the bags are put into carton boxes.
3.7 Transportation and cellar storage
There are examples of cork lots that have become contaminated during transportation, whether due to the contact with incompatible materials (pesticides, hydrocarbonates,...) or because of inappropriate shipment conditions (high cork moisture content, contamination with saline water, etc,...).
The reported cases are those in which the problem was felt at a considerable extent and the level of contamination was immediately felt after the arrival. Although contamination in small proportions does not display any visual problem, it must not be at all ignored. However, this type of contamination may lead to serious undetectable alterations in the wine.
Storage in cellars is another factor of risk. It is of the utmost importance to control the product's final quality and assess the hygienic and environmental shipment's conditions as well as the storage places in cellars.