Natural Cork Stoppers

The production of natural cork stoppers encompasses various stages. The most significant are punching, which is when the cork plank is perforated giving rise to a single piece, and the selection – meticulous choice of the corks for the different grades.

The amadia cork planks (from the third harvesting) are cooked in clean boiling water after the period of stabilization or rest. The boiling process lasts for at least one hour. The objectives of boiling are:

1. Clean the cork;
2. Extract water-soluble substances;
3. Increase its thickness, thereby reducing density;
4. Make the cork softer and more elastic.

Before boiling, the cork cells are compressed in an irregular fashion, but during this process, the gas contained in the cells expands. As a result, the structure of the cork becomes more regular and its volume increases by around 20 per cent. The boiling is an operation prescribed by the International Code of Cork Stopper Manufacturing Practice. An operation which, besides improving the internal structure of the cork, also ensures that the microflora are significantly reduced. Several cork stopper manufacturing companies use additional processes to obtain better disinfection, while others use dynamic systems in which water is constantly circulating and being decontaminated before re-entering the boiling cycle.

A period of stabilisation of the cork occurs after boiling. The planks are selected only after this period has ended, which lasts for two to three weeks. Stabilisation serves to flatten the planks and allows them to rest. This is the only way for cork to reach the necessary consistency to be transformed into stoppers. Stabilisation also enables cork to reach the ideal moisture content for processing – around 14 percent.

The edges of the planks are prepared and the corners trimmed before initial manual assessment takes place. The planks are separated into quality categories based on their thickness, porosity and appearance.

Then, they are cut into strips with a width slightly greater than the length of the corks to be manufactured.

Punching is the name of the manual, semi-automatic or automatic process of perforating the strips of cork with a drill. A cylindrical stopper is thus obtained within the defined dimensional limits. All the waste from the punching stage are used for cork granulate. Cork that is not directly used for top-of-the-range natural cork stoppers will be processed to make granulate for technical stoppers. Or it can also be used for manufacturing agglomerated cork products that are used in insulation and as building material (wall and floor coverings) and decoration.

After the punching, rectification is designed to obtain previously specified final dimensions and to regularise the surface of the stopper.

Selection, commonly known as choosing, is the operation designed to separate the finished stoppers into different grades, which are determined by automatic scanning of their surface. In some cases, selection is carried out visually and manually, relying on the human eye. During this phase, in addition to establishing quality classes, defective stoppers are eliminated.

The stoppers are washed after rectification using either hydrogen peroxide or paracetic acid. This cleans and disinfects the stoppers. Other methods can, however, be used such as microwaves or ozone.
After washing/disinfection, the moisture content of the stoppers is stabilised, ensuring optimal sealing performance while simultaneously reducing microbiological contamination.

the corks may be colmated to produce colmated stoppers. Colmation consists in sealing the pores on the surface of cork stoppers (lenticels) with a mixture of cork dust obtained from the correction of natural cork stoppers. In order to fix the dust in the pores (lenticels), a glue based on natural resin and a water-based glue are used. This process is used to improve the visual aspect of the stopper and its performance.

This operation is carried out according to customers’ specifications as to the type of marking to be applied. Printing methods available are food-quality ink printing, heat marking or also laser marking.

The surface is treated with paraffin or silicon after marking, to make the stopper easier to insert and extract from the bottle.

Once their production is finished, the stoppers are packed in plastic bags with SO2 (sulphur dioxide), a gas that blocks microbiological proliferation. Only then, the stoppers are transported to the bottling company to be used as closures for wine or spirits.