Classification of cork stoppers
The density of the cork is associated in regard with the elasticity of the stopper. Natural corks have a density between 160 kg/m3 and 220 kg/m3. Cork stoppers can have lower or higher densities.
The relative humidity in the cork stoppers should (ideally) be between 4% and 9% in order to maintain a suitable elasticity and to reduce the risk of microbial development within the cork stopper.
There are different products used in the treatment of cork stoppers. Paraffin and silicon are the most used. The treatment with paraffin is used for sealing and also to provide some lubrication for the cork stopper. Silicon is principally used for lubrication of the cork stopper in order to facilitate the cork insertion when bottling and also when opening the bottle. There are also other options available on the market which use lubricating and sealant polymers compatible with the foods industry. The type of treatment applied and dosage depend upon the type of wine, the type of bottle, the maturation period and the bottling machine. For wines that need to be aged in the bottle (more than 18 months), a paraffin surface treatment must be done first followed by silicone treatment. Whatever the treatment used, it must be of the highest quality, as there is no use in having an aesthetically attractive stopper with deficient finishing that may harm the final performance of the stopper.
The force of extraction required for the stopper tends to lessen with time in the bottle. The recommended values are between 20 and 40 kg (24 hours after bottling). The specifications vary according to the market. Nowadays, producers of cork stoppers have the means necessary for the development of surface treatments that meet requested specifications.
The visual class of the stoppers is established on the basis of the quantity and size of holes the surface has.
The size of the batches should always be taken into account when sampling and the standardized sampling tables – NP (Portuguese Standard), NP 2922 or ISO 3951 or ISO 2859, should be followed depending on the applicable mode of quality control.