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6.2.2.a What materials are available and what are their characteristics?

Cork stoppers

There are different categories of corks depending on their manufacturing process.

  • Natural cork: cork is the sole constituent of these stoppers, which are cut directly from cork planks. They are then sorted according to their visual appearance. Natural corks can be classified into different categories depending on the quality of the cork. Each cork maker adopts a designation of its own to define its respective criteria. For example, Château Latour selects categories of corks for the Grand Vin with very few lenticels on the end of the cork in contact with the wine.
Corks selected for Château Latour's Grand Vin

  • Colmated cork stoppers: another version of the natural cork stopper. However, an additional operation is necessary due to the significant number of lenticels (pores) present on the cork. These are sealed with a mixture of glue and cork powder.

  • Stoppers made from cork fragments: these stoppers are made from cork fragments and cuttings. Their advantage is their homogeneity both within the cork itself and between different corks. They are good for relatively short-term bottling.

  • Agglomerated stoppers: these are made of cork granules glued together with food-grade glue.

  • Twin-top, 1+1 stopper, or technical cork consists of an agglomerated body with natural cork discs at each end.

Technological stopper: composed of micro-granules of cork stuck together with food-grade glue.
Cork stoppers benefit from a noble and traditional image although there is a risk of cork taste in the wine. Cork can contain 2,4,6 trichloroanisole (TCA), the molecule responsible for the corked taste that can appear in wines. Quality controls make it possible to reject the batches of contaminated corks and reduce the risk of contamination of the wine once it is in the bottle. Some cork manufacturers have developed methods to eliminate potentially contaminated corks and offer a guaranteed TCA-free range.

Cork stoppers have the lowest carbon footprint. On the other hand, they require more monitoring with regard to their storage conditions. Cork is a "living" material which is sensitive to humidity and temperature. They should therefore be stored in optimal conditions. The humidity of the cork should be kept between 4 and 8%. It is often recommended to use corks a few months following their manufacture. These stoppers allow air to pass through their porous structure and each stopper is characterized by an oxygen transfer rate (OTR) (See section Management of the bottling process).

Synthetic stoppers

Synthetic closures are made of various materials, mostly cork waste or plastic (such as polyethylene or vinyl acetate). They are manufactured using different types of processes such as moulding or extrusion which is a technique that enables small solid waste or powder to be combined by passing it through a calibrated orifice under pressure.

Co-extrusion is also possible. This involves two components being extruded at the same time to form a single component. Co-extruded stoppers are composed of a central component and an outer layer.

Screw caps

Screw caps are made of aluminium. There are two seal types depending on the desired air permeability. The choice is between a low-permeability seal and a very low-permeability seal. The selection depends on the desired protection from oxygen. This type of closure enables the preservation of varietal aromas, especially for thiolated wines such as Sauvignon Blanc.
However, screw caps do not allow the natural reduction of CO2 contained in the bottle with time. This implies that dissolved gasses have been perfectly managed and/or gas has been injected at the time of bottling, which can also lower the risk that reduction will occur.
Screw caps do not always have a positive image with consumers, depending on the country's culture, and break with the tradition of using cork stoppers. In Australia, for example, screw caps are generally well accepted by consumers whereas they are not very well received in France.

Choice of stopper

Selection of the type of stopper depends on:

  • The product's turnover time: that is, the time elapsed between bottling and the wine's "ideal" time of consumption according to its storage potential.

  • The bottle filling level: the more headspace, the greater the security, especially in relation to temperature. A rise in temperature during transport or storage can lead to an increased wine level, causing spillage due to the pressure of the liquid on the cork, or even for the cork to partially come out of the bottle.

  • And also the wine's characteristics and its sensitivity to oxygen. A very tight stopper is appropriate for a wine particularly sensitive to oxidation (synthetic stopper, screw cap, etc.). The risk of TCA contamination can also be a criterion for selection.

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