Wine corks, no matter how small they may be, have a great importance in the process of wine making and preservation of wine bottles. Materials, shapes and sizes have changed (but not too much) during the course of history and every cork keeps distinctive qualities and characteristics according to the wine it is meant to preserve from external factors. A cork is nothing else than a simple cylinder made of various materials, whose task is to seal the bottle and therefore prevent wine from getting in contact with air. In fact, in case bottles were not hermetically sealed, wine could undergo degenerative processes, such as oxidation and acescence.
Wine stoppers: manufacturing materials and characteristics
The main material used for wine stoppers is cork. Light, resistant, waterproof and elastic, it guarantees a perfect isolation of liquids. During the Middle Ages, however, it fell into disuse. This was because the Iberian Peninsula, where most of cork was produced, was under Saracen rule. In its place were used pieces of wood wrapped in cloth and reinforced with pitch.
In the seventeenth century another material was used: glass. Unfortunately it was a fragile and expensive material which was almost immediately replaced by cork. In the same period were also born corkscrews. Today, with the scarcity of raw material and the increasing intolerance to cork defects, alternatives have arrived. Therefore producers have at their disposal many materials to cork their bottles. The choice of the material is however subject to the bottle to be corked and, as a consequence, to the wine stored in it. Besides the traditional cork, the materials used to make corks are: silicone, metal, glass and polyethylene.
Material par excellence for the production of wine corks, it is however subject to many problems. In fact, cork can be attacked by fungal parasites which, once their colony is attacked, release a substance called trichloroanisole (TCA). TCA is the substance responsible for the famous “corky smell”. Traditionally it is said that corks allow the passage of a micro-oxygenation. Some believe this could even help wine to evolve better. For this reason, even today, many young wines are corked with synthetic closures, whereas corks are reserved to wines destined for aging.
Actually it is not like that at all! As a matter of fact, micro-oxygenation does not happen when the bottle of wine is positioned horizontally. It is the liquid itself that acts as a compressor against the cork, preventing air from entering and circulating. Moreover the evolution of vintage wines, such as the portentous Amarone della Valpolicella, or the sublime Recioto della Valpolicella, also depends on certain reactions of polymerization of tannins. Polymerization is possible thanks to the action of some microorganisms present in the wine itself. This is the base of the decomposition of complex organic substances in other simpler ones. The latter lead to oxidoreductive reactions which take place even in absence of dissolved oxygen.
Stoppers for still and sparkling wines
The last phase of wine bottling is the application of the cork. This element varies according to the bottle it is destined to and to the wine it is going to bottle. In general, wine stoppers are divided into stoppers for still wine and stoppers for sparkling wine. The first ones must ensure the hermetic closure of the bottle and their durability varies according to the type of wine. Stoppers for sparkling wine must guarantee resistance to the overpressure to which they are subjected, which ranges from 6.5 to 8 bar.
Stoppers for still wine are divided into stoppers that close the bottle from the inside and external corking systems. The former are inserted by pressure in the neck of the bottle. The nature of their material, which is elastic and expandable, makes them swell and hermetically seal the bottle where they are inserted. They are stoppers that last in time and guarantee a good seal. The latter are called crown caps and screw caps. Both these types are made of metal and have internal coatings formed by special gaskets made of silicone material. Crown corks are disposable, whereas screw corks are reusable.
Stoppers for sparkling and semi sparkling wines must guarantee an hermetic closure and at the same time an excellent resistance to the pressure exerted from the inside by carbon dioxide. In sparkling wines the pressure is usually around 2.5-4 bars, whereas in sparkling wines it is higher and exceeds 6.5 bars. Therefore the constitutive materials are endowed with great elasticity and capacity of expansion. Very often these corks are supported by a safety cage and held by a metal wire, or by a string binding.