What seals the bottle
Cork, wire cage, crown cap: closures and their influence
The closure plays a part in how a sparkling wine ages. The mushroom-shaped sparkling wine cork lets in tiny amounts of oxygen over the years, while the wire cage holds it in place against the pressure. The crown cap is used mainly during the second fermentation. In Germany, Sekt b.A. must be sealed with a cork and wire cage.
How is a sparkling wine cork built?
A sparkling wine cork looks different from a still wine cork, and it's built differently too. Its body is usually made of compressed cork granulate. At the bottom, where it touches the wine, sit one or two discs of natural cork. Before insertion, it's a thick cylinder, considerably wider than the bottle neck. A machine compresses it and pushes it about halfway into the bottle. The upper part stays outside and becomes the familiar mushroom head.
Over the years, the cork loses its elasticity. A freshly inserted cork visibly springs open after removal and widens again at the bottom. An old cork stays narrow and keeps the shape of the bottle neck, something referred to as a "peg" shape. If you open many bottles, you can roughly judge from this how long a wine has rested under cork. For vintage Champagne, the year is additionally printed on the cork itself.
Wire cage, plaque, foil capsule: what holds the pressure?
At around 6 bar of pressure inside a Champagne bottle, a cork without support would eventually get pushed out. That's why the wire cage, also called a muselet, sits over it, twisted under the lip of the bottle neck. Between the wire and the cork sits the plaque, a small metal disc. It keeps the wire from cutting into the cork and is popular among collectors. Over everything goes the foil capsule, which covers the closure.
In Germany, this closure is mandatory for Sekt b.A. Wine regulations require a sparkling wine bottle with a mushroom-shaped stopper, a retaining device, and foil; only bottles up to 0.20 liters may be sealed differently. A Prosecco Frizzante, on the other hand, often carries a string tie, the spago. As a semi-sparkling wine, it holds less pressure and doesn't need a wire cage. Tax law also plays a role here, since a sparkling wine closure with a wire cage can classify it as fully sparkling for tax purposes.
Crown cap: fermentation closure and final closure
In the traditional method, nearly every bottle begins its life under a crown cap, the kind familiar from beer bottles. Underneath often sits a small plastic cup, the bidule, which collects the lees during riddling. The crown cap seals tightly, is cheap, and can be removed quickly during disgorgement. Only afterward does the bottle receive its dosage and cork. So lees aging happens under crown cap, while the aging that follows happens under cork.
Some wines keep the crown cap right to the end. This includes many Pét-Nats and Prosecco Sui Lieviti, which are sold with their lees still inside. Some producers of traditional-method sparkling wines also use it as a final closure. It's considered very airtight, the wine develops more slowly and evenly beneath it, and cork taint is ruled out entirely. For a Sekt b.A. in a standard bottle, however, it remains off-limits, since the law requires cork and wire cage there.
Cork taint and oxygen: the risks of natural cork
Natural cork is a natural product, and therefore never entirely uniform. The best-known flaw is cork taint. It's usually caused by the compound 2,4,6-trichloroanisole, known as TCA, and smells of musty cellar, wet cardboard, or damp newspaper. Sparkling wine is just as susceptible as still wine. According to French trade press, a 1996 study by the Comité Champagne found no difference in risk between bottles stored lying down versus upright.
The second issue is oxygen. Every cork lets in a slightly different amount of air. With young wines, this barely matters, but after ten or twenty years, it becomes quite noticeable. That's why two bottles of the same old vintage often taste different, something experts call bottle variation. Many cork producers today treat their granulate against TCA. If you open a suspect bottle, smell the cork first, then the wine.