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Corks seal a wine’s fate: aging under natural vs synthetic closures

By Andrew WaterhouseProfessor of Enology at University of California, Davis | The Conversation | Published December 30, 2014

Most foods are best as fresh as possible. I remember picking peaches at my grandfather’s ranch in Northern California and eating them on the spot. What a taste! But the exceptions to this rule are the many wines that actually need some aging to taste their best. Winemakers know this, and work to control the aging process including decisions they make about how to bottle up their product.

Aging and oxygen

One aspect of aging has to do with the reaction of fruit acids with the alcohol. This process reduces sourness in the wine, but it’s really only important for very tart wines, the ones coming from cold climates.

The complex oxidation process is the second aspect of aging. When oxygen interacts with a wine, it produces many changes – ultimately yielding an oxidized wine that has a nutty aroma. This is a desired taste for sherry styles, but quickly compromises the aromas in fresh white wines.

However the oxidation process provides benefits along the way to that unwanted endpoint. Many wines develop undesirable aromas under anaerobic – no oxygen – conditions; a small amount of oxygen will eliminate those trace thiol compounds responsible for the aroma of rotten eggs or burnt rubbber. Oxidation products also react with the red anthocyanin molecules from the grapes to create stable pigments in red wine.

The way a bottle is sealed will directly affect how much oxygen passes into the wine each year. That will directly affect the aging trajectory and determine when that wine will be at its “best.”

Stick a cork in it?

Glass is a hermetic material, meaning zero oxygen can pass through it. But all wine bottle closures admit at least a smidgen of oxygen. The actual amount is the key to a closure’s performance. A typical cork will let in about one milligram of oxygen per year. This sounds like a tiny bit, but after two or three years, the cumulative amount can be enough to break down the sulfites that winemakers add to protect the wine from oxidation.

There are three major closure options available: natural cork and technical cork, its low budget brother made of cork particles, the screw cap and synthetic corks. Natural cork closures appeared about 250 years ago, displacing the oiled rags and wooden plugs that had previously been used to seal bottles. It created the possibility of aging wine. Until 20 years ago natural corks were pretty much the only option for quality wine. It’s produced from the bark of the tree, and harvested every seven years throughout the life of a cork oak tree, Quercus suber. The cork cylinder is cut from the outside to the inside of the bark…

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