Corks and Closures

The mechanism used to seal the bottle is a major decision made by winemakers. Natural corks, synthetic corks, and screwcaps are the current bottle closures available. Screwcaps are difficult for amateur and home winemakers to use as they require special equipment to fasten the cap to the bottle. They are an increasingly popular closure for commercial wineries though as they eliminate the problem of cork taint and preserve the wine from oxidation very well.

Natural corks are the most common closure and come in several forms and grades of quality. Cork is the spongy bark from a species of oak tree. Most commercial cork comes from forests in Portugal. The biggest problem with natural cork is that it can develop off flavors in the wine. Natural cork is subject to microbial degradation and requires a cleaning process before use. A specific spoilage called trichloroanisole (TCA) is the major concern with natural cork. TCA is an aromatic compound that can develop during the cleaning process when molds present in the cork metabolize some of the chlorine cleaning agents used to bleach cork. TCA smells similar to moldy wet newspaper and is sensed in extremely small quantities in wine.

The difficulty with spoilage of natural cork has lead to the development of synthetic corks. These corks are some form of plastic polymer that is mostly inert. Some synthetic corks can impart an unpleasant flavor to wines, but the incidence is far less than natural corks. The difficulty with synthetic cork closures is that they usually don’t provide as tight a seal and allow quicker oxidation of wine over time. Some synthetic corks can also be very difficult to remove from the bottle.

The tops of wine bottles quite often are covered with a melted bead of wax or a decorative metal foil to better seal the bottle and provide a more aesthetic appearance.