One of the many “mysteries” surrounding the story of wine is the use of cork as the primary closure for fine vintages. The ritual of smelling the cork before a sample is poured contributes to the “uniqueness” of the wine experience. However, beyond this somewhat pretentious role, the cork does perform an important function in preserving wine quality.

After bottling, the most important thing standing between a great bottle of aged wine and vinegar is the small plug of tree bark from the cork oak tree. It allows the very slow transpiration of oxygen that helps the wine mature in the bottle, yet it prevents direct exposure to air that would induce spoilage.

Well over half of the world’s cork is grown in southern Portugal and is harvested from trees that are at least 25 years old. Once the cork oaks are fully mature, their bark is stripped into large slabs about every nine years. Corks, like grapes, are natural products and subject to individual variation. The longer and finer corks, the more expensive because they are cut from the bark with the fewest flaws. A tree also needs more than the minimum renewal phase for a 3 ½-inch fine grade cork as opposed to the time needed for a 2-inch average grade cork.

After being stripped from a tree, the cork slab may spend a full year in the forest before it is transported for processing. Once they reach the factory, each section is graded and boiled in water to kill any existing microorganisms. They are then stacked, flattened and allowed to air dry. Slabs are cut into strips and corks of varying lengths are punched out. Any excess pieces are collected and ultimately used for the ubiquitous corkboards, shoe soles and tiny slides that keep chairs from scratching wooden floors.

Most corks are bleached after grinding, shaping and shaving. They are rinsed and sun or vacuum dried. Ideally, their moisture content is 6-8 percent to keep the cork somewhat pliant, yet inhibit the growth of molds and unwanted microbes. The finished products are graded, bagged and vacuum-sealed before shipment to various distributors.

Sometimes tiny microorganisms buried deep within the bark resist boiling, occasionally bleach residue remains despite extensive rinsing and periodically mold will collect as the corks slowly air dry. In these cases, corks may cause “wine taint,” which results in musty odors or other off aromas in finished wines. About 2-3 percent of all bottled wines show some degree of cork taint. These problems, along skyrocketing prices and occasional political instability in Portugal, account for the growing interest contemporary winemakers are showing in plastic “corks,” screw caps and other new types of closures.

However, despite the potential problems, cork remains the closure of choice for the vast majority of international winemakers. Tradition, along with the average consumer’s fascination with the accoutrements surrounding the world of wine, assure that corks will have a central role in the industry for years to come.

This summer, the OWPA is launching an “Ohio, Uncorked!” visitation program showcasing our wineries and the various closures they use. It will be a fun project. More information will be coming soon.


DONNIELLA WINCHELL is the executive director of the Ohio WIne Producers Association. She can be reached at

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