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Dear Dr. Vinny,

What are synthetic wine corks made of?

—Adrian, United Kingdom

Dear Adrian,

The two main types of synthetic corks are made from either petrochemical-based plastics or plant-based plastics.

The petrochemical-based plastics are made from low-density polyethylene, a pliable type of plastic. Plastic pellets are melted down, and then turned into a foam consistency so they’ll mimic natural cork’s spongy texture, typically then covered with a smooth outer skin.

The plant-based plastic corks are similar in production, except that they are made from biopolyethylene, a type of renewable polyethylene that’s made from ethanol derived from the dehydration process of raw materials like sugarcane and sugar beets.

The plant-based synthetics are increasing in popularity, since they have a low carbon footprint and are renewable. Bioplastics like these are also commonly used to make water and soda bottles.

Why do some wineries choose synthetics over traditional natural corks, screwcaps or composites? Synthetic corks are cheap: They cost about a dime to 15 cents each, about the same as a composite cork; screwcaps can cost up to 25 cents, and good-quality natural corks can cost anywhere from 75 cents to $2. Some folks opt for synthetic corks to eliminate the risk of "cork taint," or the potential for irregularities that arises from using a natural product like cork.

I don’t mind synthetic corks, but they can be really hard to get out of the bottle—they can be so stiff sometimes that I’ve actually broken a corkscrew on a couple of them. I’ve also heard anecdotal reports that wine aged for a long time (years) under synthetic cork can take on some off odors or flavors that may be linked to the plastic.

—Dr. Vinny

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