What's the appeal of using a concrete "egg"–shaped tank for wine fermentation?

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Dear Dr. Vinny,

I was getting a tour of a winery in Napa and they showed us some concrete “eggs” in the cellar next to the barrels. Concrete? Really? I get stainless steel and barrels, but what is the appeal of concrete for winemakers?

—Ana, Trenton, N.J.

Dear Ana,

Concrete fermentors are one of the hottest trends in winemaking right now, especially in California, but the phenomenon started in France.

But why? Concrete’s functionality as a fermentation vessel lies somewhere between that of stainless steel and oak barrels. Concrete fermentors are neutral, like stainless steel, which means they don't impart any flavors to the wine (although some winemakers use local indigenous rocks like limestone in their concrete mix, claiming that mineral notes may be transferred to the wine). Concrete fermentors' thick walls stabilize temperatures, as well, which is key during fermentation.

Some winemakers believe that concrete can soften a wine’s texture the way a barrel does, or that minute amounts of oxygen may penetrate the concrete.

Concrete tanks come in a variety of shapes, but the “egg” is particularly popular. They look like some sort of cool teleportation pod from a science-fiction movie, and some proponents of the "egg" shape believe that it facilitates the movement of fermentation gases and/or sediment, depending on which end of the egg is up.

—Dr. Vinny

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