Where does the extra wine come from for "topping off" a barrel?

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Dear Dr. Vinny,

What does it mean to “top off” a wine barrel? Why is it done, and where does the extra wine come from?

—Donna, New York

Dear Donna,

Simply put, “topping off” a wine barrel, sometimes referred to simply as “topping,” is the process of refilling any wine that has evaporated. Barrels aren’t completely airtight, so after a while the evaporation creates an increased headspace, or ullage. Winemakers fill this headspace both to avoid any bacterial spoilage and to reduce the amount of air the wine in barrel is exposed to, which can both result in oxidized notes and further evaporation.

How quickly evaporation happens depends on temperature, relative humidity, the movement of air in the barrel room and, of course, the integrity of the barrel. A still, humid cave will have less evaporation than a dry, breezy, air-conditioned room. Most winemakers probably check their barrels and top off anywhere from every couple of days to every few weeks.

The evaporated wine is sometimes romantically referred to as the “angels' share,” but I’ve heard that as much as 5 percent of a wine’s production can disappear into the air. That can add up to a lot of potential sales that the angels enjoy.

As far as what’s poured back in, anticipating the inevitable evaporation, winemakers will typically set aside some of the same wine, in either another barrel or stainless steel tank. If not the same exact wine, another, similar wine is blended in. I’ve even heard of bottled wine being used in a pinch.

—Dr. Vinny

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