There are many activities that take place in a winery. One of those activities is topping off barrels of aging wine. Wine in barrels will evaporate. This is referred to as the angel’s share. I don’t know if angels are happier in California with all of its wine regions or Kentucky with its bourbon industry, but in either case there is evaporation. One difference, though, between winemaking and distilling bourbon is that winemakers top off. Law does not permit distillers to top off a barrel of bourbon. It is not uncommon to hear of a twenty-year old barrel of bourbon being opened just to discover that it has all evaporated. To make matters worse, the distillery had to pay taxes on that barrel all those years. Perhaps this is why many bourbons are bottled after eight to 14 years and you pay more for the longer it was aged.
The winemaker will check the barrels and add more wine to the barrel to keep it full. Home winemakers who age their wine in carboys have less evaporation, but may still have to top off. You may have noticed glass carboys at wineries. Often the wine in these carboys is used to top off barrels. Other wineries may keep the wine used for topping off in a variable capacity stainless steel tank.
The reason to keep the barrels topped off as well as the carboys filled to the neck is to reduce the amount of oxygen that can contact the wine. At this point in the winemaking process, oxygen contact is a bad thing. Not all winemakers use wine to top off barrels. Some will use an inert gas that fills the space in the barrel replacing the oxygen.
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