The first time a new barrel is filled with wine, more than three gallons of wine soaks into the wood surfaces. Later, when a barrel is stored empty, the wine soaked into the wood quickly acetifies. The wine in the wood turns to vinegar, and the barrel becomes contaminated with vinegar bacteria. Some home winemaking books recommend cleaning barrels contaminated with vinegar bacteria with soda ash. However, sterilizing contaminated barrels is practically impossible.
Experienced winemakers use their noses, and they discard any barrels smelling of vinegar.
Empty barrels can be safely stored for several weeks if they are gassed with sulfur dioxide and kept tightly sealed. Home winemakers and small commercial wineries burn sulfur wicks in their barrels. Large wineries purchase sulfur dioxide gas in high pressure cylinders. Barrels gassed with sulfur dioxide should be washed with clean water before they are filled. Winemakers face another serious problem when empty barrels are stored for long times.