What is "bâtonnage"?

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Dear Dr. Vinny,
What is bâtonnage?
—N.W. Wine Guy, Tacoma, Wash.
Dear N.W. Wine Guy (if that is your real name),
Bâtonnage is the French term for stirring settled lees back into wine. To remind you, “lees” are the sediment of winemaking, usually made up of dead yeast and bits of grape seeds and solids. Winemakers sometimes like to keep some of these solids in contact with the wine as a way to extract flavor, aroma and texture. The solids can then be filtered or fined out before bottling, or the wine can be racked, leaving the solids behind.
As you might imagine, if left alone, these solids will settle to the bottom of a barrel, which can be bad. If the lees are left undisturbed, they run the risk of stinky hydrogen sulfide forming. Bâtonnage helps prevent this, as well as extract some of the texture and complexity the lees can offer. Just like when I put sugar in my coffee, it’s a good idea to stir that sugar in, instead of just letting it sit on the bottom.
But a winemaker can’t just keep stirring the lees endlessly—all that stirring will eventually make the wines taste less fresh, so it’s a bit of a balancing act. A winemaker can use a specially designed tool, a long baton that will fit in the hole of a barrel to bâtonnage, but I’ve also heard of barrels designed to be rolled or rotated to upend settled lees.
—Dr. Vinny