Lees: the wine's larder
After fermentation, dead yeast cells settle as lees. Ageing a wine on its fine lees (sur lie) adds body, savoury/bready complexity and helps protect it from oxidation.
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Bâtonnage
Stirring the lees (bâtonnage) increases the effect — more texture and creaminess. It's a hallmark of many white Burgundies and is also key to Champagne's character.