Lees stirring, or bâttonage, is the practice of stirring the sediment in the barrel, cask or tank. In the case of a barrel a curved, stainless steel, sickle-like instrument is fed into the barrel through its bung-hole before being swept back and forward to mix wine with sediment. I recall Kumeu River (tastings) winemaker, Michael Brajkovich MW, telling me that a visiting South African winemaker told him that he used a golf club to stir the lees. “I normally use a seven-iron”, he explained, “but if I want greater length I use a three-iron”.
During fermentation, yeast cells feed off grape sugar and nutrients. When fermentation stops they sink to the bottom of the tank or barrel (or bottle in the case of bottle-fermented sparkling wine). Now it’s the wine’s turn to feed off the yeasts. Lees stirring increases lees/wine contact resulting in an extra layer of brioche, yeasty flavour complexity and greater body or richness. Lees contact (amplified by stirring) encourages the release of mannoproteins, which can bind to tannins and improve the wine’s mouthfeel.
When the yeasty sediment in wine is more than 10 cm thick and is left for over a week the wine is likely to develop off-odours such as hydrogen sulphide, disulphide or mercaptan. Stirring the lees helps to prevent off-odours.
Stirred lees act as a buffer between wine and oak flavour (if in barrels) reducing oak influence.
In summary, lees stirring is a winemaking technique that can be used to add desirable flavours, reduce the development of undesirable flavours, increase the richness or body of a wine and manage the amount of oak influence.