Lees ageing and bâtonnage

After fermentation the dead yeast cells settle out. Leaving the wine in contact with them — rather than racking it off — adds body, a creamy texture and bready, savoury notes.

Stirring the lees back into suspension (bâtonnage) increases the effect. It is central to white Burgundy and the styles that follow it.

The lees also consume oxygen, giving some protection against oxidation, and can contribute to reduction if taken too far.

Lees are the sediment left after fermentation, principally spent yeast cells. Gross lees — the coarse material that settles quickly — are usually removed. Fine lees are what a winemaker leaves the wine on deliberately.

What happens next is autolysis: the dead yeast cells break down and release mannoproteins, polysaccharides and amino acids into the wine. That adds body and a rounder, creamier texture, and a savoury, bready or biscuity flavour.

It is a slow process. A few months on lees adds texture; the brioche character of Champagne requires years, which is why minimum lees-ageing periods are written into sparkling wine appellation rules.

Stirring the lees — bâtonnage — keeps them in suspension and speeds the exchange. It also increases the risk of reductive off-aromas, so it is a technique with a cost as well as a benefit.

Lees also protect the wine. Yeast consumes dissolved oxygen, so a wine ageing on its lees is partly shielded from oxidation, which is one reason Muscadet sur lie ages better than its price suggests.