What are 'lees' in the context of winemaking?

In the context of winemaking, 'lees' refers specifically to the sediment that consists of dead yeast cells, along with other solids such as grape particles and tartaric crystals that settle at the bottom of the fermentation vessel after fermentation has occurred. After fermentation, yeast cells die and become inactive, contributing to the sediment that can impart flavors and mouthfeel characteristics to the wine.

Winemakers often leave the wine in contact with the lees for a period of time—an approach known as "sur lie aging"—which can enhance the complexity and richness of the wine through a process called autolysis, where the dead yeast cells break down and release flavors and compounds into the wine. This technique is especially common in the production of certain styles of wine, like sparkling wine and some white wines, to add texture and depth of flavor.

While grape skins, barrels, and the fermented grapes themselves are all important components in winemaking, they do not define 'lees.' Instead, it's the accumulation of dead yeast cells and other particulates that characterizes this crucial phase after fermentation.