For many wines, ageing on the lees is fundamental to their production, most notably in traditional-method sparkling wines and white Burgundy. Producers of the former tout the time spent on lees as a quality indicator, and many still wines call out their sur lie production on the label. What the lees actually do to the wine, however, is complex—and even debated. The lees themselves are a natural by-product of the winemaking process, a mix of spent yeast cells, material from the grapes’ skins, and other detritus that settles in the fermentation vessel when that process is complete. According to the textbook White Wine Technology, during the first six months, these consist of 35 to 45% microbial cells, 25 to 35% tartaric salts, and 30 to 40% organic plant debris, and they typically make up two to four percent of the wine’s total volume. It’s the microbial cells that are the most dynamic and the most studied. Primary among them are spent yeast cells and lactic acid bacteria; the latter will typically induce malolactic fermentation if not inhibited by sulfur or other means.
Source: SevenFiftyDaily – The Science of Lees Aging