Science

The Science of Lees Aging

Lees aging not only yields attractive aromas and textures for still and sparkling wines alike—it also prevents oxidation and structural instability. Here’s how

Visit of undergrounds caves, showing of sediment in bottle, traditional making of champagne sparkling wine in Cote des Bar, Champagne, France.
Known for its role in the traditional method of sparkling winemaking, lees aging has myriad benefits for a wine’s flavor and texture. Photo credit: Adobe Stock.

For many wines, aging 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 percent microbial cells, 25 to 35 percent tartaric salts, and 30 to 40 percent 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.

It’s also important to differentiate between the primary, or gross, lees, and the fine lees. The former are generated in the first 24 hours of fermentation. “There you have a high volume of grape particulates, tannins, tartaric acid, and so forth,” says Bruce Zoecklein, a professor emeritus at Virginia Tech

The composition of the no-longer-active yeast cells changes over the course of years, starting with the degradation of the inner cell walls in the first six months. The components released from this process, called yeast autolysis, include polysaccharides, glucans, mannoproteins, and chitin. 

“The mannoproteins make up a good portion of the macromolecules that are particularly germane to lees storage and have the greatest impact on the end result,” says Zoecklein. Mannoproteins are complex proteins with significant amounts of carbohydrates in their formation, most notably the mannose sugars that give them their name. Given their complex nature, they, along with other components of the lees, interact with the aging wine in multiple ways that yield the aroma characteristics, mouthfeel, stability, and aging capacity that we associate with lees aging.

From the Barrel to the Nose and Palate

Many wine drinkers notice the impact of lees aging on the nose of the wine, particularly when it comes to traditional-method sparkling wines, which gain toasty, brioche-like aromas and flavors from yeast autolysis in the bottle. Those aromatic benefits are fundamental to Champagne’s character. “Lees aging provides plenty of aromas that you find in most Champagne,” says Émilien Boutillat, the chef de cave at Piper-Heidsieck, “especially aged examples … All of that is coming directly from the yeast because the yeast we use is Saccharomyces cerevisiae, which is the same yeast, actually, that we use in bakeries or in beer making.”

Lees aging offers textural benefits for traditional-method sparkling wines as well. “The effect of compounds like yeast mannoproteins on foam stability is similar to that of the soap or other surfactants added to make stable bubble solutions for blowing bubbles,” says Gavin Lee Sacks, a professor of food science in the College of Agriculture and Life Sciences at Cornell University. “Unperturbed bubbles ‘pop’ because they dehydrate and lose water from the film that creates the bubble. Mannoproteins and related long-chain molecules can sit on both sides of the film surface and slow the rate of dehydration.” 

From left to right Gavin Lee Sacks, professor of food science in the College of Agriculture and Life Sciences at Cornell University (photo courtesy of Gavin Lee Sacks); Frederic Barnier, winemaker at Maison Louis Jadot (photo credit: Thomas Alexander).
Left: Gavin Lee Sacks, the professor of food science in the College of Agriculture and Life Sciences at Cornell University (photo courtesy of Gavin Lee Sacks); Right: Frédéric Barnier, the winemaker at Maison Louis Jadot (photo credit: Thomas Alexander).

“The bubbles will seem more silky, less aggressive, and creamier,” adds Boutillat, “and will also bring some roundness besides just the effervescence.”

The mannoproteins also affect the aroma and mouthfeel of still wines. Zoecklein says they and other components of the lees can bind with barrel-derived elements such as vanillin and some tannins, reducing the perception of both oaky aromas and astringency. “You enhance that harmony by having the mannoproteins present,” he says. “They act as sweet elements that round out the palate, especially the mid-palate, and allow the wine to be much more integrated.” 

He notes that lees aging for reds became especially popular a couple of decades ago, when it became common practice in California and other regions to open up the canopy to allow more sunlight onto the grape bunches. This greater exposure to UV light leads to a buildup of bitter flavanols, so lees aging is then used to mitigate that bitterness.

Ensuring Stability and Consuming Oxygen

The most fundamental and most studied benefits of lees aging have little to do with the aromas or taste of the wine. “Probably the most well-established effect is that wines which are aged sur lie tend to have less problems with potassium bitartrate instability,” Sacks says. Potassium bitartrate instability can lead to small crystals, sometimes called “wine diamonds,” forming in the wine later. While the exact mechanism is unclear, research suggests that some of the  compounds in the lees bind to the crystals as they form, deterring their growth and precipitation in the wine. 

While harmless, Sacks says wine diamonds “are the number one reason for consumer rejection of wines and returns, because people think it’s glass.” According to Sacks, this characteristic of lees aging has been studied more in-depth than many others due to the potential for financial loss.



Lees also offer other protections to the finished wine. “One of the most important contributions of the lees is to protect white wines during aging, especially aging in cask,” says Frédéric Barnier, the winemaker at Maison Louis Jadot in Burgundy. “The lees have the capacity to catch the oxygen and protect the whites from oxidation.” Recent studies suggest that lees could consume as much as four milligrams per liter of oxygen. 

Bringing Out the Best From the Lees

Generally, the effects of lees aging are considered positive, and winemakers may work to accelerate or heighten them. The most traditional technique for doing so is bâtonnage, whereby the lees are stirred regularly to suspend them in the aging wine for a time. While stirring does risk introducing more oxygen into the wine, Zoecklein says the process heightens the breakdown of glucans, the structural components of the cell walls, and thereby increases the concentrations of mannoproteins. 

However, producers can also add fresh lees, or add an enzymatic supplement such as glucanase to speed up the breakdown of the yeast cell walls. Some research has shown that the lees could also be removed, subject to ultrasonic waves that hasten the breakdown of microbial structures, and then reintroduced into the wine.

One must also take care to ensure lees are healthy. “When you rack your first tank or settle your first juice,” Barnier says, “you need to look at the color of your lees, smell it, or sometimes taste it.” Grapes with botrytis or oidium could introduce off-aromas into the lees, and from there into the wine. In those cases, Barnier says they would probably introduce clean lees from the same plot, but which were pressed in a different machine.

From left to right: Bruce Zoecklein, professor emeritus at Virginia Tech (photo courtesy of Bruce Zoecklein); Émilien Boutillat, chef de cave at Piper-Heidsieck (photo courtesy of Piper-Heidsieck).
Left: Bruce Zoecklein, a professor emeritus at Virginia Tech (photo courtesy of Bruce Zoecklein); Right: Emilien Boutillat, the chef de cave at Piper-Heidsieck (photo courtesy of Piper-Heidsieck).

If the lees are clean, there is no known limit where lees exposure can go astray and create off-aromas or flaws in wine, but the ability to absorb oxygen seems to fade after 3.5 years. Until that time, however, the oxygen absorption is so strong that wines aged in stainless steel, which is impermeable to oxygen, risk developing sulfurous aromas and other reductive characteristics. Some winemakers accept these aromas in the young wines with the understanding that the wine’s long-term age-worthiness will still benefit from the lees’ impact. 

“[The gross lees] are particularly notable for their ability to absorb oxygen and therefore create a strong reductive atmosphere,” says Zoecklein. For this reason, it’s more important to distinguish between gross and fine lees for red wines; reds are typically racked off the former, and only the later-developing, finer lees are used for aging. 

Even there, care must be taken, as mannoproteins can bind with anthocyanins and decrease color stability; in fact, some research has demonstrated that lees could be used on Champagne Pinot Noir musts to remove “pinking” of the juice that might occur during pressing.

The work of the lees may not end when the wine is racked. “We sometimes make Fine de Bourgogne, which is distilled from the lees,” Barnier says. “When they’re good lees, they’re a concentration of the top flavors from the wine you have aged with them, and very interesting in terms of complexity, richness and flavors. We obtain beautiful Fine de Bourgogne that way.”

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Jim Clarke writes about wine, beer, and spirits for trade and consumer publications, including Beverage Media, Fortune, and World of Fine Wine. He is a sommelier and the U.S. marketing manager for Wines of South Africa.

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