Yeast is indispensable in winemaking. It is only through the presence of yeast that grape juice can transform into wine.
While yeast may be considered the protagonist of winemaking, its stage of activity is not limited to the period of alcoholic fermentation alone. Even after completing its major role of alcoholic fermentation, yeast continues to significantly influence wine aging in the form of lees.
For example, there is a winemaking method called Sur Lie. This technique involves keeping wine in contact with lees during the aging period after alcoholic fermentation, allowing the wine to receive stronger influence from the lees. In sparkling wines exemplified by Champagne, it is well-known practice to age wine with lees for extended periods ranging from several months to years, both during base wine production and after completing secondary fermentation.
The influence that wine receives from lees is sometimes explained simply as "umami dissolving into the wine." However, in reality, the impact of lees on wine is extremely broad, affecting not only taste but also aroma. This influence is so significant that basic winemaking practices typically involve transferring wine to separate containers once alcoholic fermentation is complete, separating it from the lees.
Recently, however, there has been a growing trend to more actively incorporate lees contact even in the production of white wines and rosé wines, where contact with lees was traditionally minimized. To understand the reasons behind this trend, it is necessary to know the true nature of lees and understand the mechanisms by which they influence wine.
This article provides a detailed explanation of how lees are involved in wine aging.
What Are Wine Lees?
Lees can be written as either 滓 or 澱 in Japanese. They refer mainly to residual matter produced during winemaking or sediment that accumulates at the bottom of containers.
While there are no clear rules for distinguishing between these two terms, the general tendency is to refer to matter produced during the winemaking process as "lees" and that observed in bottles after bottling as "sediment." This article will maintain this distinction, using "lees" for matter produced during winemaking and "sediment" for material found in bottles.
Both lees and sediment are precipitates related to wine, but they have different compositional structures.
The Identity of Sediment (Precipitate in Bottles)
What wine consumers most commonly encounter is overwhelmingly sediment. The majority of sediment consists of polyphenolic compounds, proteins, and tartrate. When these components contained in wine meet multiple conditions, including the presence of oxygen, they bind together to form larger substances that increase in weight. Components that become sufficiently large and heavy can no longer remain dissolved in wine and settle at the bottom of bottles. This is the true nature of sediment.
Polyphenolic compounds, one of sediment's main components, include representative substances such as anthocyanins and tannins. Therefore, sediment is most commonly observed in red wine bottles.
The Composition of Lees (Precipitate During Winemaking)
On the other hand, what winemakers constantly deal with is lees. When transferring wine from tanks after alcoholic fermentation is complete, large quantities can be observed accumulated at the bottom of tanks.
The majority of lees consists of dead yeast cells, with small amounts of tartrate, inorganic matter, and tissue fragments from grape skins that became mixed in during pressing. During alcoholic fermentation, when grape juice transforms into wine, yeast cell counts reach 50 million to 100 million per milliliter at their peak. While yeast multiplies actively, many individual cells simultaneously die. Lees are the accumulation of these dead yeast cells.
The term "dead cells" may evoke negative images. However, in reality, dead yeast cells are rich in nitrogen, proteins, and amino acids, serving as an ideal nutrient source for living yeast that are actively metabolizing. Yeast remain important to wine not only while alive, but from the moment they die and even after becoming lees.
Two Important Functions of Lees: Adsorption and Supply
The influence that wine receives from lees during aging can be broadly classified into two categories: "adsorption" and "supply."
Lees adsorb certain components contained in wine, and through the later separation of lees from wine, these components are removed from the wine. Conversely, lees also supply components to wine that were not originally present. The phenomenon of umami being added to wine through Sur Lie is precisely an example of this supply function. Playing a major role in this supply function is what is called autolysis—the self-digestion process of yeast.
Components Adsorbed by Lees
Removal of Harmful Substances
The targets of lees adsorption from wine are diverse, but most are substances that are undesirable in wine. Lees perform the extremely convenient role of removing things that should be eliminated while leaving things that should remain. Representative examples of such undesirable substances include Ochratoxin A (OTA), 4-ethylphenol and 4-ethylguaiacol, certain pesticide residues, and oxygen. Conversely, representative substances that we prefer not to be adsorbed include phenolic compounds, particularly anthocyanins.
Removal of Toxins
OTA is a toxin that becomes contained in grape juice due to certain molds. It does not exist in juice extracted from healthy grapes, but is often found as a secondary component in grapes infected with Botrytis, the causative agent of gray mold disease.
While the use of fining agents is known to be effective for OTA removal, adsorption and removal by yeast is also possible. Although fining agents are superior in terms of simple removal rates, using lees for removal enables treatment of wines where fining agents are difficult to use, or allows for winemaking without fining agents altogether.
Reduction of Off-Odors
Lees also possess the potential to reduce off-odors caused by Brettanomyces.
Brettanomyces, also called a spoilage yeast, produces compounds called 4-ethylphenol and 4-ethylguaiacol—referred to as volatile phenols—through its metabolism. Each of these becomes a cause of off-odors, but reports indicate that their content decreases through contact with lees.
If lees contact could reduce Brettanomyces contamination, this would be extremely beneficial. However, the adsorption of volatile phenols is closely related to multiple factors including pH, temperature, alcohol content, and lees condition, so it is not simply a matter of leaving Brettanomyces-contaminated wine in contact with lees.
Removal of Pesticide Residues
For example, within EU territories, strict rules are established for pesticide handling. Among such rules are those that set residual periods for each pesticide and prohibit harvesting during those periods. If a certain agent has a residual period of 40 days, after spraying this agent, harvesting of the treated crops cannot be performed for at least 40 days.
Due to such rules, even with wine grapes that undergo no washing process after harvest and are processed directly, there is not significant concern about large amounts of sprayed agents contaminating wine. However, since pesticide residual periods are basically set using concentrations that indicate harmfulness as indicators, the reality is that pesticide residues do not necessarily become completely zero after this period.
Lees protect wine from such concerns as well. Lees can adsorb some of the agents sprayed for pest and disease control of grapes.
The amount of adsorption varies for each type of agent confirmed to be adsorbed by lees. Some are adsorbed almost entirely, while others remain at about 20% of their original amount. Additionally, verification has confirmed that the adsorption amount for each varies depending on the type of yeast that became the source of the lees.
Oxygen Adsorption
Oxygen, which can be harmful to wine, is also one of the substances that lees can adsorb. Utilizing this characteristic makes it possible to reduce the addition of antioxidants during aging.
However, the oxygen adsorption capacity of lees is not particularly high. It has been determined that lees adsorb dissolved oxygen in wine only for several months after the start of aging. Additionally, it has been suggested that when oxygen supply during alcoholic fermentation was high, the subsequent oxygen adsorption capacity of lees decreases.
While lees certainly have aspects as antioxidants, they do not have effects sufficient to completely eliminate sulfur dioxide addition. There are also suggestions that lees adsorbing oxygen makes wine reductive, resulting in reduction odors. While this seems plausible at first glance, regarding this point, the sulfur-containing amino acid supply capacity is considered to have a greater impact than the oxygen adsorption capacity.
Adsorption of Phenolic Compounds
Anthocyanins are the pigments that cause red wine to be red. They are classified as phenolic compounds. Lees also adsorb multiple phenolic compounds including these anthocyanins.
The adsorption of phenolic compounds related to color, such as anthocyanins, has both positive and negative aspects. Positive aspects include the ability to prevent undesirable coloring called pinking that occurs mainly in white wines, and the possibility of using lees as an alternative to adsorbent materials like activated carbon that might be used during Blanc de Noir production. The negative aspect is that red wine color becomes lighter.
Regarding the reasons for anthocyanin reduction, in addition to the possibility of adsorption to lees, the influence of enzymes contained in lees has also been suggested. While research reports present multiple claims, it is currently believed that each factor interacts with the others.
It has also been determined that lees adsorb some tannins. This weakens wine's astringency and is expected to reduce wine's harsh qualities.
Yeast Autolysis and Component Supply Capacity
Lees' supply capacity follows two patterns. One is the pattern where substances that were adsorbed when lees still existed as living yeast are released again into wine after becoming lees. The other is the pattern where yeast that have become lees undergo autolysis—that is, decompose their own cells—thereby supplying various components that had accumulated within cells to the wine.
Yeast autolysis is called "self"-digestion because this decomposition process is carried out by enzymes that the yeast themselves produced. In the initial stages when autolysis begins, progress is gradual because enzyme quantities are small, but as cell walls dissolve, enzymes that existed within cells are released into wine, and these enzymes cause the decomposition process to progress further. During this process, not only are various components contained within cells supplied to wine, but high-molecular compounds within cells undergo enzymatic hydrolysis and are supplied to wine as low-molecular products.
Among the components supplied through lees decomposition, particularly important are lipids, amino acids, and polysaccharides called mannoproteins.
Major Supply Components
Lipids
Lipids have a major impact on the aroma of wine during aging. They are released into wine as lees undergo autolysis, increasing their content. The reason this substance has such a significant impact on wine aroma is that, as a source of fatty acids, it is involved in the formation of aromatic volatile compounds such as esters, ketones, and aldehydes.
While lees have the characteristic of adsorbing some volatile compounds, they simultaneously supply volatile fatty acids such as terpenoids and lactones that have floral and fruity aromas to wine. Additionally, the important brioche aroma in sparkling wines exemplified by Champagne is also created based on these lipids.
Lipids hold even greater significance for sparkling wines.
When lipids dissolve into wine, the characteristics related to surface tension change. Simply put, positive effects appear on foam formation and foam persistence—which could be called the lifeline of sparkling wines. Aging sparkling wine base wines and wines that have completed secondary fermentation with lees for extended periods has great significance not only for aroma but also for subsequent effects on foam.
Amino Acids
Through yeast self-digestion of their cells, large quantities of amino acids are supplied to wine. Among these amino acids is glutamic acid, which becomes the source of umami components. This is why wine aged with lees is said to have umami dissolved into it.
However, amino acid supply is not necessarily entirely beneficial.
There is debate about whether it is appropriate to include umami in wine that was not originally present in grapes, but even more problematic are the occurrence of sulfur odors (reduction odors) from the supply of sulfur-containing amino acids and the formation of biogenic amines caused by amino acids. Excessive sulfur odors are recognized as fault aromas, and biogenic amines including histamine and tyramine are causative substances for strong allergic reactions such as headaches and rapid blood pressure changes. Amino acids supplied to wine through lees become one of the causes for such substance groups to be contained in wine.
Biogenic amine formation occurs through amino acid decarboxylation reactions and through microbial metabolism. While it has been determined that microbial production is greater in terms of formation quantity, it has been confirmed to increase markedly when bacteria are present in lees. Lees contain all of the following: amino acids, decarboxylase enzymes that change amino acids, and microorganisms that cause such decarboxylation reactions. To prevent increased biogenic amines in wine due to lees contact, management of lees condition is essential.
Mannoproteins
Mannoproteins are polysaccharides that come to exist in wine mainly through lees autolysis. As the name "protein" suggests, they are a type of protein containing mannose at a certain percentage or higher and are a major component forming yeast cell walls.
Polysaccharides that may be contained in wine can be broadly divided into two types based on their origin. One type derives from grape cell walls. They become contained in juice through crushing of grape skins during destemming and pressing. Arabinans and AGPs (arabinogalactan-proteins) are included here. Pectinase, one of the enzymes that may be used in winemaking processes, aims to decompose these substances.
The second type is mannoproteins derived from yeast. While they are supplied from yeast to wine during alcoholic fermentation, large quantities are also supplied from lees during aging. It has been determined that supply quantities vary depending on yeast type.
Mannoproteins supplied to wine stabilize red wine color and suppress astringency by binding with phenolic compounds, while also having effects as a type of stabilizing agent that prevents precipitation and sedimentation of tartrate, tannins, proteins, and other substances. Additionally, some mannoproteins present in wine become peptides through enzymatic decomposition. Reports indicate that the amount of these peptides present affects wine sensory evaluation.
Summary: How to Utilize the Diverse Characteristics of Lees
By aging wine with lees, wine receives diverse influences across a wide range. Moreover, while there are certain tendencies, there are no absolute rules. Both adsorption and supply by lees vary greatly depending not only on the type and condition of the yeast that became the source of lees, but also on the condition of the wine.
While lees adsorption effects occur from relatively early stages, supply effects that presuppose autolysis require a certain amount of time. However, autolysis itself has already begun from the point of alcoholic fermentation, and influences at imperceptible levels are constantly being received.
In this sense, at what point in winemaking to reduce lees contact becomes an important decision in winemaking.
As Sur Lie has gained popularity in some circles, actively pursuing lees contact makes wine distinctive, for better or worse. The absence of fixed rules simultaneously means that executing the same procedures will not yield identical results.
Both yeast and lees inevitably exist in winemaking. Since they are not substances that are newly added, there are low psychological barriers to their use, and they can be easily incorporated. Moreover, their effects are pronounced. If one wants to quickly change wine, this is a method with no reason to ignore.
However, as we have seen, the influence of lees on wine is extremely large and diverse. The possibility of easily overwriting wine's original character with lees character cannot be ignored. What exists there can also feel like excessive self-assertion by the winemaker.
Using lees has aspects of recent trends, and many winemakers implement it without deep consideration. While I do not deny incorporating longer lees contact itself, regarding the results, it might be worthwhile to reconsider after reorganizing and re-understanding lees characteristics once more.
