Aging Wine on the Lees
Recently the question was asked of me, “What are lees, and why do winemakers age wine on the lees?” First of all, lees are essentially dead yeast cells that have already done their job of fermenting sugar in grape juice into alcohol in wine. Once the fermentation process is completed, these dead yeast cells, let’s call them lees from this point on, begin to settle to the bottom of tank or barrel, allowing cloudy wine to become clear over time. Discussed below are several options that the winemaker now has regarding handling lees, and some technical descriptions of what lees actually do for and to a wine.
Once fermentation is completed, the lees as well as other solids left in the wine begin to settle to the bottom of tank. Heavier material, called gross lees, such as bits of grape skins and seeds, dirt, and some of the lees, settle to the very bottom, often in just a day or two. The fine lees, essentially just dead yeast cells, require a number of days to settle into a next layer, leaving the liquid wine above. Most often, before beginning an aging regimen, the winemaker will either rack off (separate) the wine from the lees, or rack down to the gross lees keeping the fine lees with the liquid.
One technical consideration of keeping lees with the wine is the fact that lees readily absorb oxygen and can lead to a reductive state. Although difficult to fully describe, and also to appreciate, the final result of creating a reductive state is that some hydrogen sulfide (rotten egg smell) and other problematic sulfur compounds can form, creating a distinctive flawed aroma in the wine. The best way to manage this is to frequently check the wine’s aroma and allow periodic oxygen contact, as needed. Of course, one must guard against too much air exposure which can cause premature oxidation of the wine. It’s a balancing act, for sure.
Removing lees from wine before aging will provide a clean, fruit-forward character, avoiding both detrimental and positive impacts that lees can have on a wine during aging.
Keeping lees with the wine can have several positive benefits. One, the absorption of oxygen, can help protect wine early in the aging process. This can also help with malolactic transformation since malolactic bacteria prefer low oxygen conditions. The transformation of malic acid into lactic acid, often called secondary fermentation, produces a softer, rounder mouth-feel in wine and can be accompanied by additional chemical transformations, such as production of diacetyl which adds buttery notes to a wine. Again, the amount of oxygen (from air) needs to be controlled throughout the aging process.
One way to add small amounts of oxygen to wine and enhance the benefits of aging with the lees is to stir the wine periodically to lift the precipitated lees into the liquid. Each time the tank or barrel is opened, air can enter and get stirred into the liquid. The lees, over time, begin to degrade as their outer membranes break open, allowing compounds called mannoproteins to enter the scene. These mannoproteins can improve the texture (mouthfeel) and enhance lees-y or bready aromas that are often associated with Champagne or other sparkling wines. Both white and red wines can develop a more rounded, mellow mouthfeel while the tannic character in red wines can be mitigated to an extent as tannins become bound to these mannoproteins, thus lowering astringency.
Frequent stirring of the lees, or batonnage in French, helps the lees to break down and release mannoproteins into the wine. A wine that has been aged on its lees with batonnage often carries the term “sur lie” on the label. Aging a white wine in this manner will certainly create a different style from one aged without the lees. Most red wines, especially those aged in barrels, are kept in contact with their fine lees.
One final technical comment relates to the positive impact that lees can have on wine stability. Lees can help limit the amount of protein colloids that form in a wine, improving heat stability. At the same time, cold stability can be improved as less tartrate crystals (“wine diamonds”) tend to form after aging on the lees. This allows a winemaker to use less bentonite (1), for example, to improve stability, thus leaving more of the natural aromas and flavors in a wine.
Eventually, having done their duty, lees will be removed via settling followed by racking off the wine, and/or filtration before bottling so that a clear, clean wine can be offered to the customer.
Background information on lees in wine can be found in the following references:
Lees and You, Dead yeast can be your friend by Alex Russan, Winemaker Magazine, p63-66, Apr-May, 2019
Lees (fermentation) @ Wikipedia.com
What Are Wine Lees? Sur Lie Explained by Madeline Puckette @ WineFolly.com, 9-Jun-16
(1) Bentonite is an activated clay material used to bind molecules like protein colloids and tartaric acid which can create cloudiness or precipitates in wine. A future Carl’s Corner is planned to address this subject.