Winemaking decisions regarding lees are as important to the final composition of a wine as yeast selection, temperature control, and cap management. Following fermentation, winemakers need to decide how they are going to age and mature their wines. Any decision that involves wine maturation should also consider lees management, and should be planned-out in the same manner you plan any other step in vinification. It is essential that winemakers understand what constitutes “lees” in both grape must and wine, and how the lees should be managed throughout fermentation and aging.
First, what are they?
Lees are simply a deposit that forms in your wine after allowing it to settle. While that seems like a simple enough definition, there are many different types of particles that make up the lees, and while some can be beneficial to a wine, others can produce bitter or herbaceous compounds. In French, “lees” refers to a very specific kind of deposit – mainly dead yeast and bacteria cells. Thus, there is less confusion as to what type of particles are involved when using lees as an enological tool. In English speaking countries, where lees can contain a number of different deposits from the wine, winemakers often use the term “light” lees when talking about lees containing yeast and bacteria (the good lees), and “heavy” lees when referring to the lees that should be discarded during racking. From now on, when using the word “lees,” I’ll be referring to “light lees.” I’ll refer to “heavy lees” as dregs. This will hopefully help avoid any confusion.
So lets talk about the positives of using lees as part of your aging program. A good rule to remember is that the lees are light, fluffy particles that generally remain in suspension 24 hours after moving your wine (e.g. racking, stirring, or pumping), as long as you didn’t use pectins. They are typically very small in size (from 1 to a few dozen micron). In contrast, dregs are particles that will settle at the bottom of your vessel after 24 hours, and can be very large (100 microns to several millimeters). Because lees are essentially dead yeast and bacteria, they contain different types of molecules such as proteins and polysaccharides that can benefit the taste and aroma of your wine when correctly managed.
Polysaccharides are involved in the sensation of roundness and fullness in a wine. They also create chemical bonds with tannins, color pigments, and aromatic molecules, thus making them unable to precipitate from your wine. This chemical bond is what makes barrel-aged wines less susceptible to protein instability. The binding of polysaccharides with aromatic molecules is what causes a wine to have a good “length” on the palate, as the aromatics will be released more slowly when you taste the wine. Wines aged in oak need lees contact to help integrate the flavors of the oak with the flavors of the wine.
Other products released by the yeast cells include amino acids and nucleic acids. Both are used as flavor enhancers in the food industry, and work the same way in your wine. Yeast cells will also release esters as they start to break down. Many of the yeast esters have floral and spicy aromas that will contribute to the overall bouquet of a wine.
Lees also have the ability to remove certain undesirable compounds from wines. It is well-known that they help bind diacetyl, the buttery-smelling aroma produced during malolactic fermentation, but they have also shown some ability to remove the volatile phenols produced by Brettanomyces. Not only do they remove unpleasant odors, but they also help remove potentially harmful compounds such as residual pesticides and fungicides that may appear in wines, as well as a class of compounds known as biogenic amines (e.g. histamine, tyramine, phenylethylamine). These compounds are known to cause headaches, nausea, hypotension or hypertension, and cardiac palpitations in people who are sensitive.
At this point you may be wondering why you haven’t been using lees all along! However, even the “light” or “fine” lees can cause some problems with your wine. While the chemical bonds created by the polysaccharides help make a wine more protein stable, it also makes them more stable against tartrate precipitation. While this is a good thing in most areas of the world, in Minnesota most wines benefit from the drop in acidity that results from tartrate precipitation. Thus, cold stabilization may want to be carried out prior to leaving your wine age on lees.
Also, while the amino acids liberated by the dead yeast can contribute positively to the wine, often these same amino acids can contain a sulfur-group that can lead to reduction odors and sulfur-off odors in your wine. Regularly stirring your lees will help minimize the risks by ensuring that the bottom of your tank or barrel doesn’t become a concentrated zone of reduction. Stirring is also essential to breaking up the cell walls of the yeast to ensure that all of the polysaccharides are liberated. Thus, it is essential that wines are stirred regularly while being aged on lees.
What about the dregs?
Heavy lees essentially have no benefit to a winemaker. They are, in fact, detrimental to wine quality. They are made up of large particles and agglomerations of different particles in wine. They can be vegetal particles, crystals, large tannins, coloring matter, and any combination of the above along with yeast and bacteria cells! Wines that are left in contact with the dregs can become bitter and herbaceous. It should be noted that dregs will continually form in your wine through various reactions that occur over the course of maturation. Thus, wines should be evaluated over the course of their maturation. Each wine will vary somewhat in the frequency that it should be racked. Generally, once wines are initially racked following alcoholic and malolactic fermentations there is no reason to rack before a 3 month aging period.
J.A. Pérez-Serradilla, M.D. Luque de Castro. 2008. Role of Lees in Wine Production: A Review. Food Chemistry. Vol 111:2