At the end of the fermentation(s) there is a large population of yeast (and possibly ML bacteria) in the wine. Once they run out of sugars or are inhibited by an SO2 addition these organisms become inactive and settle out at the bottom of the fermenter forming a layer of solids called the "lees".
If during a white wine fermentation; A) the yeast did not get the proper nutrition it required, B) the wine was not stirred daily to release the volatile sulfur compounds created by the yeast during fermentation, and/or C) the temperature was allowed to run warm (≥70 F) the yeast become stressed and produce H2S as well as other undesirable compounds. If this happens, when the unhappy yeast settle out to form the lees at the end of fermentation, the source of these negative sulfur compounds is localized and concentrated at the bottom of the vessel. The longer the wine stays in contact with these undesirable compounds the more it will incorporate them- to the point of spoiling the wine pretty quickly. Therefore a good safe-guard to avoid creating sulfur problems in our wines is to quickly rack off of the lees as soon as fermentation is over. This is the approach that is recommended in most white winemaking texts.
However, it would be a shame to just leave it at that and proclaim that "all lees are bad". If we adopt proper white winemaking protocols like removing the press solids pre-fermentation, use nutrients, stir the entire lees daily, and control the fermentation temperature, chances are you will not have any negative, sulfur compounds at the end of fermentation! This is a good thing because when there are no undesirable sulfur compounds in the lees they can be a fantastic source of positive attributes. If managed correctly, working with clean, untainted lees can bring complementary flavors and aromas of honey, nuts (hazelnut, almond), toasted bread, and spice to a wine. The lees can be used to create complexity and a creamy mouthfeel in the wine while helping to better integrate oak, fruit, tannins, and acidity into a unified whole. The yeast lees maintain a reductive capacity for 6 weeks post primary fermentation which means their presence in the wine helps to protect it from oxidation at the beginning of the ageing/storage period. Finally working with the lees can actually help a wine become both more heat and cold stable when preparing for bottling.
The compounds responsible for creating most of these positive reactions are proteins that come from the yeast (and ML bacteria), called "mannoproteins" and "polysaccharides". Yeast (and ML bacteria) are like little water balloons in that they have an outer skin that retains a liquid center. This liquid center contains various proteins. When the cell dies, the balloon eventually breaks open in a process called "autolysis," releasing proteins into the wine. Much in the same way cream cheese can smoothly coat the rough texture of a piece of toast, making it more unctuous in the mouth, autolysis proteins help round-out and smooth-over the potential edginess that tannins, firm acidity, and undesirable polyphenols can sometimes create in our wines. Working with these proteins is a very powerful tool that white winemakers can use to raise the quality of their wines.
As fantastic as all the benefits of working with the lees are, it is important to remember this technique may not be suited for every wine. If you are trying to create a very delicate, light wine that just highlights the gossamer essence of your grape, having a noticeable impact from the lees may actually detract from the targeted style. In this case you may be best served by not doing any lees contact during the ageing/storage period. One winemakers "complexity" may be another’s "masking-over/shellacking of the original fruit"! In the end, just how much lees contact (if any!) we choose to employ comes down to personal preference and the style of wine we are making. There is no right answer. The important thing to come away with after reading this section is to understand that working with the lees is just a tool and as long as the benefits and drawbacks of using this tool are understood, it can be modified to fit our needs. Often in winemaking, the best decisions are not purely black or white, but lie between the two extremes in a well-informed shade of gray! Experiment, taste often, and if you don't like what is going on, all you need to do is simply rack off the lees.
If you will be ageing on the lees ("sur-lie") here are some important points to keep in mind:
Note: If you do have a sulfur problem and need to rack off of the lees, it is still possible to get some beneficial protein impact into your wine: Specially inactivated yeast products such as Opti-White, Sur Lie (Bio-Lees) and Noblesse can be used to help positively modify the aromas and flavors of your wine during ageing/storage. Keep in mind the amount used post fermentation will be much less than the amount added at the beginning of fermentation: 0.1 - 0.5 g/gal A quick bench trial will help find the best ratio for your wine. Remember, be conservative with your addition amount: as with any addition, it is easy to add more later but hard to remove if you use too much!
If you will not be ageing on the lees then you will be racking 2 or three times during the entire ageing process (maybe once every 2-3 months) to separate the wine from the lees and help clear it out.
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