I was recently hosting a wine tasting that involved a white wine. On the back label, it stated that the wine had been “Aged on lees.”  I was asked the obvious question “What are lees?”

In the broadest sense, “lees” refers to all the stuff that settles on the bottom of a wine fermentation or aging vessel (e.g., the tank or barrel). This “stuff” can include grape skin fragments, grape seeds, grape stems, pieces if grape leaves and, after fermentation, dead yeast. 

This collection of “stuff” is generally referred to as the “Gross Lees.” Not exactly because it might be considered ‘gross’ in its consistency, but because it is not the collection of stuff that you want your wine to be aged on. These items can to lead to some rather offense odors and flavors in a wine.

Wine makers generally “rack off” their wine (transfer it) to a fresh vessel or barrel that leaves behind the really chunky stuff and only moves the fermented wine, along with a bunch of dead yeast cells that don’t readily settle and remain mixed in with the newly fermented wine.  With aging, the dead yeast cells do settle to the bottom of the tank or barrel (see image).

When a wine is “Aged on Lees,” this contact with the dead yeast cell is allowed to continue throughout the aging process or until the wine maker determines they’ve achieved the desired wine characteristic. Wine makers even encourage the wine’s contact with the dead yeast cells, or “Fine Lees” by stirring the wine to move the lees off the bottom so that it can have more contact with the wine. The French term for this stirring technique is “Bâttonage.”

As the dead yeast cells break down during aging, they can bring out flavors of nuts, honey, bread, etc. They can also bind with tannins in the wine and to help smooth out the wine. So then, when the wine maker is happy with their wine’s characteristics, they let the Fine Lees settle and again carefully transfer (rack off) the wine and leave behind the layer of dead yeast cells that lay on the bottom of the aging vessel.